The orange star Aldebaran – the fiery eye of the Bull in the constellation Taurus – is easy to find. It’s part of a V-shaped group of stars – the Hyades – that forms the Bull’s face. You can locate Aldebaran using the famous constellation Orion as a guide. Notice the three stars of Orion’s Belt. Then draw an imaginary line through the Belt to the right. The first bright star you come to will be Aldebaran with its distinctive reddish-orange glow.

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When to spot Aldebaran

Aldebaran is the 14th brightest star, but five of those that outshine it are only barely visible or not visible at all from much of the Northern Hemisphere. Aldebaran is primarily a winter and spring star for us on the northern part of Earth. That’s when this orange star is most easily visible in the evening sky. By early December, it rises shortly after sunset and is visible all night. Three months later it is high to the south at sunset, and sets at around midnight. By early May, it hangs low about the western sunset glow – and before the end of the month, it’s lost altogether. It returns to the predawn sky around late June.

By the way, although it appears among them, Aldebaran is not actually a member of the V-shaped Hyades cluster. It is actually much closer to us in space than the other Hyades stars.

History and mythology of Aldebaran

Artists often depict Aldebaran as the fiery eye of Taurus the Bull. Because it is bright and prominent, ancient Persians honored Aldebaran one of the Four Royal Stars, the other three being Regulus, Antares and Fomalhaut.

The name Aldebaran is from the Arabic for the follower, presumably as a hunter following prey, which was likely the star cluster we call the Pleiades. Some viewed the latter as a flock of birds, perhaps doves. According to Richard Hinckley Allen in his classic book Star Names: Their Lore and Meaning, people once applied the name Aldebaran to the entire Hyades star cluster, a large loose collection of faint stars.

More star lore

In Hindu myth, Aldebaran was a beautiful young woman named Rohini, disguised as an antelope and pursued by her lecherous father, disguised as a deer, Mriga. Several other ancient peoples associated the star with rain. In a Dakota Sioux myth, Aldebaran was a star which had fallen to the Earth and whose killing of a serpent led to the formation of the Mississippi River. Allen notes a number of other alternate names, but precious little mythology is known for Aldebaran separately.

Also, Aldebaran is the name of one of the chariot horses in the movie and book “Ben Hur.”

On a different note, astronomer Jack Eddy has suggested a connection with the Big Horn Medicine Wheel, an ancient circle of stones atop a mountain in Wyoming. Eddy wrote that the ancient Americans may have used this site as a sort of observatory to view the rising of Aldebaran just before the sun in June to predict the June solstice.

Interestingly, in about two million years, the NASA space probe Pioneer 10, now heading out into deep space, will pass Aldebaran.

Science of Aldebaran

Aldebaran is a huge aging star. The diameter is about 44 times the size of our sun. If Aldebaran replaced our sun, its surface would extend past the orbit of Mercury.

Aldebaran glows with the orangish color of a K5 giant star. In visible light, it is about 153 times brighter than the sun, although its surface temperature is lower, roughly 4,000 K (about 3,700 degrees C or 6,700 degrees F) compared to 5,800 K (about 5,500 C or 10,000 F) for the sun.

Although Aldebaran is associated with the stars of the Hyades, it’s much closer at 65 light-years distant. The Hyades are about 150 light-years away.

Aldebaran is an erratic variable with minor variations too small to see with the eye. Also, five faint stars are visible near Aldebaran, but so far none have been confirmed to be gravitationally bound to Aldebaran.

Aldebaran’s position is RA: 4h 35m 55s, dec: 16°30’35”

Bottom line: Aldebaran is an enormous, orange-colored star that marks one of the eyes of Taurus the Bull. It also marks one point of the V-shape of the Bull’s face.

The post Aldebaran is Taurus the Bull’s bloodshot eye first appeared on EarthSky.

Look for Achernar from southerly latitudes

The 9th-brightest star in all the heavens, Achernar, is well known to observers in the Southern Hemisphere. But many northern stargazers know this star by its name only. That’s because – although it shines at magnitude +0.45, making it one of our sky’s brightest stars – it’s extremely far south on the dome of stars surrounding Earth. If you’re north of about 33 degrees north latitude, Achernar never rises above your horizon at all. And yet this star remains one of the sky’s most famous stars as the star at the end of the River.

The River is – of course – the constellation Eridanus, which is large and easy to see in a dark-enough sky, even if you’re fairly far north on Earth’s globe. The northern part of this constellation is located near the extremely prominent constellation Orion the Hunter. Eridanus appears to swell up in a great loop near Orion, then meander southward. Finally – for most in the Northern Hemisphere – it drops out of sight below the southern horizon before it reaches its end.

But if you are far enough south – below 33 degrees north latitude – you’ll easily spot the River’s end as the bright star Achernar.

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How to see Achernar

For all practical purposes, you must be even further south – around 25 degrees north latitude – to see Achernar well. That is a line drawn around the entire globe passing through Miami in the U.S. and Taipei in Taiwan.

Nowhere in North America has it easy, seeing this star. For example, from Key West, Florida, Achernar rises only about 8 degrees above the southern horizon. Even farther south, from the southern tip of Hawaii’s Big Island, Achernar never quite makes it to 14 degrees.

And yet, if you are far-enough south, you can see Achernar easily. After all, this star is very bright!

