Oddly shaped suns and moons are great photo opportunities

Sunrises, sunsets, moonrises and moonsets are excellent opportunities to capture a particularly beautiful photograph. When you see them near the horizon, the sun and the moon can look distorted in the most fascinating ways. Their edges may appear jagged. Their bottom areas may flatten out or shrink into a pedestal. Nearby clouds and twilight color help make the artistic view even better.

But why does it happen? What causes the distortion in the appearance of a low sun or moon?

The answer is atmospheric refraction, the effect of light traveling through different densities and temperatures of air. Refraction is the same effect that causes a spoon in a glass of water to appear broken in two.

The fact is, when you gaze toward any horizon, you’re looking through more air than when you gaze overhead. It’s this greater quantity of air that causes oddly shaped suns and moons. At zenith (straight up) the atmosphere will be at its thinnest. That’s why professional astronomers prefer to observe their objects of interest as high up on the sky as possible (and as their telescopes allow). And that’s because it diminishes the effects of any atmospheric distortion lower in the sky.

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More atmosphere = more distortion

So we know there’s more air in the direction of a horizon. Now consider all the different ways refraction affects a sunrise, sunset, moonrise or moonset.

But it’s not only the amount of atmosphere that plays a role. There’s also the pressure, the temperature and the humidity. They all affect the air density and thereby how much light rays will be bent, or refracted, along their path.

Thus, temperatures varying with different layers of air can spread the light so you see a layered image of the object you’re looking at. In other words, the light refracts more in some layers than in others.

More distortion = oddly shaped suns and moons

The bending of light rays in this manner is known as atmospheric refraction. Without any kind of disturbance, light would travel in a straight line, and give your eye a true image of what you see.

For objects with a small angular size – like stars – atmospheric refraction causes them to twinkle more the closer they are to the horizon.

But what about an object with a fair amount of surface area like the moon and the sun? For them, there is a change in the refractive effect along the height of it. Thus, the upper part travels through less atmosphere than the lower part, which makes the lower part more distorted.

What is a green flash?

When atmospheric refraction is at its most extreme, you might see a mirage. It’s the exact same situation, the light is bent and distorts the image. But here it can be refracted so much that there’s a mirroring effect and you will see drawn out or multiple images. Or it may show displaced images so the moon appears higher on the sky than it actually is.

A well-known mirage for the sun is the sought-after green flash.

Why sunsets are red

Additionally, light of different wavelengths reacts differently. For example, blue light (which has more energy, a shorter wavelength and higher frequency) is more affected by refraction than red light. That means red colors have a larger chance of coming through to you than blue. That’s why sunsets, sunrises and the moon appear redder near the horizon.

The result of refraction is nature’s own form of art, perhaps reminiscent of impressionism. Maybe that is why we find it so appealing. The video below, captured by Mike Cohea, beautifully shows the effect of the thicker atmosphere as the young moon sets over Newport.

So, go out, bring your camera and keep watching the horizon (but never stare directly, or through a camera, at the sun). Then submit your best results to EarthSky Community Photos. We love seeing your photos!

Photos of oddly shaped suns

Photos of oddly shaped moons

Bottom line: The amount of atmosphere between your eye and what you observe determines how much distortion you will see. This phenomenon – atmospheric refraction – is why the sun or moon may appear flattened near the horizon.

Read more on atmospheric refraction and mirages, with images and explanations, at Les Cowley’s website Atmospheric Optics

The post Oddly shaped suns and moons on the horizon first appeared on EarthSky.

To measure the mass of a star, use 2 stars

There are lots of binary stars – two stars revolving around a common center of mass – populating the starry sky. In fact, a large majority of all stars we see (around 85%) are thought to be part of multiple star systems of two or more stars! This is lucky for astronomers, because two stars together provide an easy way to measure star masses.

To find the masses of stars in double systems, you need to know only two things. First, the semi-major axis or mean distance between the two stars (often expressed in astronomical units, which is the average distance between the Earth and sun).

And second, you need to know the time it takes for the two stars to revolve around one another (aka the orbital period, often expressed in Earth years).

With those two observations alone, astronomers can calculate the stars’ masses. They typically do that in units of solar masses (that is, a measure of how many of our suns the star “weighs.” One solar mass is 1.989 x 10³⁰ kilograms or about 333,000 times the mass of our planet Earth.)

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Sirius is a great example

We’ll use Sirius, the brightest star of the nighttime sky, as an example. It looks like a single star to the unaided eye, but it, too, is a binary star. By the way, you can see it yourself, if you have a small telescope.

The two stars orbit each other with a period of about 50.1 Earth-years, at an average distance of about 19.8 astronomical units (AU). The brighter of the two is called Sirius A, while its fainter companion is known as Sirius B (The Pup).

Finding the mass of Sirius A and B

So how would astronomers find the masses of Sirius A and B? They would simply plug in the mean distance between the two stars (19.8 AU) and their orbital period (50.1 Earth-years) into the easy-to-use formula below, first derived by Johannes Kepler in 1618, and known as Kepler’s Third law:

Total mass = distance³/period²
Total mass = 19.8³/50.1²
So total mass = 7762.39/2510.01 = 3.09 times the sun’s mass

Here, the distance is the mean distance between the stars (or, more precisely, the semi-major axis) in astronomical units, so 19.8, and the orbital period is 50.1 years.

The resulting total mass is about three solar masses. Note that this is not the mass of one star but of both stars added together. So, we know that the whole binary system equals three solar masses.

Then finding the mass of each star

To find out the mass of each individual star, astronomers need to know the mean distance of each star from the barycenter: their common center of mass. To learn this, once again they rely on their observations.

