What’s a red dwarf star?

Red dwarf stars are extremely common, at least in our Milky Way galaxy. They make up some 60 to 70% of all stars in our galactic home. In fact, the closest star to Earth, Proxima Centauri, is a red dwarf. And yet, you can’t see it with your eye alone – or any other red dwarf – because these stars are too dim. Red dwarf stars’ main characteristics are that they’re small, cool and live a long time. And, of course, they have a distinct red color.

The famous Hertzsprung-Russell diagram (or H-R diagram for short) lets you visualize where stars rank compared to other stars and throughout a star’s lifetime. Red dwarfs earn the classification of Type M. The red color is a sign of their low temperature. Cooler stars in the universe radiate light toward the red, long-wavelength end of the spectrum. Meanwhile, the hottest stars radiate toward the blue, shorter-wavelength end and shine blue or blue-white. In the same way, a poker put into a fire will start glowing with a dim red color. It will then glow orange, yellow and finally white as its temperature increases.

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Characteristics of a red dwarf star

Temperature: The surface temperature of red dwarf stars ranges from 2,000 to 3,500 degrees Kelvin (3100-5800 F or 1700-3200 C). Our sun is much hotter, at 5,500 degrees (9400 F or 5200 C), and it glows yellow as a result. A lower temperature also means lower luminosity: The largest, most luminous red dwarfs are only about 10% of the brightness of the sun. The smallest, dimmest ones are only around 0.075% our star’s brightness.

Size: The size of the smallest red dwarf stars is about 9% the radius of the sun. The largest red dwarf known, DH Tauri, has 1.26 times the radius of our sun. So, if the largest red dwarf can be bigger than our sun, then is our star called a dwarf star? And, in fact, astronomers do consider the sun a yellow dwarf star on the main sequence of the H-R diagram. While some ancient cultures worshipped the sun as the most powerful thing in the universe, in reality it’s a small and insignificant star. From just a few light-years away, extraterrestrial travelers might not even give it a second glance!

Lifespan: Red dwarfs are incredibly long-lived. They can live for tens of billions up to trillions of years. In other words many times the current age of the universe! But why is this?

The evolution of a red dwarf star

The key to understanding red dwarfs’ incredible longevity is their mass. Nuclear fusion is at the heart of every star, converting hydrogen into helium and producing heat, light and electromagnetic radiation. This nuclear fusion works at a rate governed by the mass of the star. The more massive the star, the greater the temperatures and pressures at its core, and the faster the fusion process proceeds. And vice versa. Red dwarfs typically have less than half of the mass of our sun. So, hydrogen is converted into helium at a slower rate. The end result is that red dwarfs evolve in slow motion compared to more massive stars.

However, red dwarfs, just like our sun, will one day exhaust their supply of hydrogen. In the case of the sun, this will happen when our star is around 8 to 10 billion years old; in other words, around 5 billion years from now. But because of the slow-motion fusion processes at the core of a red dwarf, this stage won’t arrive until the star is trillions of years old!

In the case of stars like our sun, the exhaustion of its hydrogen supply results in the star slowly inflating into a red giant star, many times its original diameter. But with red dwarfs, this doesn’t happen. Why? Once a star like our sun has exhausted its hydrogen, it starts to fuse helium, which triggers the inflation. However, red dwarfs don’t have enough mass to start fusing helium. Instead, the red dwarf stars bypass the red giant phase. Instead, they’ll slowly shrink and cool at the end their lives, becoming white dwarf stars. This is also our sun’s destiny after its red giant phase.

Exoplanets around red dwarfs

Astronomers have discovered planets orbiting red dwarf stars. What would it be like to live on one?

To an observer on a planet orbiting a red dwarf star, the star would appear to be much larger than the sun in our sky. But why, if red dwarfs are so much smaller than the sun? So far, the planets astronomers have discovered orbiting red dwarfs orbit much closer to their star. The red dwarf would therefore appear much larger than the sun does in our own sky.

The color of the red dwarf star would also be very different from our sun. Our sun emits most of its light in the yellow and green wavelengths of the spectrum, which is why it appears yellow to us. Red dwarf stars, on the other hand, emit most of their light in the red and infrared wavelengths. Thus, they would appear orange-red in the sky. The longer-wavelength light would also mean that the planet’s surface illumination would be far less. Everything on the planet would be dimmer and cast in red tones. Scientists think daytime on planets orbiting red dwarfs would never get any brighter than a sunset does on Earth.

Surveys have discovered that most of the planets orbiting red dwarfs are either comparable in size to Earth or are super-Earths. Scientists estimate that gas giant planets, like Jupiter, Uranus and Neptune, make up just one in 40 planets orbiting red dwarfs. In addition, computer simulations of red dwarf exoplanets indicate that at least 90% of them are at least 10% water by volume, meaning they may have global oceans.

Life on red dwarf exoplanets?

Because a red dwarf is much lower in temperature than stars like our sun, the red dwarf’s habitable zone is much closer to the star than in a planetary system like ours. Therefore, even though the planets we’ve found are closer to their red dwarf star, they might still be in the habitable zone.

However, before we get too excited about this possibility, there is one problem. Red dwarf stars are known for their violent solar flares. These flares can be up to a thousand times more powerful than the largest flares from our sun. Red-dwarf flares can emit intense radiation that can strip away the atmospheres of planets and make them uninhabitable. However, studies have shown that these flares may not be as destructive as previously thought. Flares tend to occur at high latitudes on the surface of a red dwarf, which means they may not strike planets that are orbiting closer to the star.

Solar flares on red dwarf stars are caused by magnetic activity. That’s the same thing that causes them on our sun. Red dwarf stars have very strong magnetic fields, which can become tangled and release huge amounts of energy in the form of a flare. Flares on red dwarf stars can last for hours or even days. And, they can release enough energy to power the entire Earth for centuries!

Is there a reasonable chance, therefore, that Earth-sized exoplanets orbiting red dwarfs, with their apparent abundance of water, might be hosts for life? Let’s look at one example of such a planetary system. This is a system that has astronomers and astrobiologists excited with its possibilities for life: the TRAPPIST-1 system.

The TRAPPIST-1 system

The TRAPPIST-1 system lies about 40 light-years from Earth. It’s home to seven Earth-sized planets, all orbiting the ultracool red dwarf. The planets are all quite close to their star, with orbital periods ranging from 1.5 to 19 Earth days.

Three of the TRAPPIST-1 planets are in the star’s habitable zone. This location makes them some of the most promising candidates for life outside our solar system that we’ve yet found.

When astronomers give names to exoplanets, they do so by designating each planet a letter, where “a” is the star itself, “b” is the planet orbiting closest to the star, “c” the next most distant, and so on. Observations with the James Webb Space Telescope have shown that:

• The surface temperature of TRAPPIST-1b is around 230 degrees Celsius (450 degrees Fahrenheit), making it too hot for liquid water to exist on its surface.
•  TRAPPIST-1c likely has a very thin atmosphere, or no atmosphere at all.

A chance for life near TRAPPIST-1?

However, intense and constant magneto-solar activity on TRAPPIST-1 is interfering with the Webb’s ability to obtain reliable spectra of the star. And from these spectra, scientists tease out the spectra of the planets’ atmospheres. So the spectra will probably need future observations and reanalysis. But, at the moment, it looks as if neither TRAPPIST-b or TRAPPIST-c is a likely candidate for life. Webb observations of the remaining planets, TRAPPIST-d to TRAPPIST-h, are still to come. Then, perhaps, we’ll have a profile of the whole system and understand a little more about red dwarfs, their activity and the effect they have on their planets.

Of course, all this does not mean that the TRAPPIST-1 system is typical of red dwarf planetary systems. It’s a mistake to draw general conclusions from what Webb discovered about its planets. We need to observe other planets orbiting these small, red stars. For example, astronomers have confirmed the existence of three exoplanets in the Gliese 581 system. This red dwarf is the oldest, least active M-type star currently known. Unfortunately, the three planets seem to orbit closer to their star than the inner edge of the habitable zone, so are likely too hot to support life.

The closest star to the sun, Proxima Centauri, is also a red dwarf. It has two confirmed Earth-sized exoplanets, one of which orbits in the habitable zone. However, little is known about this planet at the moment.

Understanding red dwarfs

Understanding red dwarf planetary systems is important, both for studying stellar evolution and in the hunt for extraterrestrial life. If it is true that exoplanets orbiting red dwarfs are never capable of supporting life, then at least 60% of the stars in our galaxy have lifeless systems. And that is significant.

Astronomers will continue to study these small, red, abundant stars in order to understand exactly how they behave and whether they’re capable of giving birth to exoplanets where we might find life. Red dwarfs are the obvious place to search for life-bearing planets because they’re cool, and therefore their habitable zones are close-in. That means that the length of a year for these planets – the time it takes to orbit their star – is much shorter. So, we can make repeated observations of them over a comparatively short time period as they transit across the face of the red dwarf.

Red dwarfs are an interesting area of study for astronomers and planetary science. They seem to be our best bet for finding planets with life. However, there is still a lot about them which we don’t understand. But with incredible tools such as the Webb and the upcoming planet-finding instruments, our knowledge of red dwarfs can only increase.

Bottom line: A red dwarf star is the smallest and coolest type of star known. They’re also extremely common, making up around 60 to 70% of the stars in the Milky Way.

The post What’s a red dwarf? Only the most abundant Milky Way star first appeared on EarthSky.

What are red giants?

Red giants are stars going through their death stages. It has slowly swollen up to many times its original size. Once a star becomes a red giant, it might stay that way for up to a billion years. Then the star will slowly contract and cool to become a white dwarf. The opposite of red giants, white dwarfs are Earth-sized, ultra-dense corpses of stars radiating a tiny fraction of their original energy. Eventually, after billions of years, these stars will become totally cold and radiate no energy. They’ll end their lives as a so-called black dwarf: a tiny, burned-out, virtually-invisible cinder.