Just as Achernar marks the end of the River, the River also has a beginning. The star Beta Eridani or Cursa, which itself is easily visible from the Northern Hemisphere, shines near Orion’s brightest star, Rigel.

It is visible from North America?

On most nights of the year, Achernar is not visible from anywhere in North America. However, around October 20 it skirts the southern horizon around midnight, never getting very high. Then as the months pass, it is visible earlier at night, around 10 p.m. in November, 8 p.m. in December and just after sunset in January. Being far to the south with no bright stars around it, Achernar stands out in its isolation. If you have a dark sky, and are far enough south, you’ll easily see Achernar’s constellation Eridanus making its loop under the constellation Orion.

@GEO101SUNY Here is a beautiful image of the meandering Owens River out in California!https://t.co/NbKG4o7GYt pic.twitter.com/XHawVHkoKr

— hayden (@haydenmhoyt) November 18, 2023

Earthly rivers are sometimes known for meandering. In the sky, the stars representing Eridanus the River – Achernar’s constellation – have a similar quality.

Achernar’s history and mythology

In fact, the name Achernar derives from an Arabic phrase meaning End of the River.

Interestingly, in early classical times the name Achernar was given to the star we now know as Theta Eridani, or Acamar. At that time Acamar was the brightest star of the constellation visible from Greece, and thus was considered the River’s end.

When voyagers discovered the brighter star farther to the south, it became Achernar, and the former Achernar became Acamar.

Apparently both names derive from the same phrase, “Al Ahir al Nahr,” according to Richard Hinckley Allen, whose 1899 book Star Names: Their Lore and Meaning is still the best around.

Science of Achernar

Data from the Hipparcos mission placed Achernar at about 144 light-years away. It is a B3V star, meaning that it belongs to the main sequence of stars. And, the B means the star is blue and it’s the bluest star among the top 10 brightest stars.

Achernar is much hotter and brighter than our sun. In fact, it’s estimated to be over 3,100 times the luminosity, of our sun. It’s about six times the mass of the sun.

Brighter, hotter (and bluer) than the sun, Achernar produces more energy in the non-visible ultraviolet (UV) wavelengths. When you take this into consideration, it pumps out some 3,000 to 5,000 times the solar energy level. The discrepancy is due to an uncertainty in the amount of UV radiation it produces.

Achernar is also a binary star system. The companion star – called Alpha Eridani B – is a white main sequence star with about two solar masses. It orbits the primary star at a distance of 12 astronomical units every 14 to 15 years.

Its rapid rotation results in a flattened star

As mentioned above, Achernar’s mass is about six times that of our sun, and its average diameter is nearly eight to 10 times that of the sun. But, while our sun spins on its axis once about every 25 days, Achernar completes one rotation in slightly more than two days, or nearly 15 times faster than our sun. This fast rotation produces an odd, flattened shape, first discovered by the European Southern Observatory’s Very Large Telescope (VLT) in 2003. Up close, Achernar would look more like a blue M&M, while our sun would look more like an orange. Read more about Achernar’s flattened shape from ESO.

This flattening of Achernar makes an exact surface temperature for this star hard to determine. The flattening causes the star’s poles to be hotter than the equator. Estimates range from about 14,500 to 19,300 kelvin (about 14,200 to 19,026 C or around 26,000 to 34,200 F).

In today's episode of fun space objects:

Archernar. The currently "least spherical" known star in the Milky Way, owning its shape to its rapid rotation.

Here it is compared in shape (and size) with our Sun. pic.twitter.com/CTXPckWsSs

— George Ioannidis (@Qupeplex) February 6, 2023

Achernar’s position is RA: 01h 37m 42.8s, dec: -57° 14′ 12″.

Bottom line: Achernar is the 9th brightest star and flattest star known. It marks the end of Eridanus the River. Here’s why much of Earth never sees it … and how you can.

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The post Achernar is the End of the River of Eridanus constellation first appeared on EarthSky.

Fomalhaut, bright and lonely

Fomalhaut, aka Alpha Piscis Austrinus, is also called the Loneliest Star. It’s because Fomalhaut is the only bright star in a wide stretch of sky. From the Northern Hemisphere, Fomalhaut arcs in solitary splendor across the southern sky in autumn. Therefore, some call it the Autumn Star. From the Southern Hemisphere, you’ll look higher up to see Fomalhaut in your season of spring. In 2023, Fomalhaut isn’t so solitary. A bright planet, Saturn, appears near it in the sky. Of course, Fomalhaut will be the one that’s twinkling since Saturn will shine with a steady light.

Keep reading to learn more about this young star. It’s of special interest to astronomers because it has a debris ring around it. As a matter of fact, astronomers think new worlds are forming in Fomalhaut’s ring, at an early stage in the planet-forming process.

How to see it

Fomalhaut is the 18th brightest star in the night sky. It’s part of the faint constellation Piscis Austrinus the Southern Fish. In a dark sky, you’ll see a half-circle of faint stars of which bright Fomalhaut is a part. This star pattern marks the open mouth of the Southern Fish.

In early September, Fomalhaut is opposite the sun. So, it shines in the sky all night. It reaches its culmination – its highest point in the sky – around local midnight in mid-September.