It turns out that Sirius B, the less massive star, is about twice as far from the barycenter than is Sirius A. That means Sirius B has about half the mass of Sirius A.

Thus, you know the whole system is about three solar masses by using Kepler’s Third Law. So now you can deduce that the mass of Sirius A is about two solar masses. And then Sirius B pretty much equals our sun in mass.

What about the mass of a star not in a binary system?

But what about stars that are alone in their star systems, like the sun? The binary star systems are once again the key: Once we have calculated the masses for a whole lot of stars in binary systems, and also know how luminous they are, we notice that there is a relationship between their luminosity and their mass. In other words, for single stars we only need to measure its luminosity and then use the mass-luminosity relation to figure out their mass. Thank you, binaries!

Read more: What is stellar luminosity?

Read more: What is stellar magnitude?

Bottom line: For astronomers, binary star systems are a quite useful tool to figure out the mass of stars.

The post How do you measure the mass of a star? first appeared on EarthSky.

Milky Way and Andromeda merger has begun

The Andromeda galaxy, the nearest spiral galaxy to our Milky Way, isn’t noticeable in our night sky, unless you look for it. Under dark skies, however, you can see it without optical aid, but only as a barely visible fuzzy patch of light. But one day, far in the future, Andromeda will be bright in our sky, growing larger and larger … as it gets closer and closer to us. And even though the two galaxies are still 2.5 million light-years apart, the eventual merger of our two galaxies has, in fact, already begun.

The great extent of galactic halos

The Andromeda galaxy is currently racing toward our Milky Way at a speed of about 70 miles (113 km) per second. With this in mind, our merger will occur five billion years from now. But, in August 2020, the peer-reviewed Astrophysical Journal published new research revealing that the collision between our galaxies is already underway.

The news about the Andromeda galaxy came from Project AMIGA, which uses the Hubble Space Telescope to look at the deep-space surroundings of the Andromeda galaxy. AMIGA stands for Absorption Map of Ionized Gas in Andromeda. NASA called it:

… the most comprehensive study of a halo surrounding a galaxy.

The Andromeda galaxy, our Milky Way and other galaxies all sit enshrouded in a large envelope – called a galactic halo – which consists of gas, dust and stray stars. The halos of galaxies are faint, so faint, in fact, that detecting them is not an easy feat. These astronomers measured the size of the halo of the Andromeda galaxy by looking at how much it absorbed light from background quasars. They were surprised to find that the Andromeda galaxy’s halo stretches much, much farther beyond its visible boundaries.

Indeed, it extends as far as half the distance to our Milky Way (1.3 million light-years) and even farther in other directions (up to 2 million light-years).

Are the halos touching yet?

So, does this mean the halos of the Andromeda and Milky Way galaxies are touching?

It turns out that, from our vantage point inside the Milky Way, we cannot easily measure the characteristics of our galaxy’s halo. However, because the two galaxies are so similar in size and appearance, scientists assume that the halo of the Milky Way would also be similar.

In other words, it’s the faint halos of the galaxies that indeed appear to have started to touch one another. Thus, in a manner of speaking, the collision between our two galaxies has already started.

Visualizing Andromeda’s halo in our sky

So what will the Andromeda merger look like?

NASA released the images below in 2012. They are artist’s concepts of what someone on Earth might see as the Andromeda galaxy hurtles toward us.

The depictions below are based on painstaking Hubble Space Telescope measurements of the motion of the Andromeda galaxy, with computer modeling of the inevitable collision between the two galaxies. Also, a series of studies published in 2012 showed that – rather than glancing off each other, as merging galaxies sometimes do – our Milky Way galaxy and the Andromeda galaxy will in fact merge to form a single big elliptical, or football-shaped, galaxy.

Roeland van der Marel, an astronomer at the Space Telescope Science Institute, told Discover Magazine in February 2022:

Whether it’s fully a head-on collision or more of a glancing blow doesn’t really affect the end result.

And that is a new, giant elliptical galaxy.

Another video of the Andromeda merger

The Milky Way and Andromeda galaxies, however, won’t be the only ones involved in this merger. As shown in the video below, the other large galaxy in our Local Group of galaxies, that is, M33, aka the Triangulum galaxy, will also play a role.

In the video below, you’ll recognize the Triangulum galaxy as the smaller object near the Andromeda and Milky Way galaxies. Although the Triangulum galaxy likely won’t join the merger, it may, nevertheless, at some point strike our Milky Way while engaged in a great cosmic dance with the two larger galaxies.

What happens to stars and planets when galaxies merge?

Across the universe, galaxies are colliding with each other. Astronomers observe galactic collisions – or their aftermaths – with the aid of powerful telescopes. In some ways, when a galactic merger takes place, the two galaxies are like ghosts; they simply pass through each other. That’s because stars inside galaxies are separated by such great distances. Thus the stars themselves typically don’t collide when galaxies merge.

That said, the stars in both the Andromeda galaxy and our Milky Way will be affected by the merger. The Andromeda galaxy contains about a trillion stars. Meanwhile, the Milky Way has about 300 billion stars. Stars from both galaxies will be thrown into new orbits around the newly merged galactic center. For example, according to scientists involved in the 2012 studies:

It is likely the sun will be flung into a new region of our galaxy …

And yet, they said,

… our Earth and solar system are in no danger of being destroyed.

Will humanity see the Andromeda merger?

So, how about life on Earth? Will earthly life survive the merger? Well, the sun will eventually become a red giant in about 7.5 billion years, when it will increase in size and consume the Earth. But even before then, the luminosity, or intrinsic brightness, of the sun will increase. This will happen, ultimately, in a timeline of about four billion years.