To become a red giant, a particular star must have between half our sun’s mass up to eight times our sun’s mass. Astronomers call such stars low- and intermediate-mass stars. So you can see that our sun is one of the stars that will inevitably, someday, become a red giant.

Our sun will become a red giant

In fact, it’s our sun’s destiny to become a red giant star (and afterwards a white dwarf, and then a black dwarf). But what processes will drive the sun’s evolution to the red giant stage? And what will happen exactly, inside the star, as it changes? Let’s examine the fate of low- and intermediate-mass stars such as our sun, as they evolve to the red giant phase.

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Hydrogen: A star’s 1st fuel

Stars radiate energy by converting hydrogen to helium via nuclear fusion. It’s this process that causes our sun to radiate light, heat and other forms of energy as a byproduct. But nuclear fusion in stars at first requires hydrogen. And stars don’t have an infinite supply of hydrogen.

Our sun converts around 600 million tons of hydrogen into helium every second. If that sounds as if the supply should therefore soon exhaust itself, just remember that the sun is a star nearly a million miles across. And if you have trouble visualizing that, imagine boarding a jet airliner for a flight that is going to last 226 days. That is how long it would take you to fly around our local star.

In truth, our sun, as an average star of its type, was born with enough hydrogen to last for around 10 billion years. Astronomers estimate our star is now around 4.5 billion years old. So the sun is leaving the days of its youth behind it. It’s entering into middle age. And like people, it won’t be too long until its processes start to change and falter.

Hydrogen burning and the main sequence

We call the current stage of our star’s life the hydrogen-burning phase. That’s because its energy source is the fusion of hydrogen atoms. But burning is a bit of a misnomer. It’s nuclear fusion, not chemical burning. Stars do not burn in the conventional sense of the word. Still, astronomers do use the term burning to describe the type of fusion going on inside a star. Hence, you might hear of carbon burning or helium burning. Both are stages of nuclear fusion, consuming different elements, when a star nears the end of its life.

Stars that mostly burn hydrogen are in what’s known as the main-sequence phase. As a main sequence star, our sun is in what’s called stellar equilibrium. That means the outward radiation pressure from the sun’s internal fusion reactions exactly balances the inward push of the sun’s own gravity.

It’s important to realize that, when the sun’s on the main sequence, even the consumption of hundreds of millions of tons of hydrogen per second does not immediately deplete the sun’s hydrogen. Only 0.7% of our sun’s hydrogen consumed in the fusion process will ultimately be radiated as energy. The rest is used up converting the hydrogen atomic nuclei into helium atoms. That tiny percentage of energy has been giving us all the light and heat we get from the sun for the last 4.5 billion years!

Read more about this video: An all-sky red giant star symphony

The star begins to die

Eventually, as its nuclear fires falter, a star starts to contract under its own gravity. At the same time the star is shrinking, its temperature is increasing. So the star becomes brighter.

In an aging star, this phase of shrinking and brightening can last for several million years. The shrinking core, which is heating up as it shrinks under gravity, brings more hydrogen towards the center of the star, into the place previously occupied by the now-shrunken core. Eventually, temperatures and pressures are sufficient to ignite this shell of hydrogen around the core: radiation from this new hydrogen-shell burning pushes outward through the star, causing its outer layers to expand.

There are complex physical processes at work here, but the laws of the conservation of energy, in conjunction with the way gravity behaves, mean that if the core of the star shrinks, the rest of the star must expand. The star has started evolving into what is known as a subgiant star, representing an intermediate phase between the main sequence and the red giant stage.

It becomes a red giant

The hydrogen-shell burning occurs through fusion processes that are far more intense than they were when the star was on the main sequence. The result is that the star brightens by a modest amount. But the outer layers of the expanding star, now being further away from the hydrogen shell around the core, cool at the same time, dropping from a maximum of between 6,000 and 30,000 degrees down to 5,000 K. This also means that the star’s light reddens, in the same way that a cooling poker removed from a fire will cool from white through yellow to red over time.

Sun-like stars get bigger and brighter

The hydrogen-burning phase can last for between a few hundred million to a billion years, depending on the initial mass of the star. For stars between 0.8 and two times the mass of our sun, this results in a subgiant which is 10 times the diameter of our sun. Stars of mass outside this range may then follow different evolutionary paths, but for a star like the sun the next phase will be a massive increase in size, a huge rise in brightness and more cooling.

The driving energy for this will arise from the helium core, collapsing, getting denser until, at the end of the subgiant phase, it becomes hot enough to burn its helium. This causes a large increase of energy output which forces the expansion of the star. Eventually, after perhaps hundreds of millions of years, the star will be a hundred times the diameter of the sun and distinctly red in color.

And so a red giant is born.

How long do red giants last?

A star will be in the red giant phase for typically around a billion years. What happens next will depend on the star’s mass. High-mass stars will explode as supernovae. Low- to intermediate-mass stars like our sun will slowly shrink and cool into white dwarf stars.

Red giants vs red supergiants in the sky

We can look up and see several red giants with our unaided eyes. Aldebaran is one example. Keep in mind though, that two other well-known red beasts, Antares and perhaps the most famous one, Betelgeuse, are not red giants, but red supergiants. Red supergiants are the end stages of much larger stars, and will explode in a supernova before they end up as a neutron star or even a black hole (depending on their mass).

Betelgeuse is famous because it made headlines a few years ago when it suddenly started getting dimmer, over a period which lasted for several months in 2019. Its brightness dropped by more than 60%, meaning it was noticeably dimmer in the night sky. Read more about Betelgeuse’s extraordinary dimming.

The eventual fate of our sun

So what about our sun? Over the next few hundred million years, it will slowly increase in brightness and start to radiate more energy across the electromagnetic spectrum, as it heads towards its subgiant phase. That’s bad news for the Earth. In about a billion years, increasing radiation from our star will have sterilized our planet, extinguishing all life.

Eventually, as our sun completes its changes from a modest G-type star into a red giant, it will expand to swallow Mercury, Venus and perhaps Earth too. And that will be the end of our world.

Stars die, too

The study of the end phase of stars is complex, as there are many variables and exceptions. It can throw out the unexpected, like the dimming of Betelgeuse. But these giant stars are just going through a natural phase of life, getting old and dying. By the time our sun, for example, ends its life as a white dwarf, it will have lived for ten billion years. And perhaps, when our star swells up to enormous size, it will be studied by alien civilizations looking from afar, as we study red giants in our skies. They will have little idea that, once, a tiny blue dot orbited that star, whose inhabitants looked to the stars and wondered, too.

Bottom line: What are red giants? Most main sequence stars, like our sun, will become one. Red giants swell to such a large size they can swallow their inner planets.

The post What are red giants? Our sun will become one! first appeared on EarthSky.

The sun – our blazing star – has a metaphorical dark side. It has the potential to cause our modern technological civilization to falter. We had a taste of our sun’s destructive effects on September 2, 1859. On that day, around the world, compasses at sea failed to work, causing some ships to become lost. Telegraph networks experienced disruption, with some telegraph lines catching fire. Tellingly, people as far south as the Caribbean and Mexico saw auroras. Scientists now believe that what happened on that day – 164 years ago – was an extreme geomagnetic storm. Since then, the 1859 storm has become known as the Carrington Event.

Many scientists and others wonder … what would happen if a Carrington Event took place today?

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The Carrington Event

Richard Carrington was a well-known British astronomer of the 19th century. His focus was the sun. He determined the position of the sun’s axis of rotation (the location of its north and south poles) and was the first to learn that the sun doesn’t rotate as a solid body, but that solar material goes around faster at the sun’s equator than at its poles. He discovered that the dark spots on the sun’s surface, called sunspots, vary in latitude over the 11-year solar cycle. He and Richard Hodgson saw the first bonafide solar flare.

On the first day of September 1859, he was observing sunspots when he saw a bright flash of light. Scholars now believe he saw the mighty coronal mass ejection (CME) – a powerful eruption near the sun’s surface, driven by kinks in the solar magnetic field – whose resulting shocks rippled through our solar system. One day later, on Earth, a great storm occurred in our world’s magnetic field. The effects of that great geomagnetic storm are now called the Carrington Event.

Coronal mass ejections (CMEs)

CMEs are common on the sun, especially when the sun is near the peak of its 11-year solar cycle. And aurora-watchers welcome them, because they cause the beautiful displays of auroras, aka northern or southern lights, seen at high latitudes. Nowadays, our spacecraft routinely record CMEs. But, in the 19th century, CMEs hadn’t been discovered yet (although there’d been hints that they existed).

Not until 1971 did the Helios spacecraft discover CMEs from ultraviolet observations.

Carrington Event report

Carrington immediately reported the flash to the Royal Astronomical Society. He probably didn’t give it much more thought until the next day … when the fast-moving solar particles had had time to travel across space to Earth, causing the geomagnetic field to go haywire. Wrapping Earth in a seething, writhing mass of high-energy particles, the blast of solar particles buffeted, squeezed and distorted Earth’s magnetosphere, releasing an estimated 10³⁵ electron volts of energy. That’s a ten followed by 35 zeroes. This amount of energy is equal to a 10-megaton nuclear bomb. It’s also equal to the amount of energy the sun releases in about 10 seconds.

It was the most powerful solar event ever yet recorded.