Fomalhaut culminates (reaches its highest point in the sky) at different times on different dates. Here are just a few approximate times and dates of culmination:

July 15: 4:30 a.m. daylight saving time (DST)
August 15: 2:30 a.m. DST
September 15: 12:30 a.m. DST
October 15: 10:30 p.m. DST
November 15: 8:30 p.m. DST
December 15: 5:30 p.m. standard time

The view from different hemispheres

From the Northern Hemisphere, you can see Fomalhaut from as far north as 60 degrees latitude (southern Alaska, central Canada, northern Europe), where it just skims the southern horizon. From the Southern Hemisphere, Fomalhaut appears much higher in the sky. You can use one of several stargazing smartphone apps, some that are free, to help you find it. Or visit Stellarium-Web.org, the free online planetarium, and enter your location and time.

Rings of dust and gas

Fomalhaut is a hot white star about 25 light-years away. It’s almost twice the mass and size of our sun but radiates over 16 times the sun’s energy. Fomalhaut has a companion star less than a light-year away from it. The companion is an orange dwarf star, about 70% the mass of our sun. A third member of the Fomalhaut star system was announced in 2013, a small reddish star about 2.5 light-years from Fomalhaut. From Earth, we see the third star located in the constellation Aquarius instead of Piscis Austrinus.

Fomalhaut itself is a young star, just 440 million years old. That’s in contrast to 4 1/2 billion years for our sun. Fomalhaut is of special interest to astronomers because it has several rings of dust and gas around it, early indications of planets in the process of formation around this star. Astronomers have detected inner debris disks close to the star, within a few astronomical units (AU) from the star.

There’s a much larger, thicker debris ring about 133 AU from the star. A study published in 2008 generated a lot of excitement when Hubble Space Telescope images, taken in 2004, 2006 and 2008 showed an apparent planet very close to this debris ring. Astronomers first thought it was the first directly imaged exoplanet. But data from other telescopes brought that conclusion under scrutiny. And, by 2014, this object was no longer visible to Hubble.

A possible explanation

So what happened? Astronomers think that the “planet” was actually a large dust cloud generated by the collision of two large bodies near the ring. And over time, that dust cloud may have dissipated. And even though it turned out not to be a planet, astronomers were pleased. Catching the aftermath of a collision in a planet-forming disk was good, too! The event provided clues to a deeper understanding about how planets form.

Fomalhaut in history and mythology

The name Fomalhaut derives from the Arabic Fum al Hut, meaning Mouth of the Fish.

In the sky visible from the Northern Hemisphere, the constellation Aquarius the Water Bearer resides above Fomalhaut’s constellation Piscis Austrinus. You can see a zigzag line of stars from Aquarius to Piscis Austrinus. In sky lore, this line of stars represents water from the Jar of the Water Bearer, trickling into the open Mouth of the Fish.

According to Richard Hinckley Allen, Fomalhaut was one of the four guardians of the heavens to the ancient Persians, in 3,000 BCE, called by them Hastorang. (The other guardians were Aldebaran in Taurus, Antares in Scorpius, and Regulus in Leo.) Around 2,500 BCE, Fomalhaut helped mark the location of the winter solstice, meaning that it helped to define the location in the sky where the sun crossed the meridian at noon on the first day of winter. Also Allen also says that in 500 BCE, people worshipped Fomalhaut at the temple of Demeter in Eleusis, in ancient Greece.

Bottom line: Fomalhaut is relatively easy to spot as it shines brightly in an area of sky with no other bright stars nearby. And it’s of special interest to astronomers because of debris rings around it that are possibly the beginnings of a planetary system.

Read more: Fomalhaut has 3 nested belts around the star

The post Fomalhaut is the loneliest star in the southern sky first appeared on EarthSky.

The Big Dipper

The Big Dipper is a famous and favorite group of seven bright stars. It’s supposed to be easy to find. So why can’t you find it? In the northern autumn months, the Big Dipper rides low in the northern sky during the evening hours. For some parts of the U.S. and similar latitudes, it’s below the horizon in the evening at this time of year. To see it now, look north shortly before dawn!

The Big Dipper forms a highly recognizable star pattern that resembles an old-fashioned water dipper. The Dipper has a bowl made of four stars, and a handle made of three stars. It is part of the constellation Ursa Major the Great Bear.

The northern sky is like a large celestial clock, with Polaris – aka the North Star – at its center. In other words, the entire northern sky wheels in a great circle throughout the night (although it’s wheeling in a counter-clockwise direction). But the northern star Polaris stays still (or nearly so). That’s because Earth’s northern axis nearly points to it. And so Polaris is the famous North Star, used by sea navigators and scouts to find the direction north. Want to find it? You can use the famous Big Dipper asterism to locate Polaris.

Use Big Dipper to find Polaris

Notice that a line from the two outermost stars in the bowl of the Big Dipper points to Polaris. And notice that Polaris marks the tip of the handle of the Little Dipper. However, go look soon, because, in September, the Big Dipper is headed for its least noticeable time of year. The reason is that the Big Dipper swings full circle – 360 degrees – around Polaris in about 23 hours and 56 minutes. In 24 hours, the Big Dipper actually swings more than a full circle, or 361 degrees. Does that make a difference? Yes! It means that – if you look at the same time each autumn evening – the Big Dipper will appear just a little bit lower in the northwestern evening sky.