As solar radiation reaching the Earth increases, Earth’s surface temperature will increase. We may undergo a runaway greenhouse effect, similar to that going on now on the planet next door, Venus. So there’s a good change that earthly life won’t be around when the merger concludes.

But by that time, maybe some earthly inhabitants will have become space-faring. Perhaps we’ll have left Earth, and even our solar system. We may still get the view of Andromeda crashing into the Milky Way, just from a slightly different perspective.

Read more: Hubble Shows Milky Way is Destined for Head-On Collision

Bottom line: The Milky Way and Andromeda merger has already begun. The two spiral galaxies will form one giant elliptical galaxy in 5 billion years.

Source: Project AMIGA: The Circumgalactic Medium of Andromeda*

Via Discover Magazine

The post Andromeda and Milky Way galaxies are merging first appeared on EarthSky.

UPDATE NOVEMBER 4, 2021, AT 19:30 UTC (3:30 PM EDT): Aurora!! There are a phenomenal number of fantastic images people from around the world have been sharing. Here is one shared on Spaceweather.com by Markus Varik in Tromsø, Norway.

UPDATE NOVEMBER 4, 2021, AT 12:00 UTC (8:00 AM EDT): What a great show! A G3 storm (strong storm level) is still in progress and is predicted to stay near this level for the next 24 hours. Over the last 12 hours, the storm level has fluctuated between G3-G2-G1-G3 as can be seen in the NOAA Kp Index chart. G1 (minor) is Kp 5, G2 (moderate) is Kp 6, and G3 (strong) is Kp 7.

This activity has produced some amazing aurora especially closer to the polar region. The NOAA Ovation model is driven by solar wind measurements from the DISCOVR spacecraft. It gives a good indication of the true extent of the auroral oval on a timescale of 30-90 minutes. It does not provide detailed information at specific locations but gives a good idea of where one can expect to see great aurora (assuming the skies are clear).

UPDATE NOVEMBER 4, 2021, AT 00:10 UTC (8:10 PM EDT, NOVEMBER 3): The geomagnetic storm level reached the G3 level (Strong Storm) at 23:59 UTC (7:59 PM EDT), on November 3, 2021.

UPDATE NOVEMBER 3, 2021 AT 22:20 UTC (6:20 PM EDT): The combined CME has arrived, slightly earlier than forecast. The CME from November 2 overtook the CME from November 1, and the combined event was first detected as a shock wave at 19:57 UTC (3:57 PM EDT) by the DSCOVR spacecraft using its solar wind monitor (DSCOVR is located 1 million miles – 1.6 million km – from Earth in the direction of the sun.) Then a strong compression of Earth’s magnetosphere due to this shock was detected at 21:29 UTC (5:29 PM EDT). This set off a G1 geomagnetic storm at 21:38 UTC (5:38 PM EDT) increasing to a G2 geomagnetic storm at 22:05 UTC (6:05 PM EDT). NOAA has issued a warning for a geomagnetic storm of G3 or greater!

Aurora happening right now! Loch Lomond @VisitScotland @BeingScots @StormHour @ScotsMagazine #Aurora #Scotland pic.twitter.com/WnXLe7Dy95

— john anderson (@john_a_photo) November 3, 2021

UPDATE NOVEMBER 3, 2021 AT 17:00 UTC (1 PM EDT): Computer modeling by the NOAA Space Weather Prediction Center now suggests that the CME from the November 2, 2021, solar flare will first strike Earth’s atmosphere at 06:00 UTC (2 a.m. EDT) on November 4. So geomagnetic storming – and possible low-latitude auroras – should begin around the first half of November 4. Moreover, the G1 geomagnetic storm has been updated to a G2.

Aurora alert on an increasingly active sun

Only days after the last coronal mass ejection (CME) from the surface of the sun, with resulting aurorae, it’s time for a new one! This bump-up in activity is expected as Solar Cycle 25 continues to ramp up. On November 2, 2021, a solar flare in the now-visible sunspot region called AR2891 launched another CME toward Earth. The culprit solar flare behind the CME is classified as an M1.7 flare. And the movie above shows the CME as it emerges from the surface of the sun like a halo. More about that below.

Researchers expect the incoming solar charged particles will interact with Earth’s magnetic field to cause category G2 geomagnetic storms and a subsequent increased display of aurorae, or northern lights.

Turns out this cloud of charged particles from the sun isn’t just an ordinary CME, but also a cannibal CME sweeping up older and slower CME’s in front of it. More about that below, too.

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About these CMEs: Halos and cannibals

We mentioned the word “halo” above. This particular CME is considered a halo CME, because it appears as an expanding halo around the sun as we see it leaving the sun’s surface. Imagine you’re face to face with someone who blows a giant bubble of chewing gum the size of their head. The bubble frames their face in a halo from our point of view, because the bubble is aimed directly at us. In other words, this time, Earth is in the bullseye. Even though Earth will have traveled a bit in its orbit by the time the CME arrives a few days later, the CME is so much larger in extent that we will still be well within it.

And not only that, it is also a cannibal CME! A cannibal version of a CME is a CME that moves faster and thus able to sweep up and assimilate slower CMEs in front of them. Thus when they pile up on their way toward us, these combined CMEs contain strong magnetic fields and can spark even more impressive geomagnetic storms.

Spaceweather.com reported that:

… the slower CMEs, in this case, were hurled into space on November 1 and 2 by departing sunspot AR2887.

Bottom line: If you’re at a high latitude, keep your eyes open for aurorae in the next few days. The sun sent out a halo coronal mass ejection on November 2, 2021, due to arrive at Earth early in the day on November 4, with aurorae commencing that day.