The effects of the Carrington Event

The effects of the September 2, 1859, solar storm were unprecedented. People saw auroras as far south as the Caribbean and Mexico. At some more northerly latitudes, it’s said the sky was so bright with auroras that birds, thinking it was morning, began to sing. But it wasn’t all awe and beauty. There were widespread stories of people receiving shocks from doorknobs and other metal objects, thanks to the induction of electrical currents. Around the world, compasses at sea failed to work, causing some ships to become lost. Telegraph networks experienced disruption, with some telegraph lines catching fire.

One apocryphal tale tells of a telegraph operator who received a shock from his machine, knocking him unconscious and awaking later to find his arm paralyzed. This story, while remaining uncorroborated, is certainly not beyond the realm of possibility.

The extreme geomagnetic storm subsided the following day. Work began to repair telegraph networks. The brilliant auroras faded from view, and the world returned to normal.

But the stories of the event remain to this day.

A bit of background

The Carrington Event was an extreme geomagnetic storm. To understand the 1859 event, we must understand the solar cycle. German amateur astronomer Samuel Heinrich Schwabe had just discovered the 11-year cycle in the year 1843. Schwabe had been observing the sun for over 17 years when he noticed that the number of sunspots on the sun’s surface varied over time. He also noticed that the period of this variation was about 11 years.

Schwabe’s discovery was a breakthrough in our understanding of the sun. It showed that our star is not a static object, but rather dynamic and ever-changing.

And – thanks to Schwabe’s tracking of the solar cycles – we know that the peak of Solar Cycle 10 was in February 1860. The Carrington Event happened just months earlier, in September 1859.

The sun’s magnetic field

The sun’s magnetic field creates the 11-year solar cycle, which peaks when the north and south magnetic poles of the sun swap places. Around the peak of each cycle, for a few years on either side, the sun can experience violent events, including increased coronal mass ejections (CMEs). We’re in such a time now, by the way. The peak of the current solar cycle is expected in the mid-2020s. You can read the sun news each day at EarthSky’s daily sun post.

When a CME leaves the sun, the sun expels around a billion tons of matter. And sometimes this solar material is directed toward Earth. When it arrives, the Earth experiences a geomagnetic storm, usually not an extreme one, but an awesome event nonetheless. At such times, the solar wind slams into our planet’s magnetic field, infusing Earth’s magnetosphere with high-energy particles.

From our viewpoint on the surface of Earth, one immediate effect is beautiful, bright auroras as the particles collide with atoms in the upper atmosphere, imparting their energy and causing the atoms to glow. This is a geomagnetic storm, and it can last for many hours.

News reports from the time

The Carrington Event was a hot story in newspapers of the day. The September 2, 1859, edition of The New York Times reported:

Last night the city was visited by one of the most brilliant displays of the aurora borealis that has been witnessed for many years. The sky was clear, and the stars shone with unusual brilliancy. About nine o’clock a faint light appeared in the north, which gradually increased in brightness until it reached the zenith. The aurora then assumed a variety of forms, and the sky was constantly changing. At times the whole heavens were illuminated with a brilliant light, and the stars were entirely obscured. The aurora continued for several hours and disappeared about midnight.

On September 3, 1859, The Boston Globe reported:

Yesterday there was a great magnetic storm which affected all the telegraph lines in the country. The telegraph lines in Boston were all interrupted for several hours, and some of them were so badly injured that they will not be repaired for several days. The storm also affected the magnetic compasses on ships, and some vessels lost their way.

And on September 5, 1859, The London Times reported:

On the night of the 1st and 2nd of September … the magnetic compasses were so much affected that it was impossible to steer by them. The aurora borealis was seen in many places where it is rarely seen, and in some places it was so bright that it was possible to read by it.

If a Carrington Event happened today

Today we live in a completely different world. Our technology is advanced, complex and ubiquitous. Where once telegraph lines sang their messages across the flat midwestern plains of the United States, now it’s the internet that connects us and everything we do.

The first undersea transatlantic telegraph cable came just a year before the Carrington Event, in 1858. It connected North America with Europe for the first time, allowing news to propagate around the world faster than ever before. Today, most of the world’s internet traffic flows through undersea cables of vast capacity. Existing cables flow with ever-multiplying streams of ones and zeroes, the telegraph songs of the digital age.

Computers manage our society. They affect every single aspect of our lives, from traffic control to power grids to banking to healthcare to entertainment. The birth of the integrated circuit gave us the modern world, appearing in all modern devices from toasters to televisions and cellphones to cars. What might another Carrington-type event do, if it were to induce large electrical currents in Earth’s magnetic field? What might happen to national power grids? 

There would almost certainly be widespread burnout of electronic circuits and the failure of power grids on a much bigger scale than the 1989 Quebec blackout from a solar storm. Many, many millions of people would likely be without power and unable to use phones or other devices.

The effects on satellites

In space, satellites would also fail as their electronics fried. This has happened several times during geomagnetic storms on a scale far smaller than the Carrington Event. The most recent was in March 2022, when 40 SpaceX Starlink satellites failed after a CME. They launched the day before the storm hit. But it wasn’t their electronic systems that failed. One effect of a geomagnetic storm is to increase atmospheric drag on the satellites. It pulled the satellites back toward Earth, where they burned up in the atmosphere.

Only about 1% of the world’s internet traffic transmits via satellite. However, in the banking industry, ATM and credit card transactions, the transfer of funds and banking messages all travel through satellites. Widespread communication loss would be inevitable. There would be utter chaos for a while. Recovery might take years.

Predicting the next one

If all this sounds frightening to you, let’s ask an important question to put it all in perspective. Just how likely is another Carrington Event? After all, it’s been 164 years since the last one. So do we view it as a blip, or do such events recur on longer timescales, or perhaps even at regular intervals? Can we predict the next storm and what its effects might be? And – perhaps the most important question – just how much notice might we get of an extreme, Carrington-like event?

Let’s start with the source of the problem: coronal mass ejections or CMEs. Yes, we know much more than we did about them since their discovery in 1971. But CMEs are unpredictable. Apart from the fact that they occur more frequently around solar maximum, due to that reorientation of the sun’s magnetic field, we don’t yet know enough about the mechanisms that generate CMEs to say when they will occur.

So, we have no way of knowing when a event similar to the Carrington Event might occur again. We also don’t know how often these events occurred before 1859. Before there were electric grids or devices, such storms probably went unrecorded apart from mentions of brilliant auroras.

Preparing for the next one

How much notice might we receive of an impending, society-changing, potentially catastrophic storm? Well, you’ll be pleased to know that scientists are fully aware of the dangers. They’re working hard using artificial intelligence to model when and where they could hit worst. NASA heliophysicists have created a system called DAGGER, but it could only give us an estimated 30 minutes’ warning of an approaching storm.

We now have the sun under constant observation from Earth and satellites. But when the sun releases a CME, it’s difficult to work out exactly how much material will hit us. Put simply, we don’t know which ones are the dangerous ones.

Hope rests on attaining a greater, more holistic and in-depth understanding of the sun’s magnetic field. One day, we might be able to predict the destructive geomagnetic storms of the future. We can harden our technology and power grids against damage in the same way that spacecraft have their electronics hardened against electrical currents. But that requires lots of money and the world’s politicians to recognize the dangers and act.

So far, they have not allocated nearly enough money and resources to protect us from civilization-destroying asteroids or, of course, the effects of climate change. There’s little reason to be optimistic that those in power will take the threat of another Carrington Event seriously.

Learning lessons from the Carrington Event

The Carrington Event, in the end, caused minimal damage in an age when there was little which could be damaged. But were it to occur today, it would be catastrophic. We really need to learn the lessons from our ancestors and treat the sun seriously as a threat as well a life-giver. If we do not, we will only have ourselves to blame when the next extreme geomagnetic storm hits.

Bottom line: The Carrington Event of 1859 was a massive geomagnetic storm triggered by activity on the sun. People saw auroras at low latitudes that were bright enough to read by.

Read more: Biggest solar superstorm yet, glimpsed in ancient tree rings

The post What was the Carrington Event, and why does it matter? first appeared on EarthSky.

Found! Long-sought gravitational wave background

What are gravitational waves?

Gravitational waves are ripples in the structure of spacetime. Much as a ship traveling across the surface of a calm sea leaves a wake behind it, so moving objects in the universe create gravitational waves. The “ships” in the case of gravitational waves are extremely violent and cataclysmic events far off in the cosmos: black hole mergers, neutron star collisions, supernovae. All of these generate waves in the structure of spacetime, stretching and squeezing it as the ripples travel across the universe.

Because gravitational waves are extremely weak as observed from our earthly vantage point, the technology to detect them has become available only in recent years. Like all waves, gravitational waves diminish in size with distance, shrinking to faint echoes of those distant “shipwrecks” – those distant violent events in the cosmos – by the time they reach us.

From our location, many light-years from a black hole merger or a neutron star collision, the waves compress and stretch space, and everything in it, by a thousandth of the diameter of a proton as they pass through the Earth. That’s a billionth of a billionth of a meter. We require very advanced technology indeed to see that change. So, it’s like seeing the distance between the sun and its closest neighbor among the stars – Alpha Centauri, 4.3 light years distant – change by the thickness of a human hair.

Albert Einstein and his General Theory of Relativity

It was Albert Einstein who, in his General Theory of Relativity of 1915, first postulated the existence of gravitational waves. His suggestion that gravity travels in waves seemed logical: every type of light on the electromagnetic spectrum, from ultraviolet to visible to radio, travels in waves. Sound travels in waves.

Why should gravity not be propagated in the same way? Einstein calculated that extremely violent events in the cosmos would cause space to ring like a bell. This was distinct from the idea of the static, unchanging gravitational fields that are generated by any object that has mass, like a star or a planet.