In other words, the Dipper is descending in the northwestern evening sky, from one night to the next. And that means that, a month from now at mid-evening (say around mid October), the Big Dipper will be noticeably lower in the northwest. For some months in autumn and winter, parts or even all of the Big Dipper is beneath the horizon in the evening, as seen from the southernmost latitudes in the United States. That might be why, if you’re just learning the sky, you sometimes look for the Dipper and can’t find it.

The Big Dipper is circumpolar from mid-northern latitudes

On the other hand, the Big Dipper is circumpolar, or always above the northern horizon, from more northerly latitudes. You’ll find it in your sky throughout the year as seen from the northern U.S., Canada and similar latitudes.

Watch the Big and Little Dippers circle around Polaris tonight!

The Dipper throughout time

The constant motion from night to night of these stars circling Polaris is a bit like a bear circling its prey, looking for a way to attack. Several ancient cultures from the Greeks and Romans to the Mi’kmaq Indians likened these stars to a bear.

In Greek mythology, the Big Dipper asterism represents the hindquarters and tail of the constellation Ursa Major the Great Bear. The Mi’kmaq saw the three stars of the Big Dipper handle as hunters chasing the bear.

Bottom line: To locate Polaris, the North Star, just draw a line between the two outer stars in the bowl of the Big Dipper.

The Big and Little Dippers: How to find them

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The post Why can’t I find the Big Dipper in September? first appeared on EarthSky.

They call it the Flying Star

61 Cygni is a double star in the constellation Cygnus the Swan. It’s not a standout in brightness. Why go to the trouble of finding it? Stars are individuals, and there’s something interesting about each one. But 61 Cygni is particularly cool because it has one of the highest proper motions of any visible star. That’s its sideways movement across the dome of the sky.

If you took photos of 61 Cygni over the course of several years, you’d see it shift position in the sky with respect to the more distant stars around it.

This unusual motion across our sky earned 61 Cygni the nickname the Flying Star.

The movement of the double star 61 Cygni over 8 years. pic.twitter.com/2HujtmYayl

— Dave Eagle FRAS ? Keep Looking Up. (@Dave_StarGeezer) June 14, 2023

61 Cygni has a high ‘proper motion’

So why does this star have such a high proper motion? Think of two people who are running, one near you, and the other farther away. In relation to the more distant landscape, the person closer to you would appear to cover more ground – more objects would pass behind them – than the person farther away.

Then in a similar way, very distant stars appear “fixed” in relationship to each other. However, they’re actually all moving through space in their various journeys around the center of our Milky Way galaxy. But most are so far away that we can’t easily detect their proper motions. On the other hand, 61 Cygni is different. It moves relatively rapidly in front of the fixed stars because 61 Cygni is relatively near Earth.

While not the closest star to the sun (that honor goes to the Alpha Centauri system), 61 Cygni is just 11.4 light-years distant. That makes it the fourth-closest star visible to the unaided eye, after Alpha Centauri, Sirius, and Epsilon Eridani. And it’s the 15th nearest known star system to the Earth.

Science of 61 Cygni

First, 61 Cygni isn’t just one star. In fact, it’s a binary system with an orbital period of about 659 years. So, to the unaided eye and through most binoculars, it appears as one star. However, if you look at it through a modest-sized telescope, you’ll see it resolved as two stars. They have apparent magnitudes of 5.21 and 6.03.

The 61 Cygni binary system is the 15th-nearest known star system to us. Both are K-type dwarf stars in the main sequence, thought to have formed 6 billion years ago (the sun, in comparison, is 4.6 billion years old). The more massive star of the pair has 70% of the sun’s mass and puts out 15% of the sun’s total electromagnetic energy. Its companion has 63% of the sun’s mass and shines at just 8.5% of the sun’s luminosity. Both are a bit over half the size of the sun. They’re also variable stars, exhibiting small changes in brightness over time.

The history of 61 Cygni

61 Cygni has no role in classical mythology. Of course, since it’s barely visible to the eye, the ancients apparently left no written reference to it at all. But its role in the history of astronomy is assured.

The motion of 61 Cygni across our sky, while large compared to other stars, can’t be easily detected with the eye alone over the span of a human lifetime. It was with the arrival of telescopes and through meticulous observations that astronomers discovered high proper motions of stars.

Astronomer Giuseppe Piazzi, in 1792, first noticed that 61 Cygni had a high proper motion when he compared his observations to those taken by another astronomer 40 years earlier. By 1804, he had gathered enough information to be the first to publish about this extraordinary star that he nicknamed the Flying Star.

Piazzi correctly noted that this high proper motion indicated that 61 Cygni was a nearby star, and that parallax measurements could be used to figure out its distance. German astronomer F. W. Bessel was the first to get reliable measurements of 61 Cygni stars’ parallaxes that gave a distance of 10.4 light-years, which is pretty close to the actual distance we know today, 11.4 light-years. It’s also the first time a star’s distance was reliably measured.

How to see it

As a matter of fact, 61 Cygni is roughly halfway between two other stars that you can probably identify. First, the brighter one is Deneb, the brightest star in the constellation Cygnus the Swan. And the other star is Zeta Cygni, at one end of the Swan’s wing. You’ll find 61 Cygni between these two. Several other similarly dim stars are located nearby, so you’ll need a detailed finder star chart to properly identify 61 Cygni.