Via Spaceweather.com

Via NOAA Space Weather Prediction Center

The post A storm on the sun, and a great week for auroras first appeared on EarthSky.

Voices of Apollo 11

As we celebrate the 52nd anniversary of the first human footsteps on the moon – which took place on July 20, 1969 – we’d like to bring some new astronomy art to your attention. Finnish astronomy artist J-P Metsavainio collected all the Apollo 11 transmissions and used them to construct an image of a full moon. He did this as a tribute to lunar astronaut Michael Collins, who died this year, on April 28, 2021. Metsavainio described his new image on his blog. He wrote:

I downloaded NASA’s original full transcript of Apollo 11’s onboard voice conversations. The idea was to turn this text into an image of the moon. After a few weeks of intense work at a feverish pace, my tribute was ready. Now the moon is made up entirely of Apollo 11 voice transcription letters. 

This is also a tribute to the entire Apollo 11 team: Commander Neil A. Armstrong, Command Module Pilot Michael Collins, and Lunar Module Pilot Edwin E. [Buzz] Aldrin Jr.

Michael Collins died just nine days after tweeting about a previous work by Metsavainio (an incredible Milky Way image that took 12 years to make). Metsavainio recounted:

I was most gratified and deeply moved when Michael Collins – the Apollo 11 and Gemini 10 astronaut, author, explorer and artist – tweeted […] about my work on April 19, 2021. The news of his passing, just nine days later, hit me all the harder. It was a very emotional moment for me. Out of the blue, I got inspired to create this artwork. I absolutely had to do it right away, which I did.

Michael Collins was also an artist. His iconic photos made from moon orbit are true art and part of mankind’s greatest cultural heritage treasure.

A beautiful mosaic of the Milky Way.
?: @JP_Metsavainio https://t.co/ocuXxCGug4 pic.twitter.com/9WYutIF9dU

— Michael Collins (@AstroMCollins) April 19, 2021

The loneliest man in history

At the same time Neil Armstrong was making history as the first human to set foot on the moon, Michael Collins was all alone aboard a spacecraft orbiting the moon. He was the astronaut who stayed behind on the command module, Columbia, while his two crew mates were testing out the moon’s surface in person.

And that’s why many affectionately called Michael Collins the loneliest man in history.

When he flew solo behind the moon, he was without radio contact with anyone. Meanwhile, his colleagues, Buzz Aldrin and Neil Armstrong, were making history with their first steps on the moon.

So, here at EarthSky, we think that this is a fitting image to celebrate the July 20 moon landing. Like most astronomical images, it is not only a beautiful image, but full of information. Apart from the Apollo 11 communications, Metsavainio also pointed out the landing site. If you zoom into the image far enough at the right spot, you can find it marked by two letters in red.

Working in solitude

Metsavainio recounts how he connected to Collins’ lonely and otherworldly experience as he was working:

A similar solitude gripped me while I was creating this tribute image. For being an astronomical photographer and a visual artist often is a very lonely job. Especially this time as I was deeply emotional throughout my creative process for this artwork. Even though I never met him personally, the end of his Earthly mission meant more to me than I was prepared for. I needed to make this photo-based artwork to process the inner storm of my thoughts and feelings.

Bottom line: Astronomy artist J-P Metsavainio has created an image of the moon, using all the words spoken to Earth by Apollo 11 astronauts, while on the moon. The image serves as a tribute to Michaels Collins, who died in April, 2021, as well as the entire Apollo 11 crew on this the 52nd anniversary of the Apollo moon landing.

The post Astronomy art: Voices of Apollo 11 first appeared on EarthSky.

For the first time, astronomers have found convincing evidence for a new type of supernova – a new sort of stellar explosion – powered by electron capture. They announced their discovery in late June 2021. It’s a type of supernova predicted 40 years ago, but never observed until now. Astronomers designate this supernova SN 2018zd. It lies in a remote galaxy, NGC 2146, 21 million light-years away.

The findings solve a mystery about one of the sky’s most famous objects, the Crab Nebula. Astronomers believe this cloud in space may be the remnant of a supernova that exploded in the year 1054 A.D. These scientists said the Crab likely stemmed from an electron-capture supernova, too, like SN 2018zd.

The team published their research in the peer-reviewed journal Nature on June 28, 2021.

Daichi Hiramatsu, lead author of the study, said:

We started by asking ‘what’s this weirdo?’ Then we examined every aspect of SN 2018zd and realized that all of them can be explained in the electron-capture scenario.

Type Ia and Type II supernovae

The names and distinctions between different types of supernovae might seem bewildering. But it’s worthwhile to remember two primary types of supernovae whose physical explosion mechanisms are different from one another (The type names are, however, based on differences in their light spectra).

In other words, Type Ia and Type II supernovae start out as different sorts of objects, explode for different reasons, and leave behind different things.

Type Ia supernovae happen when a small, dense white dwarf star exists in a double system with another star. Before the white dwarf became a white dwarf, it was a star with a mass up to eight times that of our sun. In this scenario, the white dwarf pulls matter from its companion. When it’s pulled enough mass to reach a critical limit, it undergoes runaway nuclear fusion and explodes as a Type Ia supernova. In the process, the white dwarf most likely disintegrates completely, leaving nothing behind.

Type II supernovae happen when a larger star – more than 10 times the mass of our sun – has reached the end of its life and its fuel. It then explodes in a so-called iron core collapse. The end product after this type of explosion is an exceedingly dense and small neutron star, or an even denser and smaller black hole.