However, for decades after 1915, Einstein himself was unconvinced of the existence of gravitational waves.  In 1936, he and colleague Nathan Rosen published a paper entitled Do Gravitational Waves Exist? which, initially, was rejected by one journal because of a mathematical error.

It was the error that had caused the authors to conclude that gravitational waves don’t exist. When Einstein had corrected the error, the paper’s conclusion became exactly the opposite! Although the evidence now pointed to their existence, Einstein remained unconvinced, and believed that even if gravitational waves did exist, they would be so very weak that humans could never develop the technology to detect them.

Gravitational waves and their detection

It should be noted that Einstein was not the only theorist who worked on gravitational waves. Important contributions were made by other famous scientists, among them Robert Oppenheimer, Roger Penrose, Karl Schwarzschild, Arthur Eddington, Kip Thorne and Richard Feynman. But it was Feynman who, in January 1957, finally convinced the doubters that not only do gravitational waves do exist, but they can carry energy as well, explaining this by using something he called his Sticky Bead argument.

Feynman’s work directly paved the way for today’s gravitational wave detectors. Yet, it would be another 50 years before the first gravitational waves were detected. Developing the concepts and the technology to do so took decades of hard work by many scientists. Finally, LIGO, the Laser Interferometry Gravitational-wave Observatory situated at two sites in the United States, started observing in 2002. It took several upgrades to LIGO, between 2002 and 2015, to give it the sensitivity to make its historic first detection.

The first detection, of two black holes merging some 1.3 billion light years distant, came in September 2015 and was announced to the world in February 2016 after months of work verifying that the signal, which had lasted a mere tenth of a second in perfect agreement with Einstein’s predictions, was real. Incredibly, LIGO had not yet begun its official observing run when the detection came: after its latest in a series of upgrades to improve its range and sensitivity, LIGO had been turned on for engineering tests. The black hole merger was detected almost immediately the detector was operational.

Another key prediction of Einstein was that gravitational waves would travel at the speed of light. By measuring the difference in time between when the gravitational wave signal arrived at the two LIGO observatories – in Hanford, Washington, and Livingston, Louisiana, separated by nearly 2,000 miles (3,000 km) – scientists were able to determine that Einstein’s prediction was completely correct. Gravitational waves do indeed propagate at the speed of light.

LIGO was joined in 2018 by the European Virgo detector in Italy, which has greatly improved the ability of scientists to pinpoint the location on the sky where the gravitational waves originated. Since then, LIGO/Virgo have detected some 50 black hole mergers, but also eight neutron star collisions and six neutron star-black hole collisions. Some of these may end up being to due to so-called “terrestrial interference”: vibrations from passing traffic and even distant ocean waves can cause false positives.

On January 14, 2020, LIGO also detected an event of completely unknown origin, which does not fit any models or predictions, perhaps, excitingly, pointing to the existence of a hitherto-unknown cosmic phenomenon.

What is coming next?

Very soon, the Japanese KAGRA observatory will join Virgo and LIGO in the detection of gravitational waves. In the 2030s, the European Space Agency will launch LISA, a space-based gravitational wave detector, which should enable the detection of low-frequency gravitational waves emanating from supermassive black holes and from supernova explosions. China has started work on building three gravitational wave observatories, its avowed intent to become the world leader in Earth- and space-based gravitational wave detection.

All of the gravitational wave events detected so far agree perfectly with Einstein’s predictions and with computer simulations derived from his calculations. Einstein would surely have been amazed that he was wrong, that human intellect and ingenuity has indeed triumphed and created the technology he thought impossible. He would also probably have regretted doubting his own work in predicting the existence of gravitational waves. But he would also, surely, have been happy that the detection of gravitational waves is also a confirmation of his theory of Relativity. There are now few places left to run for those who doubt Einstein’s greatest triumph.

Gravitational-wave astronomy is a completely new science and one which promises to unlock many of the universe’s mysteries. It’s no exaggeration to say that a revolution in our view of the universe is underway. In the future, it might even be possible to detect gravitational waves from the Big Bang itself, to hear the sound of Creation ringing out across billions of years.

If you would like to keep up to date with the latest gravitational wave events, the University of Birmingham in the U.K. has created this page, which is a database of LIGO and Virgo detections during their current observing run. The database is also available as a free app for Android/Apple phones, downloadable from their respective stores.

Bottom line: First postulated by Albert Einstein in 1916 but not observed directly until September 2015, gravitational waves are ripples in spacetime.

Read more and watch a video explainer: What are gravitational waves?

The post What are gravitational waves? Ripples in spacetime first appeared on EarthSky.

When a massive star explodes as a supernova at the end of its life, its core can collapse into a tiny and superdense object with not much more than our sun’s mass. These small, incredibly dense cores of exploded stars are neutron stars. They’re among the most bizarre objects in the universe.

A typical neutron star has about 1.4 times our sun’s mass. And they can range up to about two solar masses. Now consider that our sun has over 100 times Earth’s diameter. In a neutron star, all that mass is squeezed into a sphere that’s only about 12-25 miles (20-40 km) across, or about the size of an earthly city.

So perhaps you can see that neutron stars are very, very dense! A tablespoon of a neutron star material would weigh more than 1 billion U.S. tons (900 billion kg). That’s more than the weight of Mount Everest, Earth’s highest mountain.

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Here’s how a neutron star forms.

Throughout much of their lives, stars maintain a delicate balancing act. Gravity tries to compress the star while the star’s internal pressure exerts an outward push. And nuclear fusion at the star’s core causes the outer pressure. In fact, this fusion burning is the process by which stars shine.

In a supernova explosion, gravity suddenly and catastrophically gets the upper hand in the war it has been waging with the star’s internal pressure for millions or billions of years. With its nuclear fuel exhausted and the outward pressure removed, gravity suddenly compresses the star inward. A shock wave travels to the core and rebounds, blowing the star apart. This whole process takes perhaps a couple of seconds.

But gravity’s victory is not yet complete. With most of the star blown into space, the core remains, which may only be twice our sun’s mass. Gravity continues to compress it, to a point where the atoms become so compacted and so close together that electrons are violently thrust into their parent nuclei, combining with the protons to form neutrons.

Thus the neutron star gets its name from its composition. What gravity has created is a superdense, neutron-rich material – called neutronium – in a city-sized sphere. The exact internal structure of this sphere is the subject of much debate. Current thinking is that the star possesses a thin crust of iron, perhaps a mile or so thick. Under that, the composition is largely neutrons, taking various forms the further down in the neutron star they are located.

It’s all about mass.

If, after the supernova, the core of the star has enough mass, scientists believe that the gravitational collapse will continue, and a black hole will form instead of a neutron star. In terms of mass, the dividing line between neutron stars and black holes varies by sources. Typically, astronomers consider the mass of a neutron star to range from 1.4 to 2.9 solar masses. Then, if the collapsed core has more than three solar masses it becomes a black hole.

What are the properties of a neutron star?

Because neutron stars are so dense, they have intense gravitational and magnetic fields. The gravity of a neutron star is about a thousand billion times stronger than that of the Earth. Thus the surface of a neutron star is exceedingly smooth; gravity does not permit anything tall to exist. Neutron stars may have “mountains”, but they are only inches tall.

Neutron stars with abnormally strong magnetic fields are known as magnetars. The origin of these abnormal stars with ultra-powerful magnetic fields is unknown.

Unimaginably violent neutron star collisions, one of which was detected in 2017 by the LIGO gravitational wave observatories, are thought to be where heavy elements like gold and platinum are created. And that’s because normal supernovae are not thought to generate the requisite pressures and temperatures.

Neutron stars are also thought to be responsible for several little-understood phenomena, including the mysterious Fast Radio Bursts (FRBs) and the so-called Soft Gamma Repeaters (SGRs).

A neutron star does not generate any light or heat of its own after its formation. Over millions of years its latent heat will gradually cool from an initial 600,000 degrees Kelvin (1 million degrees Fahrenheit), eventually ending its life as the cold, dead remnant of a once-glorious star. It’s estimated there are more than a hundred million neutron stars in our Milky Way galaxy, but many will be too old and cold to be easily detected.

Pulsars: How we know about neutron stars.

Although neutron stars were long predicted in astrophysical theory, it wasn’t until 1967 that the first was discovered, as a pulsar, by Dame Jocelyn Bell Burnell. Since then, we know of hundreds more, including the famous pulsar at the heart of the Crab Nebula, a supernova remnant observed by the Chinese in 1054.

On a neutron star, intense magnetic fields focus radio waves into two beams firing into space from its magnetic poles, much like the beam of a lighthouse. If the neutron star is oriented precisely so that these beams become visible from our earthly viewpoint, we see flashes of radio light at regular and extremely exact intervals. Neutron stars are, in fact, the celestial timekeepers of the cosmos, their accuracy rivaling that of atomic clocks.

Neutron stars rotate extremely rapidly, and we can use the radio beams of a pulsar to measure just how fast. The fastest-rotating neutron star yet discovered rotates an incredible 716 times per second, which is about a quarter of the speed of light.

Read more about Jocelyn Bell Burnell, who discovered pulsars

Sci fi alert!

“Dragon’s Egg” by Robert L. Forward (out of print) depicts the imaginary inhabitants living on the surface of a neutron star. Claudia commented: “They were tiny and dense (of course) and lived at a tremendous speed. It’s been a while, but I remember it as a good read.” Andy added: “Yes, I remember that book! Very entertaining. It’s incredible to think that if the surface of a neutron star slips by as little as a millimeter, it causes a starquake.”

More resources on neutron stars:

Read more: Introduction to neutron stars

Five extreme facts about neutron stars

Read more: How high are pulsar mountains?

Bottom line: Neutron stars are the collapsed cores of formerly massive stars that have been crushed to an extreme density by supernova explosions. A neutron star isn’t as dense as a black hole, but it’s denser than any other known type of star.