61 Cygni’s position is RA: 21h 06m 55s, Dec: +38° 44′ 57″
Proper motion: 4″ in Right Ascension, 3″ in Declination
Parallax: 0.286″

Bottom line: 61 Cygni, while faint to the unaided eye, is one of the closest stars to Earth. It exhibits a high proper motion – or motion across the sky – compared to other stars.

The post 61 Cygni – a double star – is nicknamed Flying Star first appeared on EarthSky.

Ordinary binoculars under a dark sky can bring the Lagoon Nebula to you from 5,000 light-years away. Look for it a few degrees above and to the right of the Teapot asterism in the constellation Sagittarius. Messier 8 (M8) is the formal designation for this nebula. It’s a large gas cloud within our Milky Way galaxy, barely visible to the human eye under good conditions, but glorious with a dark sky and a bit of optical aid.

How to find the Lagoon Nebula

To be sure, you can enjoy great views of M8, but first you have to find it. Let’s start with when to look. In the Northern Hemisphere, mid-summer to mid-fall is ideal. If you’re in the Southern Hemisphere, you’ll want to look in mid-winter to mid-spring. Later, by early July each year, this object is crossing the meridian – appearing highest in the sky – at midnight. And by early September, it’s crossing the meridian as darkness falls, making it prime for early evening observations.

Next, pick a night close to new moon and an observing spot that is far from interfering lights. Visit EarthSky’s Best Places to Stargaze page to find a dark sky location near you.

Then, look for the constellation Sagittarius, which marks the direction of the Milky Way’s center. You’ll be looking southward in the evening from Earth’s Northern Hemisphere. If you’re in the Southern Hemisphere, look northward, closer to overhead, and turn the chart below upside-down. Want a more exact location for Sagittarius? Try using Stellarium, which will let you set a date and time from your exact location on the globe.

What you’ll see

The Lagoon Nebula spans an area of sky about three times the size of the full moon. As the largest and brightest of a number of nebulosities around Sagittarius, it’s widely visible throughout the Northern Hemisphere. Due to its location in the sky (-24 degrees declination), observers farther south see it even higher, which is better for observing.

The nebula is just a very faint patch to the unaided eye, but it takes on an oblong shape in binoculars. A brighter nucleus (the so-called “hourglass”) is visible on one side, separated by a dark rift from an open star cluster on the other side. While you may have seen images of the nebula with stunning color, these are attained via highly technical long-exposure photography. To the unaided eye, the faint nebulosity appears grayish, with little (if any) hint of color.

The science of the Lagoon Nebula

M8 is about 5,000 light-years away, and roughly 130 light-years in length. The Lagoon Nebula is an emission nebula, composed primarily of hydrogen. Much of it is ionized (heated or energized) by radiation from the nearby young and massive star Herschel 36. It’s also a stellar nursery: a place where new stars are born. And an open star cluster – NGC 6530 – made of hot, blue stars just a few million years old lies in this region. In addition to these young stars, there are also many dark Bok globules (dark nebulae) of condensing gas and dust. These are on their way to becoming protostars, and ultimately fully-fledged stars like those already formed nearby.

Early observations of the Lagoon

While the name Lagoon might conjure up a sense of fantasy, there is no known mythology associated with this interstellar cloud. The name apparently refers to the nebula’s appearance, with the dark lane running through the cloud evoking a sandbar between two lagoons. While visible to the unaided eye and therefore certainly seen throughout history, there is no known mention of this nebula until 1654, when Sicilian astronomer Giovanni Battista Hodierna recorded his observations of the star cluster within the nebula.

The area was then observed by several other astronomers, including Charles Messier in 1764, after which it also became known as Messier 8, or M8: the eighth object in Messier’s catalog.

If you’re looking to make some observations yourself, you may want to know that the Lagoon Nebula’s approximate center position is RA: 18h 04m, declination: -24° 22′.

Bottom line: The Lagoon Nebula, or Messier 8, is a large emission nebula in the constellation Sagittarius. You can explore it with binoculars.

The post Explore the Lagoon Nebula, M8, in Sagittarius first appeared on EarthSky.

The bright star Altair, aka Alpha Aquilae, shines as the brightest star in the constellation Aquila the Eagle. Mostly known for being one of the three Summer Triangle stars, this star is distinctive in its own right. It shines at magnitude +0.76. It’s only 16.8 light-years away from Earth, making it one of our nearest stellar neighbors. Plus, it has two more noteworthy features.

First, Altair rotates rapidly

This star requires only about 10 hours to spin once on its axis. That’s in contrast to 24 hours for our Earth to spin once, and about 27 days for our sun. In other words, this mighty star spins on its axis more rapidly than Earth! So this speedy spin tends to flatten the star a bit, much as a pizza crust flattens as it spins. Rough estimates are that Altair’s flattening is about 14%. Also, our sun is an oblate spheroid, although its flattening is difficult to measure due to the low rotation rate.

In 2007, University of Michigan astronomers combined light from four widely separated telescopes to produce the first picture (below) showing surface details on Altair. The researchers, led by John Monnier of U-M, used optical interferometry to get this image. Read more about the study at SpaceDaily.com.