An electron-capture supernova is a third type of supernova. It’s said to be a “missing link” between Type Ia and Type II. Astronomers have assumed since the early 1980s that electron-capture supernovae exist. One of the new study’s co-authors, Ken’ichi Nomoto, was one of the first to predict this possibility.

But, until now, there was no clearcut evidence anyone had seen an electron-capture supernova.

Super-AGB stars spawn electron-capture supernovae

And that might be because there was also little evidence for the progenitor stars that would cause electron-capture supernova. Astronomers call this kind of star super-AGB stars. That stands for massive super-asymptotic giant branch stars. The tongue-twisty name is a reference to where the star sits in the classic Hertzsprung-Russell diagram, which displays stars in various stages of their evolution.

A super-AGB star is a star in a late stage of its evolution. Like red supergiants, it has expanded to become large, cool and luminous.

A super-AGB star has an intermediate mass at around 7.5 to 10 times our sun’s mass. So it’s too massive to end up as a Type Ia supernova. And it’s not massive enough to become a Type II supernova.

Instead, for decades, astronomers have believed there must be a third type of supernova based on the electron-capture scenario.

What is an electron-capture supernova?

The astronomers behind this study identified six indicators necessary for a supernova to be an electron-capture supernova. SN 2018zd was the only one to agree with all six. Detailed Hubble Space Telescope images of the region, including the star before it went supernova, helped them with the identification.

Inside the core of a super-AGB star are oxygen, neon and magnesium atoms. When the core gets too dense, the neon and magnesium atoms start absorbing their electrons, which are particles found bound to the nucleus or cores of atoms. This absorption of electrons is known as an electron-capture reaction.

The electrons would normally keep the star’s core pressure up. But as the electrons are gobbled up, the core pressure reduces. Eventually the core collapses and the star explodes.

Voilà. An electron-capture supernova.

Theory and observation in agreement

There is a special gratification for a scientist when observations verify theory. Nomoto, co-author of this study, said:

I am very pleased that the electron-capture supernova was finally discovered, which my colleagues and I predicted to exist and have a connection to the Crab Nebula 40 years ago. I very much appreciate the great efforts involved in obtaining these observations. This is a wonderful case of the combination of observations and theory.

Was the Crab an electron-capture supernova?

In the year 1054 A.D., Chinese and Japanese sky-watchers recorded a supernova in our own Milky Way galaxy. It could be seen during daytime for 23 days and was visible at night for two years. In other words: it was bright. We can still see the remnant from this massive explosion, and we call it the Crab Nebula.

The supernova behind the Crab Nebula – SN 1054 – was, until now, the best candidate for an electron-capture supernova. But because the explosion happened a millennium ago, astronomers didn’t have all the details they needed to be sure. This new study helps bring up the confidence that SN 1054 was indeed the same kind of supernova as SN 2018zd. It also explains why SN 1054 was so bright. Las Cumbres Observatory reported:

Its luminosity was probably artificially enhanced by the supernova ejecta colliding with material cast off by the progenitor star as was seen in SN 2018zd.

An astronomical Rosetta Stone

The term Rosetta Stone is used too often as an analogy when we find a new astrophysical object. But in this case, I think it is fitting. This supernova is literally helping us decode thousand-year-old records from cultures all over the world. And it is helping us associate one thing we don’t fully understand, the Crab Nebula, with another thing we have incredible modern records of, this supernova. In the process it is teaching us about fundamental physics: how some neutron stars get made, how extreme stars live and die, and about how the elements we’re made of get created and scattered around the universe.

Bottom line: Astronomers have found observational evidence for an electron-capture supernova, a third kind of supernova that has been theorized for 40 years.

Source: The electron-capture origin of supernova 2018zd

Via Las Cumbres Obsrvatory

The post Finally, an electron-capture supernova first appeared on EarthSky.

Supermassive black holes help

Supermassive black holes are often described as devouring, monster, behemoth, lurking and so on. These words make it sound as if black holes are a harbinger of destruction. It’s true a star that ventures too close might get spaghettified and utterly destroyed by a black hole’s strong gravity. Plus, a supermassive black hole often sends out massive beams of destruction (better known as jets). But maybe black holes can do more than lurk and destroy? In June 2021, astronomers said that these supermassive black holes might bring about new star birth!

The newly forming stars wouldn’t be in the large galaxies where supermassive black holes themselves reside. Instead, the new stars are in the small satellite galaxies that exist on the outskirts of the larger galaxies.

The peer-reviewed journal Nature published this research on June 10, 2021.

These astronomers looked at the data of as many as 124,163 satellite galaxies that reside in the outskirts of 29,631 large galaxies, each believed to have a supermassive black hole in its centre.

This impressive amount of data come from the Sloan Digital Sky Survey (SDSS), an ambitious project that has been mapping the sky for over two decades.

Supermassive black holes not only destructive?

Not all astronomers find the oft-used descriptions of supermassive black holes to be entirely fair. Krista Smith recently expressed her thoughts about how supermassive black holes are often portrayed:

Stop demonizing supermassive black holes. They aren't "lurking" in galactic centers; that's their home! They built it! They aren't "monsters." They don't "ravenously consume" everything they see. Keep a respectful distance and you'll be fine. They are just beautiful giants!

— Krista Lynne Smith (@Quasar_Gal) May 12, 2021

And indeed, although there are unimaginable amounts of energies involved that admittedly can wreak some havoc, this new research shows something different. It describes how the energy of a central supermassive black hole can blow away – from the direction of the jets – intergalactic gas far, far out in the outskirts of the galaxy halo, and form “bubbles” in the gas. When small satellite galaxies travel through these bubbles, the astronomers noticed that they form new stars at a higher rate than they otherwise would. Why is that?