The post What is a neutron star? How do they form? first appeared on EarthSky.

What is a galaxy?

A galaxy is a vast island of gas, dust and stars in an ocean of space. Typically, galaxies are millions of light-years apart. Galaxies are the building blocks of our universe. Their distribution isn’t random, as one might suppose. Instead, galaxies are along unimaginably long filaments across the universe, forming a cosmic web of star cities.

A galaxy can contain hundreds of billions of stars and be many thousands of light-years across. Our own galaxy, the Milky Way, is around 100,000 light-years in diameter. That’s about 587,900 trillion miles, or nearly a million trillion kilometers.

The three types of galaxies are spiral, elliptical or irregular.

Galaxy sizes vary widely, ranging from very small to unbelievably enormous. Small dwarf galaxies contain about 100 million stars and giant galaxies contain more than a trillion stars.

Also, there are an estimated two hundred billion galaxies in the universe.

In 2016, using 20 years of images from the Hubble, it was estimated that there were in total two trillion or more galaxies in the observable universe.

In 2021, data from NASA's New Horizons space probe was used to revise the earlier estimate to roughly 200 billion galaxies. pic.twitter.com/J11gK0Zu4F

— swami (@C8257877) February 24, 2023

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The discovery of other galaxies

The famous astronomer Edwin P. Hubble first classified galaxies based on their visual appearance in the late 1920s and 30s. In fact, Hubble’s Classification of Galaxies is still widely used today. Although, since Hubble’s time, like any effective classification system, it’s evolved in light of ongoing observations. Hubble used several basic types of galaxies, each containing sub-types.

Spiral galaxies are more mysterious than we thought! @KarenLMasters, @vrooje and hundreds of thousands of volunteers @GalaxyZoo have shown that the ‘Hubble Tuning Fork’ method of categorisation is wrong ??https://t.co/EUu9BhBUef pic.twitter.com/QFZgrXHjtU

— Royal Astronomical Society (@RoyalAstroSoc) June 11, 2019

Before Hubble’s study of galaxies, we believed that our galaxy was the only one in the universe. Astronomers thought that the smudges of light they saw through their telescopes were in fact nebulae within our own galaxy. However, Hubble discovered that these nebulae were galaxies. Additionally, it was Hubble who demonstrated, by measuring their velocities, that they lie at vast distances from us.

These galaxies lie millions of light-years beyond the Milky Way, at distances so huge they appear tiny in all but the largest telescopes. Moreover, he demonstrated that, wherever he looked, galaxies were receding from us in all directions, and the further away they are, the faster they are receding. Thus, Hubble had discovered that the universe is expanding.

Spiral galaxies

The most common type of galaxy is a spiral galaxy. The Milky Way is a spiral galaxy. Spiral galaxies have majestic, sweeping arms, thousands of light years long, made up of millions upon millions of stars. Their spiral arms stand out because of bright stars, glowing gas and dust. Spiral galaxies are active with star formation.

Also, spiral galaxies have a bright center, made up of a dense concentration of stars, so tightly packed that from a distance the galaxy’s center looks like a solid ball. This ball of stars is known as the galactic bulge.

Also, there are two types of spiral galaxies. There are regular spirals and barred spirals. If the spiral has bars, they extend off the central bulge. Then, the spiral arms start at the end of the bar.

Elliptical and irregular galaxies

Elliptical galaxies are the universe’s largest galaxies. In fact, giant elliptical galaxies can be about 300,000 light-years across. While, the dwarf elliptical galaxies – the most common elliptical – are only a few thousand light-years across. There are several shapes of elliptical galaxies, ranging from circular to football-shaped.

Overall, 1/3 of all galaxies are elliptical galaxies. Elliptical galaxies contain very little gas and dust – compared to a spiral or irregular galaxy – and they are no longer actively forming stars. The stars in elliptical galaxies are older stars and contain very few heavier elements.

Irregular shaped galaxies have all sorts of different shapes but they don’t look like a spiral or elliptical galaxy.

Irregular galaxies can have very little dust or a lot. Plus, they can show active star-forming regions or have little-to-no star formation occurring. They seemed plentiful in the early universe.

Our Milky Way Galaxy

The Milky Way, in fact, falls into one of Hubble’s spiral galaxy sub-types: it’s a barred spiral, which means it has a bar of stars protruding out from each side of its center. As the spiral arms sweep out in their graceful and enormous arcs, the ends of the bars are the anchors. This is a recent discovery and it’s unknown how bars form in a galaxy. Our solar system is situated about 2/3 of the way out from the galactic center towards the periphery of the galaxy, embedded in one of these spiral arms.

Another recent discovery is that the disk of the Milky Way is warped, like a long-playing vinyl record left too long in the sun. Exactly why is unknown, but it may be the result of a gravitational encounter with another galaxy early in the Milky Way’s history.

It also appears that all galaxies rotate. For example, the Milky Way takes 226 million years to spin around once. Since its creation, the Earth has traveled 20 times around the galaxy.

Galaxies come in clusters

Galaxies group together in clusters. Our own galaxy is part of what is called the Local Group, and it contains roughly 55 galaxies.

Ultimately, galaxy clusters themselves group into superclusters. Our Local Group is part of the Virgo Supercluster.

The “glue” that binds stars into galaxies, galaxies into clusters, clusters into superclusters and superclusters into filaments is – of course – gravity. In fact, gravity is the universe’s construction worker, which sculpts all the structures we see in the cosmos.

The Universe is Infinite. However our existence in our current Galaxy Cluster is Finite. Time is also a factor… Our Galaxy Clusters actually expand into the void of space over time. pic.twitter.com/EAOcisPdZn

— Gustavo Suárez (@gsuarez333) February 26, 2023

Galaxies are flying apart

Although most galaxies are flying apart from each other, those astronomically close to each other will be gravitationally bound to each other. Caught in an inexorable gravitational dance, eventually they merge, passing through each other over millions of years, eventually forming a single, amorphous elliptical galaxy. Gravity shockwaves compress huge clouds of interstellar gas and dust during such mergers, giving rise to new generations of stars.

The Milky Way is caught in such a gravitational embrace with M31, aka the Andromeda galaxy, which is 2 1/2 million light-years distant. Both galaxies are moving toward each other because of gravitational attraction: they will merge in about 6 billion years. However, both galaxies are surrounded by huge halos of gas which may extend for millions of light-years, and it was discovered that the halos of the Milky Way and M31 have already started to touch.

Galaxy mergers and companion galaxies

Galaxy mergers are common. The universe is full of examples of galaxies in various stages of merging together, their structures disrupted and distorted by gravity, forming bizarre and beautiful shapes.

Then, at the lower end of the galactic size scale, there are so-called dwarf galaxies, consisting of a few hundred to up to several billion stars. Their origin is not clear. Typically, they have no clearly defined structure. Astronomers believe they were born in the same way as larger galaxies like the Milky Way, but for whatever reason they stopped growing. Ensnared by the gravity of a larger galaxy, they orbit its periphery. The Milky Way has around 20 dwarf galaxies orbiting it that we know of, although some models predict there should be many more.

Our closest neighbors: The Magellanic Clouds

The two most famous dwarf galaxies for us earthlings are, of course, the Small and Large Magellanic Clouds, visible to the unaided eye in Earth’s Southern Hemisphere sky.

Eventually, these and other dwarf galaxies will rip apart under the titanic pull of the Milky Way’s gravity. This will leave behind a barely noticeable stream of stars across the sky, slowly dissipating over eons.

Supermassive black holes lurk in galactic centers

At the center of most galaxies lurks a supermassive black hole, of millions or even billions of solar masses. For example, TON 618, has a mass 66 billion times that of our sun.

The origin and evolution of supermassive black holes remains a mystery. A few years ago, astronomers uncovered a surprising fact: in spiral galaxies, the mass of the supermassive black hole has a direct linear relationship with the mass of the galactic bulge. The more mass the black hole has, the more stars there are in the bulge. No one knows exactly what the significance of this relationship may be. However, its existence seems to indicate that the growth of a galaxy’s stellar population and that of its supermassive black hole are inextricably linked.

This discovery comes at a time when astronomers are beginning to realize that a supermassive black hole may control the fate of its host galaxy: the copious amounts of electromagnetic radiation emitted from the maelstrom of material orbiting the central black hole, known as the accretion disk, may push away and dissipate the clouds of interstellar hydrogen from which new stars form. This acts as an inhibitor on the galaxy’s ability to give birth to new stars. Ultimately, the emergence of life itself may be tied to the activity of supermassive black holes. This is an area that is undergoing extensive research.

While astronomers still know very little about exactly how galaxies formed in the first place – we see them in their nascent state only a few hundred million years after the Big Bang – the study of galaxies is an endless voyage of discovery.

We discovered other galaxies exist less than a century ago

Less than a hundred years after we realized that other galaxies exist besides our own, we have learned so much about these grand, majestic star cities. And there is still much to learn.

Bottom line: A galaxy is a vast island of gas, dust and stars in an ocean of space. There are three types of galaxies. Learn about these starry islands in space.

Read more: Milky Way’s farthest stars reach halfway to Andromeda

The post What is a galaxy? All you need to know first appeared on EarthSky.

Meet the asteroid belt, a place in our solar system where small bodies – mostly rocky and some metallic – orbit the sun. Sometimes scientists call these little worlds minor planets. One (Ceres) is technically a dwarf planet. These objects move mostly between the orbits of our solar system’s 4th planet, Mars, and 5th planet, Jupiter. Astronomers once thought they were leftovers of a rocky planet that Jupiter’s gravity tore apart long ago. Now they think differently. They think the asteroids are likely simply remnants from the formation of our solar system 4.6 billion years ago.