The bright star Altair in the constellation Aquila is spinning on its axis so fast that it's flattened almost like an M&M. While our Sun rotates at a stately pace of once every 25 days, Altair flies around its axis once every 10 hours – close to its breakup speed.#Psalm19 pic.twitter.com/9YboZV7eq1

— Sarah Salviander (@sarahsalviander) September 25, 2019

Second, it’s variable … but not in a usual way

Variable stars brighten and dim, many on a (more or less) regular schedule. But Altair has as many as nine different rates of brightenings and dimmings. You won’t see these brightness variations with your eye. They’re too small to measure without sensitive instruments. But they’re there, and they’re likely related to Altair’s fast rotation.

By the way, if Altair took the place of our sun, at the distance the sun is now, life on Earth would be doomed. That’s because Altair is over 10 times more luminous that our sun. As you might have guessed, Altair is a more massive star than our sun, with about 1.8 times the sun’s mass. Its diameter is estimated to be between 1.6 to 2 times that of the sun. And its surface temperature is between 11,960 degrees F (6,626 C) to 14,840 F (8,226 C).

Altair is a white main sequence star – with a spectral type A7 – and is the 12th brightest star in the sky. It shares that spot with the star Acrux in the constellation Crux.

The star is classified as a Delta Scuti variable star since it shows slight changes in luminosity. It has three dim companion stars visible through telescopes. And not only is it a fast spinner, it moves quickly in front of its background stars. In fact, it’ll move about a full degree over the next 5,000 years.

How to see Altair

Altair has an apparent magnitude of +0.76. So you can see Altair easily with the eye.

But how will you recognize it? If you’re outside on a July or August evening, watch for the large Summer Triangle asterism in the east (as shown on the chart above). Look near the horizon for Altair, the last of the three Summer Triangle stars to ascend over your horizon.

You will recognize Altair by the two fainter stars on either side of it.

Also, the Great Rift of the summer Milky Way passes through the Summer Triangle. In fact, it goes right between the stars Vega and Altair. In dark skies in June, July and August, you can see rich star fields with your binoculars on both sides of the Great Rift.

‘Forbidden Planet’

In modern western culture, Altair is probably best known for being the home star system of the aliens in the 1956 science fiction film Forbidden Planet.

Altair in history and myth

The name Altair is Arabic in origin and has the same meaning as the name of the constellation Aquila in Latin; that is, they both mean simply “eagle.”

In classical mythology Aquila, and by extension Altair as well, was an eagle favored by Zeus. He played a part in numerous myths, including the abduction of Ganymede in which Aquila carries off a young boy (Ganymede) to Mount Olympus on Zeus’ command to become the cupbearer to the gods. In another myth Aquila is the eagle that torments Prometheus, until Hercules shoots it with poisoned arrows.

In India, Altair with its two flanking stars, Beta and Gamma (Tarazed and Alshain), in tradition represent the celestial footprints of the god Vishnu.

Altair is separated from the similar looking (but brighter) star Vega in the constellation Lyra by the great starlit band of the Milky Way. In Asia, this hazy band across our sky is known as the Celestial River. One story common in China, Japan and Korea is of a young herdsman (Altair) who falls in love with a celestial princess (Vega), who weaves the fabric of heaven.

The princess became so enamored of the herdsman that she neglects her weaving duties. This act enrages the princess’s father, the Celestial Emperor, who decrees that the herdsman must stay away from his daughter, on the opposite side of the River. The Emperor finally listened to the princess’s pleas, however, and allowed the herdsman to cross the Celestial River once per year, on the seventh day of the seventh month.

In Japan, Altair is Hikoboshi, and Vega is Orihime (or Tanabata). If it rains on the day of the festival of Tanabata, the rain represents Orihime’s tears shed because Hikoboshi could not navigate the treacherous waters of the Celestial River.

Altairs position

The position of Altair is RA: 19h 50m 47.0s, dec: +08° 52′ 06″

Bottom line: Altair is the brightest star in the constellation Aquila, and one of the closest stars to our solar system. Although 1.8 times our sun’s mass, it spins on its axis in only about 10 hours.

Our Summer Triangle series includes:

Vega is bright and blue-white

Deneb is distant and very luminous

Altair spins fast!

The post Summer Triangle star: Altair is variable and spins fast! first appeared on EarthSky.

Deneb or Alpha Cygni is the northernmost star in the famous Summer Triangle, a prominent asterism visible in the east on July and August evenings. Three bright stars make up the Summer Triangle. They are the brightest stars in three separate constellations. Deneb’s constellation is Cygnus the Swan. In a dark sky, you can imagine the Swan, flying along the starlit trail of the summer Milky Way. The constellation Cygnus also makes an obvious cross shape, and that’s another asterism. That is, it’s another prominent star pattern. It’s called the Northern Cross.

Okay, we’ve given you a lot of names here: Summer Triangle, Cygnus and Northern Cross.

Just remember, the constellation Cygnus the Swan contains the asterism of the Northern Cross. The Cross is just another way to see the Swan. Deneb is at the top of the Cross, but at the tail of the Swan (the star name “deneb” always means “tail”). The little star Albireo is at the head of the Swan, but at the base of the Cross.

Whew!