How often do stars form?

Your average galaxy won’t form new stars often. For example, our own Milky Way galaxy forms only about two stars per year. To form a new star, you need gas. Every galaxy consists of stars and gas (and dust and dark matter). The more gas, the more stars you can form. Consequently, the ability to form stars varies from galaxy to galaxy.

Some galaxies go through growth spurts. These are called starburst galaxies. They form a lot more stars than the Milky Way, on the order of a few hundred stars per year. And then there are some galaxies that barely form any stars at all.

An example of the last kind are the small kind of satellite galaxies that revolve around bigger ones. Our Milky Way has several, the most famous ones being the Large and Small Magellanic Clouds (these two are visible from the Southern Hemisphere).

Headwind from gas in the intergalactic medium

Satellite galaxies barely form any stars at all, and for many decades, astronomers have suspected that it’s because the intergalactic medium (IGM) that is present between galaxies are stripping away their gas. The gas in the intergalactic medium is extremely hot and thin; just about one atom per cubic meter or less. You can compare that to gas between the stars within a galaxy – the interstellar medium – which has one atom per cubic centimeter (about the size of a die).

Despite how unimaginably thin the intergalactic gas is, it affects galaxies moving through it, like a headwind you’d feel when riding a motorbike. It doesn’t affect the already existing stars in the small galaxy. But, to make new stars you need gas, and the galaxy’s gas – the material for making stars – will definitely be affected by the gas it meets as the galaxy moves through it, and it is stripped away. Consequently there is not enough gas left to form new stars, and the galaxy turns barren, if you will. Astronomers call this effect of losing gas while travelling through the IGM ram pressure stripping.

This is a very time-consuming process and nothing we can observe in real time. Instead, astronomers take help from simulations to figure out what might be going on.

A lunch break and its consequences

Over a lunch break at the Max Planck Institute, two astronomers – a theorist and an observer – had a discussion. The theorist, Annalisa Pillepich, had made a special kind of simulation (see the image at the top of this article). The observer, Ignacio Martin-Navarro, was a specialist at working with large data sets like the SDSS.

In the simulation, Pillepich found bubbles in the intergalactic gas, with much lower gas density, on each side of the galaxy, caused by the activity of the supermassive black hole in its center. Its jets are stretching out from the accretion disk in each direction, blowing away gas like a leaf blower clearing leaves in the autumn. The two astronomers mused: What would happen to satellite galaxies traveling through these empty cavities?

A Max Planck Institute statement said:

Martín-Navarro took up this question within his own domain. He had extensive experience in working with data from one of the largest systematic surveys to date: the Sloan Digital Sky Survey (SDSS), which provides high-quality images of a large part of the Northern Hemisphere … He examined 30,000 galaxy groups and clusters, each containing a central galaxy and on average four satellite galaxies.

What they found was a surprise: There were more actively star forming satellite galaxies in the direction where the supermassive black hole ejects its energy.

Why is this a surprise? It’s because one would think that the energy from the supermassive black hole would disturb the star formation and blow it out, like a wind blows out a candle. Instead, these astronomers said, the black hole outflows clear the way in the intergalactic medium so that satellite galaxies traveling through are not affected by it, and preserve their ongoing star formation. On average, these galaxies were 5% less likely to have had their star formation activity quenched.

Bottom line: Astronomers found that supermassive black holes may clear large bubble-like areas in the outskirts of their host galaxies. When small satellite galaxies travel through these areas, they will form more stars than they otherwise would.

Source: Anisotropic satellite galaxy quenching modulated by black hole activity

Via Max-Planck-Gesallschaft

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Dust hides the Milky Way center

Normally, the bright center of our home galaxy, the Milky Way, is hidden behind dust. The ordinary light we see with our eyes can’t pass through. But expand our vision (via telescopes and their instruments) to see different kinds of light – different wavelength regimes of the electromagnetic spectrum – and we can see through the dust. Then go a step further and combine several different kinds of “lights.” At that point, astronomers see fascinating new details, leading to new insights.

Astronomer Q. Daniel Wang of the University of Massachusetts Amherst did just this. He used high-energy X-ray data, gathered over two decades, combined with new low-energy radio data. The result, released on May 27, 2021, is an extraordinary new image of the central region of our Milky Way.

Thread-like superheated gas and magnetic fields

A portion of the new image is shown at the top of this post. It covers a region, 2,000 light-years thick, above and below the core of our galaxy. Fascinatingly, the new image reveals intriguing threads of superheated gas and magnetic fields.

Wang published the image and accompanying results in the June 2021 edition of peer-reviewed journal Monthly Notices of the Royal Astronomical Society.

As intriguing as these results are, Q. Daniel Wang, author of the paper, wasn’t surprised. He told EarthSky:

Not really, although I felt humbled. After all, we know that the [Milky Way] galaxy is a complex ecosystem. It will be a long way to truly understand how it works.

How did astronomers make the new image?

Wang stitched together X-ray data observed at 370 different telescope pointings – different aims at the sky to increase the field of view – at three different energy levels. He gave the energy levels – different frequency bands – the colors orange, green and purple and combined them. (This is similar to how different frequencies of visible light have different colors and combine into color images in our eyes.) These X-ray data were observed with the Chandra X-ray telescope over some 20 years. Among them are data Wang was in charge of, as the successful telescope – operating since 1999 – made its first large-scale X-ray survey of the Milky Way center.

In addition, the high-energy X-ray data were combined with low-energy radio observations (shown in grey and lilac) from the new South-African radio telescope MeerKAT.