The word asteroid means starlike. Asteroids got this name because, when astronomers first discovered them in the early 1800s, they thought they looked like stars. And yet their movement was separate from stars. Because they are closer to us, they move against the starry backdrop. This showed asteroids to be something other than stars.

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Asteroids by the millions

While the graphic may make it seem like the asteroid belt is teeming with debris, if you lumped all the material together it would only create a body smaller than Earth’s moon.

The asteroid belt contains objects that vary wildly in size. It has 1 to 2 million asteroids more than half a mile (about a km) across. Plus, the asteroid belt contains untold millions of smaller ones, some probably no bigger than pebbles. In 1801, the astronomer Giuseppe Piazzi discovered the first asteroid, which is also the biggest object in the asteroid belt. It is 1 Ceres, which measures some 587 miles (945 km) across. The International Astronomical Union has reclassified Ceres from an asteroid to a dwarf planet.

Distances in the asteroid belt

Outer space is vast. And thus, despite there being many millions (possibly billions) of objects in the asteroid belt, the average distance between them is 600,000 miles (about 1 million km). This means that spacecraft can fly through the asteroid belt without colliding with any asteroids. (Although, obviously, a chance collision is never completely out of the realm of possibility and bad luck.) The asteroid belt is certainly nothing like the densely packed fields depicted in fantasies such as “Star Wars.”

Standing on any asteroid in the belt, you would likely be unable to see any other asteroids, because of their distance.

The asteroid belt lies between 2.2 and 3.2 astronomical units (AU) from our sun. One AU is the distance between the Earth and sun. So the width of the asteroid belt is roughly 1 AU, or 92 million miles (150 million km).

Its thickness is similarly about 1 AU.

Asteroids in and out of the main belt

We often call the asteroid belt the main belt to distinguish it from other, smaller groups of asteroids in the solar system such as the Lagrangians and Centaurs in the outer solar system.

What scientists once thought was a homogeneous belt they now know to be slightly more complicated. There are different and distinct zones within the main-belt asteroids. This is especially true at its edges, where astronomers now recognize the Hungaria group at the inner edge and the Cybele asteroids at the outer. Toward the middle of the belt there is the highly inclined Phocaea family.

In addition, astronomers have established that the age of asteroids in the main belt also varies. They’ve now classified several asteroid groupings by their age, including the Karin family, a group of about 90 main-belt asteroids that share an orbit and may have come from a single object some 5.7 million years ago. And there is the Veritas family, from about 8.3 million years ago. A very recent group is the Datura family, dating from a collision just 530,000 years ago.

Bottom line: The asteroid belt is a region of our solar system – between the orbits of Mars and Jupiter – where many small bodies orbit our sun.

The post The asteroid belt contains solar system remnants first appeared on EarthSky.

What are exoplanets?

Exoplanets are planets that orbit a star other than our sun. The prefix exo comes from the Greek and means outside; these worlds are far, far outside our own solar system. Astronomers have confirmed more than 5,000 exoplanets orbiting distant stars. The existence of planetary systems other than our own had been surmised for centuries. But it wasn’t until 1992 that astronomers found the first two exoplanets orbiting a pulsar. Then came the confirmation of the first exoplanet orbiting a sunlike star in 1995.

Why didn’t we see them before? It’s because exoplanets are so far away, several light-years away at their closest. And it’s because – unlike stars – exoplanets don’t shine with their own light. Like Earth, they shine only with light reflected from their local stars. In contrast to their stars, exoplanets are exceedingly dim; even the largest drown in the light of their vastly brighter stars.

Before the first exoplanet discovery, most astronomers assumed exoplanets, if found, would resemble the planets in our solar system. The great shock has been that many exoplanets are far different, with their positions and orbits difficult to explain. If astronomers thought the solar system was in any way representative of other planetary systems out there in the galaxy, they’ve been disappointed. Our solar system may be the exception rather than the rule.

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Unusual exoplanet families

As a harbinger of this realization, the very first exoplanets discovered in 1992 orbit a neutron star. In this case, it was a pulsar (a neutron star that emits beams of radio waves like a lighthouse, which may be detected from the Earth if the beams point in the right direction). Generally speaking, a neutron star is the superdense remains of the core of a massive star after it has ended its life in a supernova explosion.

It wasn’t thought possible, and it’s still not fully explained, that planets could survive such a cataclysm. Normally, the neutron stars we see as pulsars rotate with an invariance rivaling that of atomic clocks. Thus, neutron stars are some of the most accurate timekeepers in the cosmos.

Astronomers Aleksander Wolszczan and Dale Frail were trying to explain irregularities in the rotation of a particular pulsar, known as PSR B1257+12. They realized they could explain the slight variations in the star’s rotation if the gravity of two planets were pulling on it. Those planets would need to be three and four times the mass of Earth.

Historically significant as this discovery was, astronomers’ main quest in hunting exoplanets was to find one orbiting a sunlike star, not orbiting the remains of a huge star after a supernova. After all, ultimately, the quest is to find a planet like Earth, and then to find life there. Humans have always asked the question: “Are we alone in the universe?”

The radial velocity method

Finding an Earth-like planet, especially one where life resides, has been and remains the impetus for our searches and explorations of these distant worlds.

The detection of the first planet orbiting a main-sequence star like the sun came in 1995. That’s when Didier Queloz discovered a planet at least as massive as Jupiter orbiting the F-Type star 51 Pegasi, some 50 light-years from Earth. He detected it by the star’s “wobble” as an unseen planet pulled on it. For this discovery, he and colleagues Michel Mayor and James Peebles received the Nobel Prize in Physics in 2019.

In the 1990s, the available technology turned up only the largest exoplanets: those with enough gravity to induce a “wobble” in the spin of their parent stars. This method of detecting exoplanets is known as the radial velocity method, and it’s still a highly successful method for detecting exoplanets from Earth’s surface. You can read more about the radial velocity method – sometimes called Doppler spectroscopy – at this link.

The transit method

Nowadays, astronomers use another method – called the transit method, or transit photometry – with even greater success to find exoplanets. NASA’s planet-hunter spacecraft, Kepler, has discovered the most exoplanets so far, and it employs the transit method. This technique can detect smaller exoplanets. The transit method relies on the fact that, when an exoplanet crosses the face of its star as seen from Earth, it blocks the star’s light ever so slightly, dimming it. This change in brightness may only be 1%, but is nevertheless detectable with modern instruments such as those on Kepler. Read more about the transit method here.

Direct image of exoplanets

With the launch of the James Webb Space Telescope in 2022, astronomers have a new tool to help them track down exoplanets. On September 1, 2022, NASA announced that the Webb telescope had directly imaged an exoplanet for the first time. This exoplanet, HIP 65426 b, was not a new discovery. It was first found through direct imaging by the Spectro-Polarimetric High-Contrast Exoplanet Research (SPHERE) instrument in 2017. But Webb was able to look for the exoplanet and pick it up in four different filters. Read more about the direct imaging of exoplanets.

It should be noted that a famous and beloved exoplanet, Fomalhaut b – the first ever to be directly imaged – turned out not to be an exoplanet, after all, but instead a cloud of dust. Read more about the sad disappearance of Fomalhaut b.

Types of exoplanets

Twenty-five years after the discovery of the first exoplanet orbiting a sunlike star, astronomers have identified many types of planets in the exoplanet “zoo.” Some of these are listed below. See the lengthy list here for a complete classification.

• Hot Jupiters: Among the first exoplanets astronomers discovered because of their size, these are gas giant planets. They contain the mass of Jupiter or more, in close proximity to their star, and in some cases, orbiting it in just a few Earth days. Assuming such planets could not have formed in their current location, astronomers think they were born much further out and migrated inward. The study of Hot Jupiters has shed much light on the formation of the solar system.
• Super-Earths: These are planets with a mass between that of Earth and the smallest gas giants – Neptune and Uranus – in our solar system. Astronomers think the composition of such planets is largely rock rather than gas. Therefore, they’re more likely to be like our terrestrial planets. Astronomers use the term “Earth-like planets” for exoplanets that are rocky rather than gaseous and orbit in the so-called “Goldilocks Zone.” This zone is where water can exist in liquid form. “Earth-like” does not literally mean a planet is a twin of Earth, possessing an Earth-like atmosphere and possibly life.
• Mini-Neptunes: An exoplanet with up to ten Earth masses, but smaller in size than Neptune or Uranus. These are likely to be predominantly gaseous worlds.
• Ocean Worlds:  These are exoplanets that contain a substantial amount of water, either as oceans on the surface or underground.
• Ice Giants:  These exoplanets are made up of volatile compounds such as water, methane and ammonia, rather than the hydrogen and helium of Jupiter and Saturn, for example.

Looking for Earth’s twin

The quest for a true twin of the Earth continues. In June 2019, astronomers announced the discovery of the most Earth-like planet discovered at that time, orbiting Teegarden’s Star, a red dwarf just 12.5 light-years away. The exoplanet, Teegarden b, has a rating of 95% on the Earth Similarity Index.

But new exoplanets are turning up all the time. In fact, astronomers announced in December 2022 that they’ve discovered two possibly Earth-like worlds just 16 light-years away.

Kepler, for which so many exoplanets are named, is no longer active (although its data are still being analyzed). But the planet-hunting spacecraft TESS has been discovering planets since 2018. TESS is using citizen scientists to help it find worlds beyond our own.

In December 2019, the European Space Agency (ESA) launched the spacecraft CHEOPS to better characterize already-known exoplanets. The new generation of Earth-based telescopes such as the European Extremely Large Telescope (ELT), the world’s largest telescope, currently under construction in Chile, will be able to analyze the atmospheres of exoplanets directly and identify biosignatures such as oxygen and methane.