Very far away, and very luminous

The star Deneb in the constellation Cygnus the Swan is one of the most distant stars you’ll ever see with your eye alone. That’s because it’s one of our Milky Way galaxy’s most luminous stars.

Deneb is somewhere around 1,500 light-years away. That’s in contrast to most visible stars in our sky, located tens to hundreds of light-years away.

But astronomers still aren’t certain of the exact distance for this very luminous star. There are varying estimates for its distance. Why?

For some decades, ESA’s Earth-orbiting Hipparcos satellite, which operated from 1989 to 1993, provided the most important distance measurement for Deneb. Hipparcos was the predecessor to the Gaia space observatory, which is currently in space and collecting data, with the goal of creating a 3D map of our Milky Way galaxy.

Both Hipparcos and Gaia gather what’s called astrometric data on the stars. That is, they measure stars’ positions, motions and brightnesses not just once, but again and again. Those measurements let earthly astronomers calculate a distance, see how the star is moving, and much more.

Early analyses of Hipparcos data indicated a distance around 2,600 light-years for Deneb. Then, in 2009, a newer study – which used more powerful analysis techniques on Hipparcos data – gave a distance for Deneb that’s about half the widely accepted value, closer to 1,500 light-years.

Today, that value – around 1,500 light-years – is the most widely accepted value for Deneb’s distance.

When will Deneb’s distance estimate be updated?

Gaia has now released three sets of data. Why haven’t Gaia’s newer measurements let astronomers measure Deneb’s distance more precisely? It’s mainly because Gaia isn’t geared toward observing such a bright star as Deneb. Astronomer Anthony G.A. Brown of Leiden Observatory in the Netherlands – a member of the Gaia team – told EarthSky in July 2021 that Gaia data still haven’t been used to determine a new distance for Deneb. He said:

The Hipparcos distance estimate still stands.

Deneb is so bright that we can only observe it with Gaia through specially programmed observation sequences (the observing instruments on the spacecraft do not automatically pick up the star). We have observations of
Deneb in hand but these will require a dedicated processing which we have not yet started.

So, for now, the updated Hipparcos number is still the best one for Deneb’s distance. The best distance estimate for Deneb is approximately 1,500 light-years, for now.

And that’s impressive. So, for us to see a star shine so brightly in our sky from this great distance away, the star must be very powerful. Deneb is one of the most luminous stars – one of the brightest stars, intrinsically – that we can see with the eye.

Science of Deneb

Deneb is a blue-white supergiant star with a spectral class of A2Ia. It is the 19th brightest star in the sky shining at +1.25 magnitude. It’s about 196,000 times more luminous than our sun. Deneb contains about 20 solar masses, and as mentioned above its distance is uncertain. Deneb has a diameter about 203 times that of the sun. And that makes Deneb one of the largest type A spectral class stars known.

Deneb is the prototype for the Alpha Cygni variable stars. Its brightness varies due to non-radial fluctuations on the surface of the star. The fluctuations originate from areas on the surface of the star either expanding and contracting at the same time. They can last for days to weeks and their origin is unknown. The change in brightness is minimal, but detectable. For example, Deneb varies in brightness from 1.21 to 1.29 magnitude. Another Alpha Cgyni variable star is Rigel, in the constellation of Orion the Hunter.

Deneb was once a spectral class O-type main sequence star with a mass about 23 times that of the sun. Now that it’s a supergiant, it’s no longer fusing hydrogen in its core. It will evolve into a very luminous red supergiant or possibly a highly luminous blue variable star or maybe a Wolf-Rayet star. Regardless of which type of star it becomes, it’s expected to explode as a supernova sometime in the next few million years.

Deneb will be the North Pole Star around 9800 AD, but will be seven degrees from the pole. By the way, Deneb is the North Pole Star for Mars.

Deneb in history and mythology

The name Deneb derives from the Arabic Al Dhanab al Dajajah meaning Tail of the Hen. It obviously dates from an earlier incarnation of Cygnus not as a swan but as a chicken. Like many bright stars, Deneb has been called by a number of other names, but the oddest, according to Richard Hinckley Allen, who cites the Arabic name above, was Uropygium, meaning the posterior part of a bird’s body from which feathers grow, and oddly sometimes called the “Pope’s nose.”

In Chinese mythology Deneb is associated with the story of the Celestial Princess or the Weaver Girl. In this story a girl (the star Vega) is separated from her beloved (a cowherd represented by the star Altair) by the Milky Way. Once a year, the girl and the cowherd are allowed to meet briefly when a large flock of magpies forms a bridge across the starry river. Deneb represents the bridge.

How to see Deneb

You can gaze at this faraway star in the evening starting around May, or late spring in the Northern Hemisphere. From this hemisphere, in July and August, Deneb shines in the east at nightfall and appears high up in the sky around the middle of the night.

Like all stars, Deneb is found about one degree farther west at the same time each day, and climbs to its highest point about four minutes earlier per day, 1/2 hour earlier per week, or two hours earlier per month.

Deneb is circumpolar as seen from locations of about 45 degrees north latitude, roughly the northern tier of U.S. states. In other words, from the northern U.S. and similar latitudes, Deneb never sets but instead circles round and round the pole star.

This star is not visible south of about 45 degrees south latitude. That includes Antarctica, far southern Argentina and Chile, and perhaps the far southern tip of New Zealand’s South Island.