Magnetic threads in Milky Way center

Astronomers can measure the magnetic field of something when cosmic rays get caught in the field and spiral around the field lines. That spiraling causes detectable radio emission.

The combined image reveals superheated gas (X-ray) and magnetic fields (radio) of the gas forming thread-like formations near the Milky Way center. A NASA statement explains how these are thought to be formed:

Such strips may have formed when magnetic fields aligned in different directions, collided, and became twisted around each other in a process called magnetic reconnection.

This is similar to the phenomenon that drives energetic particles away from the sun and is responsible for the space weather that sometimes affects Earth.

In other words, much as there is a kind of space weather in our solar system, so there is a large-scale space weather near the center of our home galaxy. The galaxy’s weather has a different cause. It’s driven by violent happenings such as supernova explosions and outbursts of matter from the region near the central supermassive black hole, Sagittarius A*. In the big picture, the galaxy’s space weather could explain how our galaxy is evolving. Wang said:

We know the centers of galaxies are where the action is and play an enormous role in their evolution.

A magnetized thread of particular interest

There are a few cases where the radio and X-ray appear in the same feature. In particular, a long thin thread called G0.17-0.41 is 25 light-years in length, but exceedingly thin.

Wang told EarthSky that finding the cause to this thread carried the biggest surprise:

It was so difficult to explain G0.17-0.41 by any well known mechanisms (for example a pulsar wind nebula or supernova remnant). Magnetic reconnection seems to be the only feasible mechanism.

In fact, this particular thread provided the strongest evidence, Wang pointed out:

This pair of the X-ray thread/radio filament really gives the key evidence for the magnetic reconnection and annihilation, as highlighted in the paper. The radio filament is locally fattened by the presence of the X-ray thread, clearly indicating the dynamic association.  The X-ray probably traces the hot gas heated by the annihilation, while the radio is emitted by the cosmic ray electrons accelerated by the reconnection.

Now, why are the thread-like features more visible in radio? Why don’t they all have counterparts in both X-ray and radio? Wang explained:

The magnetic field has to be strong enough to produce X-rays. But reconnection events should in general accelerate cosmic rays [shown in radio]. A reconnection is a dynamic process and comes and goes, although the time scale in the interstellar medium should be much longer than in a solar flare.

There is still much to do. Wang told EarthSky the next steps:

I’d like to confirm that magnetic reconnections are indeed responsible for many of the X-ray threads and radio filaments observed in the region and to see how effectively magnetic fields are transporting the energy from the center to large-scale structures and releasing it there.

Bottom line: By stitching together two decades worth of X-ray data from the Chandra space telescope and combining with radio data, astronomer Q. Daniel Wang has created a new image of the central region of our Milky Way galaxy. The image reveals magnetized threads stretching out large distances from the inner region.

Source: Chandra large-scale mapping of the Galactic Centre: probing high-energy structures around the central molecular zone

Via NASA

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Swirly and beautiful, spiral galaxies are what we often think of when someone mentions the word galaxy. Our own Milky Way is a spiral galaxy. These galaxies are pretty common in the nearby universe. But the farther back in time and distance astronomers look, the fewer spiral galaxies they see among the multitudes of galaxies in our universe. Instead, as we go out into space – and back in time – galaxies appear more irregular in shape. And thus how and when spiral galaxies formed is one of astronomy’s classic questions. And so it was with some excitement on May 20, 2021 that astronomers reported the most ancient spiral galaxy yet found.

This galaxy is labeled BRI 1335-0417. It existed only 1.4 billion years after the Big Bang, which equals a distance from us of 12.4 billion light-years. So far away – so far back in time – and yet this galaxy has clearly visible spiral arms! Clearly, this galaxy has an important contribution to make in answering questions about spiral galaxies’ origins.

The astronomers published a paper on their findings in the peer-reviewed journal Science on May 20, 2021.

What is a spiral galaxy?

Galaxies come in many different shapes and are classified by their morphology, meaning how they look. There are elliptical, spiral and strangely irregular galaxies, all with different features. Spiral galaxies consist of a central bulge of older stars, a flat rotating disk, and arms spiraling around the disk. Spiral galaxies exist primarily in the nearby universe. As you go out far in distance, back in time, the fewer spirals you see.

Takafumi Tsukui at the university SOKENDAI in Japan is the lead author of the new paper. He said in a statement:

I was excited because I had never seen such clear evidence of a rotating disk, spiral structure, and centralized mass structure in a distant galaxy in any previous literature.

Observing the most ancient spiral galaxy

The astronomers used a radio telescope, the Atacama Large Millimetre Array or ALMA telescope, to study galaxy BRI 1335-0417. This observatory – located in the Atacama Desert of northern Chile – is able to reach a high level of resolution (detail), despite the enormous distance the the galaxy. Tsukui said:

The quality of the ALMA data was so good that I was able to see so much detail that I thought it was a nearby galaxy.

Due to both the galaxy’s distance, and the early age of the universe at that distance, galaxy BRI 1335-0417 contained a lot of dust that obscures the light from it. The dust makes the galaxy structure hard to see using visible-light telescopes like Hubble. But, at radio wavelengths, astronomers can observe specific elements within the galaxy. And so they can look past the obscuring dust.

In this case, the astronomers looked at the emission from carbon ions for information.

Using the carbon ions as a tool for tracing the galaxy’s structure, the astronomers could see the spiral shape of BRI 1335-0417. They could see this structure extends about 15,000 light-years from the center of the galaxy. This is about 1/3 of the size of the Milky Way, as a comparison. But BRI 1335-0417 is about as massive as our Milky Way galaxy, including its number of stars and amount of interstellar matter. Just because you don’t see it extend farther doesn’t mean it isn’t larger. Tsuki explained:

As BRI 1335-0417 is a very distant object, we might not be able to see the true edge of the galaxy in this observation. For a galaxy that existed in the early universe, BRI 1335-0417 was giant.