Thus the ancient dream of finding life elsewhere in the universe may soon be a reality. Stay tuned!

Exoplanet visualization

By the way, the cool video below shows all of the multi-planet systems from Kepler’s original mission as of the announcement of Kepler’s end of life: October 30, 2018. Astronomer and planet-hunter Ethan Kruse, who created this visualization using data derived from Kepler, wrote:

The systems are shown together at the same scale as our own solar system (dashed lines). The size of the orbits are all to scale, but the size of the planets are not. For example, Jupiter is actually 11 times larger than Earth, but that scale makes Earth-sized planets almost invisible (or Jupiters annoyingly large). The orbits are all synchronized such that Kepler observed a planet transit every time it hits an angle of 0 degrees (the 3 o’clock position on a clock). Planet colors are based on their approximate equilibrium temperatures, as shown in the legend.

Bottom line: Exoplanets are worlds orbiting distant stars. The history of our knowledge of exoplanets, the various types of exoplanets, how astronomers find them, and more, here.

Explore: The NASA Exoplanet Archive

Check out: NASA’s Exoplanet Exploration site

Have fun: NASA’s Exoplanet Travel Bureau

The post Exoplanets are worlds orbiting other stars first appeared on EarthSky.

Do you think of the Milky Way as a starry band across a dark night sky? Or do you think of it as a great spiral galaxy in space? Both are correct. Both refer to our home galaxy, our local island in the vast ocean of the universe, composed of hundreds of billions of stars, one of which is our sun.

Long ago, it was possible for everybody in the world to see a dark, star-strewn sky when they looked heavenward at night. In those ancient times, humans looked to the starry sky and saw a ghostly band of light arcing from horizon to horizon. This graceful arc of light moved across the sky with the seasons. The most casual sky-watchers could notice that darkness obscured parts of the band, which we now know to be vast clouds of dust.

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Myths of the Milky Way

Myths and legends grew up in different cultures around this mysterious apparition in the heavens. Each culture explained this band of light in the sky according to its own beliefs. To the ancient Armenians, it was straw strewn across the sky by the god Vahagn. In eastern Asia, it was the Silvery River of Heaven. The Finns and Estonians saw it as the Pathway of the Birds.

Meanwhile, because ancient Greek and Roman legends and myths came to dominate western culture, it was their interpretations that were passed down to a majority of languages. Both the Greeks and the Romans saw the starry band as a river of milk. The Greek myth said it was milk from the breast of the goddess Hera, divine wife of Zeus. The Romans saw the river of light as milk from their goddess Ops.

Thus it was bequeathed the name by which, today, we know that ghostly arc stretching across the sky: the Milky Way.

Observing the river of stars

When you are standing under a completely dark, starry sky, away from light pollution, the Milky Way appears like a cloud across the cosmos. But that cloud betrays no clue as to what it actually is. Until the invention of the telescope, no human could have known the nature of the Milky Way.

Just point even a small telescope anywhere along its length and you will be rewarded with a beautiful sight. What appears as a cloud to the unaided eye resolves into countless stars. Their distance and close relative proximity to each other do not permit us to pick them out individually with just our eyes.

It’s the same way a raincloud looks solid in the sky but actually consists of countless water droplets. The stars of the Milky Way merge together into a single band of light. But through a telescope, we see the Milky Way for what it truly is: a spiral arm of our galaxy.

What is the Milky Way?

Thus we arrive at the second answer to the question of what the Milky Way is. To astronomers, it is the name given to the entire galaxy we live in, not just the part of it we see in the sky. If this seems confusing, we must acknowledge the need for our galaxy to have a name.

Many other galaxies are designated by catalog numbers rather than names, for example the New General Catalogue. First published in 1888, it merely assigns a sequential number to each. More recent catalog numbers contain information of far more use to astronomers, for example, the galaxy’s location on the sky and during which survey it was discovered. Moreover, a galaxy may appear in more than one catalog and thus possess more than one designation. For example, the galaxy NGC 2470 is also known as 2MFGC 6271.

Other galaxies, particularly those brighter and closer, received names from astronomers of the 17th and 18th centuries. The names reflected their appearance: the Pinwheel, the Sombrero, the Sunflower, the Cartwheel, the cigar and so forth. These names came long before any systematic sky surveys with numerical labeling systems.

In time, the galaxies with descriptive labels received catalog numbers as well. Yet, our own galaxy does not appear in any index of galaxies. So, it needed a name for astronomers to refer to it by. Thus we call it the Milky Way instead of the galaxy or our galaxy. That name refers to both that river of light across the sky, which is part of our galaxy, and the galaxy as a whole. When not using the name, astronomers refer to it with a capital G (the Galaxy), and all other galaxies with a lowercase g.

Where is the sun in our galaxy?

Our solar system lies about 2/3 of the way out from the galactic center. We’re 26,000 light-years from the center, or 153,000 trillion miles (246,000 trillion km).

When we look toward the edge of the galaxy, we see the Orion-Cygnus Arm (or the Orion spur). The solar system is just on the inner edge of this spiral arm.

Or we can look toward the center of the galaxy, in the direction of Sagittarius. Vast clouds of dark gas hide the galactic center from us. Only in recent decades have astronomers pierced that dusty fog with infrared telescopes. A study of around 100 stars at the galactic center revealed that those giant clouds of dark dust were hiding a monster: a black hole. This black hole – known as Sagittarius A* – has a mass four million times that of our sun.

The stats on our galaxy

Our Milky Way galaxy is one of billions in the universe. We do not know exactly how many galaxies exist: a modern estimate vastly increases previous counts to as many as 2 trillion.

The Milky Way is approximately 100,000 light-years across, or 600,000 trillion miles (950,000 trillion km). We do not know its exact age, but we assume it came into being in the very early universe along with most other galaxies: within perhaps a billion years after the Big Bang. Estimates of how many stars live within the Milky Way vary quite considerably, but it seems to be somewhere between 100 billion and double that figure.

Why is there so much variance? Simply because it is so difficult to count the number of stars in the galaxy from our vantage point here on Earth. Imagine being in a banquet room full of people and trying to count everyone without being able to move around the room. From where you are standing, all you can do is make an estimate because people close to you block the view of those farther away. Neither can you see what size and shape the room is. The mass of people hides the edges of the room. It’s exactly the same from our position in the galaxy.

Seeing the city of stars

It is this inability to see the structure of the Milky Way from our location inside it that meant for most of human history we did not even recognize that we live inside a galaxy in the first place. Indeed, we did not even realize what a galaxy is: a vast city of stars, separated from others by even vaster distances.

Without telescopes, we couldn’t see most of the other galaxies in the sky. The unaided eye can only see three of them: from the Northern Hemisphere we can see the Andromeda galaxy. Also known as M31, the Andromeda galaxy lies some two million light-years from us. In fact, it’s the farthest object we can see with our eyes alone, under dark skies. The skies in the Southern Hemisphere also have the Small and Large Magellanic Clouds, two amorphous dwarf galaxies orbiting our own. They are far larger and brighter in the sky than M31 simply because they are much closer to us.

Other galaxies in the universe

Until the 1910s, astronomers had not observationally confirmed the existence of other galaxies. Astronomers long believed that those fuzzy patches of light they saw through their telescopes were nebulae, vast clouds of gas and dust in our own galaxy.

But the concept of other galaxies was born earlier, in the early and mid-18th century. Swedish philosopher and scientist Emanuel Swedenborg and English astronomer Thomas Wright apparently conceived the idea independently of each other. Building upon the work of Wright, German philosopher Immanuel Kant referred to galaxies as island universes. The first observational evidence came in 1912 by American astronomer Vesto Slipher, who found that the spectra of the “nebulae” he measured were redshifted and thus much further away than astronomers previously thought.

Edwin Hubble and distant galaxies

And then came Edwin Hubble. Through years of painstaking work at the Mount Wilson Observatory in California, he confirmed in the 1920s that we do not live in a unique location. Our galaxy is just one of perhaps trillions.

Hubble came to this realization by studying a type of star known as a Cepheid variable, which pulsates with a regular periodicity. The intrinsic brightness of a Cepheid variable is directly related to its pulsation period: by measuring how long it takes for the star to brighten, fade and brighten again you can calculate how bright it is, that is to say, how much light it emits. Consequently, by observing how bright it appears from the Earth, you can calculate its distance.

It’s like seeing distant car headlights at night and estimating how far away the car is from how bright its lights appear. You can judge the distance of the car because you know all car headlights have about the same brightness.

Cepheid variables in Andromeda

One of Edwin Hubble’s great achievements was the discovery of Cepheid variables in M31, the Andromeda galaxy. Hubble repeatedly photographed Andromeda with the Hooker Telescope. Eventually, he found stars that changed in brightness over a regular period. Performing the calculations, Hubble realized that M31 is not astronomically close to us at all. It’s 2 million light-years away, and it’s a galaxy like our own.

Hubble, for whom this discovery must have been a monumental shock, surmised that our galaxy was no different from M31 and the others he observed. Thus, he relegated us to a position of lesser importance in the universe. This was as big a revelation and diminution of our position in the universe. It was like when we learned that Earth is not the center of the universe.

We do not live in a special or privileged location. The universe does not have any vantage points which are superior to others. Wherever you are in the universe and you look up at the stars, you will see the same thing. Your constellations may be different, but no matter in which direction you look, you see galaxies rushing away from you in all directions as the universe expands, carrying the galaxies along with it.

Until the work by Slipher and Hubble (and others), we did not know the universe was expanding. It took a surprisingly long time for the astronomical community to accept this fact. Even Albert Einstein did not believe it, introducing an arbitrary correction into his calculations on relativity to achieve a static, non-expanding universe. However, Einstein later called this correction his greatest error.