How did a spiral galaxy form so early?

Simulations show that interacting galaxies can form an end-product galaxy with spiral arms. Galaxies interacted much more in the early universe, and so might explain the presence of BRI 1335-0417 so far back in time. There are more clues to that scenario as well: BRI 1335-0417 has a large supply of gas in its outskirts, for example. That’s an indication that there’s some kind of supply delivery coming in from the outside, possibly because this galaxy has been colliding with other, smaller galaxies.

The video below is a simulation that shows how many small galaxies interact to form a larger spiral galaxy.

Video ©2007 T. Takeda, S. Nukatani, T. R. Saitoh, 4D2U Project, NAOJ.

What happened next?

What happened next is the interesting question. According to conventional theory, star-forming galaxies (like BRI 1335-0417) with lots of dust in the early universe would evolve into giant ellipticals as they age. But maybe that might not happen? Maybe a galaxy like BRI 1335-0417 would remain a spiral for a much longer time? Spirals arms are of special interest to us because, as Tsukui said:

Our solar system lodges in one of the Milky Way spiral arms. Tracing the roots of spiral structure will provide us with clues as to the environment in which the solar system was born. I hope that this research will further advance our understanding of the formation history of galaxies.

Bottom line: Astronomers were surprised to discover spiral arms in a galaxy located in the very early universe. This makes the galaxy, BRI 1335-0417, the most ancient spiral galaxy found so far and provides clues to how and when spiral galaxies formed.

Source: Spiral morphology in an intensely star-forming disk galaxy more than 12 billion years ago

Via ALMA

The post The most ancient spiral galaxy yet first appeared on EarthSky.

Astronomers used a new technique – asteroseismology combined with spectroscopy – to pinpoint the ages of a sample of around 100 old red giant stars in the Milky Way. They were able to reach a much higher accuracy of the stars’ ages, they said in a statement on May 17, 2021. And they also found that a number of those red giant stars did not originate in the Milky Way! They are instead alien stars, which came here from another galaxy. Their original home in space was Gaia Enceladus (also known as the Gaia Sausage), a dwarf galaxy that collided and merged with our Milky way galaxy about 10 billion years ago.

This new research was published on May 17, 2021, in the peer-reviewed journal Nature Astronomy.

What do alien stars tell us?

So the idea here is that the Milky Way galaxy had already started forming many of its stars before a dwarf galaxy came by and merged with our galaxy, bringing its own stars with it. This event took place around 8-11 billion years ago. In contrast, the age of the Milky Way is about 13.6 billion years, give or take a few.

This merger, then, happened early in our galaxy’s history.

The dwarf galaxy – or the remnants of it – go today under the name Gaia Enceladus or the Gaia Sausage, because of the highly elongated shape it forms – like a sausage – as seen from data from the Gaia mission. In Greek mythology, Enceladus was the offspring of the goddess Gaia. It is also, incidentally, the name for one of Saturn’s moons.

In this new research, the astronomers were able to identify stars that are remnants of the merger. These stars provide a way of looking back to the distant past, when the merger took place. Josefina Montalbán at the University of Birmingham is the lead author on the paper. She said:

The chemical composition, location and motion of the stars we can observe today in the Milky Way contain precious information about their origin. As we increase our knowledge of how and when these stars were formed, we can start to better understand how the merger of Gaia-Enceladus with the Milky Way affected the evolution of our galaxy.

How did astronomers find the stars?

These astronomers had targeted a sample of 100 old stars observed with the Kepler mission. These are red giant stars, at the end of their lives.

The team used data from three Milky Way research instruments to measure the stars’ ages, all with the task of mapping and analyzing Milky Way stars. One instrument was Kepler, as mentioned previously. The other two were the Gaia satellite and APOGEE.

With data from these instruments, the astronomers used the technique of asteroseismology that studies how stars oscillate. That is, the technique measures regular variations within the star. Asteroseismology is similar to helioseismology, the study of oscillations in the sun. Learning how a star oscillates lets astronomers gain info about a star’s size and internal structure, which, in turn, will let them estimate the star’s age.

Team member Mathieu Vrard at Ohio State’s Department of Astronomy, said:

[It] allows us to get very precise ages for the stars, which are important in determining the chronology of when events happened in the early Milky Way.

In addition, the astronomers also used spectroscopy – the study of the stellar spectrum – to learn the chemical composition of the stars. This also helps with age determination, and in combination, the methods let the astronomers determine the ages to an unprecedented precision.

The astronomers noticed that a number of them were of the same age, and that this age was a bit younger than most of the stars that we know started their lives in the Milky Way.

Team member Andrea Miglio at the University of Bologna added:

We have shown the huge potential of asteroseismology in combination with spectroscopy to deliver precise, accurate relative ages for individual, very old, stars. Taken together, these measurements contribute to sharpen our view on the early years of our galaxy and promise a bright future for [Milky Way] archeoastronomy.

Now the researchers want to apply their approach to larger samples of stars to get a better view of the Milky Way’s formation history and evolution.

Bottom line: Astronomers used a new technique – asteroseismology in combination with spectroscopy – to precisely measure the age of stars, and found that a number of the red giants in their sample were not originally formed in our Milky Way galaxy, but came here as the result of a merger with dwarf galaxy Gaia Enceladus in the early history of the Milky Way.

Source: Chronologically dating the early assembly of the Milky Way

Via the University of Birmingham

Via Ohio State University

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