The Milky Way from a distance

So, what does the Milky Way would look like from the outside? How many spiral arms there are? How big is the galaxy and how many stars does it hold? These were questions still unanswered in the 1920s. It took most of the 20th century after Hubble’s discoveries to piece together those answers through a combination of painstaking work with both Earth- and space-based telescopes.

So, if you could travel outside our galaxy, what would it look like? A standard analogy compares it to two fried eggs stuck together back-to-back. The yolk of the egg is known as the Galactic Bulge, a huge globe of stars at the center extending above and below the plane of the galaxy.

Astronomers now think the Milky Way has four spiral arms winding out from its center like the arms of a Catherine wheel. But these arms do not actually meet at the center. A few years ago astronomers discovered that the Milky Way is a barred spiral galaxy. This means a “bar” of stars runs across its center, and the spiral arms extend from either end. Barred spiral galaxies are not uncommon in the universe. But we do not yet understand how that central bar forms.

New discoveries in the Milky Way

Only a few years ago, astronomers made another major discovery. The Milky Way is not a flat disk of stars but has a kink running across it like an extended S. Something has warped the disk. At the moment, the finger points at the gravitational influence of the astronomically close Sagittarius dwarf galaxy. It’s one of perhaps twenty small galaxies that orbit the Milky Way, like moths around a flame. As the Sagittarius galaxy slowly orbits around us, its gravity has pulled on our galaxy’s stars, eventually creating the warp.

Other objects are also bound to the Milky Way. A halo of globular clusters surrounds our galaxy. Globular clusters are concentrations of stars that look like fuzzy golf balls. They contain perhaps a million or so extremely ancient stars.

Discoveries about the Milky Way continue. The study of its nature and origin is accelerating as new astronomical tools become available, such as the European Space Agency’s orbiting Gaia telescope. Gaia is making a three-dimensional map of our galaxy’s stars with exquisite and quite unprecedented accuracy. Read more about Gaia’s 3rd data release.

It’s an extremely exciting time for the study of our galaxy. It is all a far cry from when, thousands of years ago, our ancestors ascribed fantastic beasts and gods to that mysterious band of light they saw as they stood in awe under the starry sky.

Bottom line: Learn about our galaxy, the Milky Way. We discuss the origin of the name, its structure, and the history of how our knowledge has developed over the centuries and continues to develop today.

The post What is the Milky Way? It’s our home galaxy first appeared on EarthSky.

Many stars are not constant

When you go out under a starry sky, the stars seem unchanging, eternal, constant. Occasionally you might see a nova or a supernova – apparently “new” stars – but such events usually last only weeks before fading from view, and they are rare (especially supernovae). Apart from those transient reminders that the universe is restless and constantly changing, the stars in the night sky seem to shine with a steady, unwavering light. But many stars are not constant. Their brightness varies over time. We classify a star as a variable star if its light, as seen from the Earth, changes in brightness. A variable star is one that’s known to dim and then brighten again.

Variable stars aren’t rare or unusual. According to the American Association of Variable Star Observers (AAVSO), astronomers had identified more than 1 million variable stars. It’s not uncommon for amateur astronomers to make interesting and useful scientific discoveries about variable stars. You might wish to join them!

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Most stars fluctuate in brightness

Most stars have at least some variation in luminosity: our own sun, for example, varies in brightness by a small amount (about 1%) over the course of its 11-year cycle. But unless the fluctuation is large enough to be seen from Earth, the star isn’t classified as variable.

The changes in brightness of variable stars aren’t generally noticeable to the unaided eye, even if the brightness does change over short timescales (say, hours). To observe most variable stars, you need to monitor the brightness of the star carefully over extended periods of time. But there are examples of stars whose brightness has noticeably faded, over short timescales.

The dramatic dimming of Betelgeuse

A famous recent example is the red supergiant star Betelgeuse in the constellation Orion the Hunter. Betelgeuse is one of our sky’s brightest stars. It’s a prominent star, in a noticeable constellation. And there was a worldwide outcry, when, in late 2019, Betelgeuse suddenly began to dim. By February 2020, Betelgeuse was only half as bright as before.

Betelgeuse is nearing the end of its life. Many are aware that – within the next 100,000 years (soon, from an astronomical perspective) – Betelgeuse might explode as a supernova. Could the dimming be a sign Betelgeuse was about to explode?

In the end, Betelgeuse did not explode, and its brightness has now returned to normal. Why did it suddenly dim? Astronomers concluded that the bright red supergiant star Betelgeuse literally blew its top in 2019. Betelgeuse lost a substantial part of its visible surface, causing a dust cloud to form and dimming the star as seen from Earth. Betelgeuse is still recovering from that outburst.

Will Betelgeuse fade noticeably again? It might, but it’s not possible to predict exactly when.

Do all variable stars brighten and dim due to obscuring clouds of gas? No. There’s more than one reason for a star to change its brightness. That’s why it’s helpful to divide variable stars into categories.

Intrinsic variables, like Cepheids

Intrinsic variable stars change in brightness due to events happening within the star itself.

Cepheid variables are the most important of this type. These stars are pulsating variable stars. They literally pulse: get bigger and then smaller in size. As they expand and contract, their brightness changes.

Cepheid variables are named for the first known example of the type, the star Delta Cephei, discovered to be variable in 1874.

It wasn’t until 1908 that astronomer Henrietta Swan Leavitt discovered a direct relationship between the rate at which a Cepheid fluctuates in brightness and its luminosity or absolute brightness. A street light will appear dimmer as you move farther from it. Likewise, more distant stars appear dimmer than closer stars, assuming both have the same absolute brightness. And that’s why Cepheids are useful. If you see a Cepheid brightening and dimming at a certain rate, you know its true brightness. So you can see how bright it looks and thereby determine its distance.

Cepheid variables are powerful tools in astronomy. They were an early stepping stone in the establishment of the cosmic distance ladder that now enables astronomers to estimate distances to objects hundreds, thousands, millions and billions of light-years away.

Read more about the cosmic distance ladder: Meet Delta Cephei, a famous variable star

Henrietta Swan Leavitt died #OTD in 1921. One of the famous « Harvard computers », she discovered the period-luminosity relation of Cepheid variable stars, providing a vital ladder on the cosmological distance scale.https://t.co/7MtUIPoTga pic.twitter.com/tvyX0w40pD

— François Levrier (@FrancoisLevrier) December 12, 2021

Cataclysmic variables and novae

Cataclysmic variable stars are also intrinsic variable stars, but have a different cause for their brightness fluctuations. These aren’t single stars getting bigger and then smaller in size. They are two stars, orbiting close to each other: a binary star system. The star whose brightness fluctuates is a white dwarf, an evolved and compact star. The other star is likely to be more ordinary, except for its closeness to the white dwarf. This closeness means that white dwarf’s gravity deforms the shape of the second star, pulling material off it and forming an accretion disk around the white dwarf. Strong emissions in X-ray and ultraviolet light often betray this disk’s presence.

As the accretion disk material falls onto its surface, the white dwarf accumulates material from the second, donor star. Once the amount of material falling onto its surface reaches a critical point, runaway nuclear fusion reactions take place around the star. They cause a dramatic brightening of the star, sometimes becoming visible to the unaided eye as a “new” star. Indeed, some single cataclysmic variable events are also called novae, from the Latin word meaning new. Once this conversion has taken place, the fusion reactions end and the star dims to its former brightness.

If enough mass collects, however, this kind of situation would cause huge thermonuclear explosions that blow the white dwarf apart. They destroy the star, which then becomes known as a Type Ia supernova.

Other sorts of intrinsic variables

In addition to the Delta Cepheids, there are approximately 30 sub-groups within the intrinsic variable classification. They differ from one another in the speed of the star’s pulsations, and also its age, type, metallicity and several other factors.

Thus we have RR Lyrae variables, long-period variables and Mira variables. All of them vary due to internal changes within the stars themselves.

Extrinsic variables

Extrinsic variable stars have brightness fluctuations due to external factors. Again, there are many different types, but they break down into two main groups: eclipsing binaries and rotating variables.

Eclipsing binaries are systems containing two orbiting stars. As seen from the Earth, one star passes in front of the other, causing the eclipsed star’s brightness to fluctuate regularly. A famous example of this type of variable is Algol in the constellation Perseus. Another group of eclipsing variable stars is the W Ursae Majoris variables, where binary stars are situated so close to each other that they whip around each other in less than a day, and the surfaces of the two stars are so close that they are nearly touching!

Rotating variables, on the other hand, are variable stars where the brightness fluctuates due to phenomena associated with their rotation. There are many kinds of rotating variables, for example stars with huge sunspots on their surface which, as these rotate into view of the Earth, block and dim the light from the star.

What is a variable star?

The study of variable stars can reveal much about the nature, history and future of stars. There are many astronomers – both amateur and professional – who study them. And organizations such as the AAVSO serve as collectors and collators of variable star observations.

Variable stars show that you don’t always need sophisticated and expensive technology to do useful and valuable science. On a base level, all you need is your eyes, although telescopes and equipment can more precisely measure the brightness of a star. But all you really need is the ability to estimate the brightness of a star by comparing it to that of others. This is an acquired skill that comes from practice.

If you wish to become a variable-star observer, visit the AAVSO.

Read more about types of variable stars from Australian Telescope National Facility

Read more: Milky Way’s farthest stars reach halfway to Andromeda

Bottom line: A variable star is one whose brightness changes regularly. Here we discuss the different kinds of variable stars and what causes their brightness variations.

The post What is a variable star? first appeared on EarthSky.