Aside from that, just about anyone should have a chance to see Deneb at one time or another. When you do see it, think of the power of this mighty star shining over such a great distance in space!

Deneb’s position is RA: 20h 41m 26s, dec: +45° 16′ 49″.

Bottom line: Information on the star Deneb, plus how to see it in your sky.

Our Summer Triangle series includes:

Vega is bright and blue-white

Deneb is distant and very luminous

Altair spins fast!

The post Summer Triangle star: Deneb is distant and luminous first appeared on EarthSky.

The Summer Triangle

On July evenings, look eastward in the evening for the season’s signature star pattern. It’s an asterism called the Summer Triangle, and, as you might guess, it consists of three stars: blue-white Vega, distant Deneb and fast-spinning Altair.

They’re the first three stars to light up the eastern half of the sky after sunset. You can see them even from light-polluted cities, or on a moonlit night.

Watch for the Summer Triangle pattern in the evening beginning around June, through the end of each year.

Vega is bright and blue-white

Blue-white Vega shines brightest of the three stars in the Summer Triangle. It’s the brightest star in the east in the evening on July evenings. And it’s the brightest light in the constellation Lyra the Harp. Thus Vega is also known as Alpha Lyrae. It shines at magnitude +0.03.

Vega is about 25 light-years away. And many people recognize Vega’s constellation, Lyra. This pattern of stars looks like a triangle of stars connected to a parallelogram.

Skywatchers around the world have a special place in their hearts for this beautiful blue-white star. Come to know it, and you will see.

How to see Vega and its constellation

Observers in the Northern Hemisphere typically begin noticing Vega in the evening around May, when this star comes into view in the northeast in mid-evening. Throughout northern summer, Vega shines brightly in the east in the evening. It’s high overhead on northern autumn evenings, and in the northwest by December evenings.

The little constellation Lyra has some interesting features. Near Vega you can see Epsilon Lyrae, which telescope users know as a famous double-double star. In other words, through small telescopes, you can see Epsilon Lyrae as double, with each of the two components also a double star.

Meanwhile, another famous telescopic sight lies between the Gamma and Beta stars in Lyra, the Ring Nebula, also called M57.

You can see Vega, Epsilon Lyrae and M57 (the Ring Nebula) marked on the chart below.

Science of the star Vega

Vega is the fifth-brightest star visible from Earth, and the third-brightest easily visible from mid-northern latitudes, after Sirius and Arcturus. At about 25 light-years away, it is the sixth-closest of all the bright stars, or fifth if you exclude Alpha Centauri, which most of the Northern Hemisphere can’t easily see.

The star’s distinct blue color indicates a surface temperature of nearly 17,000 degrees Fahrenheit (9,400 Celsius), which is is about 7,000 degrees F (4,000 C) hotter than our sun. This star is roughly 2.5 times the diameter of the sun, and about twice its mass. But Vega’s internal pressures and temperatures, far greater than our sun’s, will cause it to burn its internal fuel faster. At only half a billion years old, Vega is already middle-aged. That’s in contrast to our middle-aged sun, which is 4 1/2 billion years old. Vega is only about a tenth our sun’s age, but it will run out of fuel in only another half-billion years.

In astronomer-speak, Vega is an “A0V main sequence star.” The “A0” signifies its temperature, whereas the “V” is a measure of energy output (luminosity), indicating that Vega is a normal star (not a giant). “Main sequence” means it’s in the category of normal stars, and produces energy through stable fusion of hydrogen into helium. With a visual magnitude of +0.03 (apparent brightness), Vega appears only marginally dimmer than Arcturus, but with a distinctly different, cool-blue color.

Vega rotates so fast it’s flattened

Vega rotates rapidly, making a single full rotation about its axis once about every 12.5 hours. In contrast, our sun requires 27 days to spin once. As a result, if you could visit Vega in space, you’d find it noticeably flattened, as shown in the computer simulation below. Though a fast spinner, Vega isn’t the fastest of the three Summer Triangle stars. Altair rotates in only about 10 hours!

Vega appears to have an asteroid belt

In 2018, astronomers announced it appears Vega has a large asteroid belt surrounding it. NASA’s Spitzer Space Telescope and the European Space Agency’s Herschel Space Observatory detected a ring of warm, rocky debris. NASA said:

In this diagram, the Vega system, which was already known to have a cooler outer belt of comets (orange), is compared to our solar system with its asteroid and Kuiper belts. The relative size of our solar system compared to Vega is illustrated by the small drawing in the middle. On the right, our solar system is scaled up four times.

The comparison illustrates that both systems have inner and outer belts with similar proportions. The gap between the inner and outer debris belts in both systems works out to a ratio of about 1-to-10, with the outer belt 10 times farther away from its host star than the inner belt.

Astronomers think that the gap in the Vega system may be filled with planets, as is the case in our solar system.

In tradition and myth

In western skylore, Vega’s constellation Lyra was a harp played by the legendary Greek musician Orpheus. According to legend, when Orpheus played his harp, neither god nor mortal could turn away.

In western culture, Vega is often called the Harp Star.

But Asia has the most beautiful stories relating to Vega. In China, the legend speaks of a forbidden romance between the goddess Zhinu – represented by Vega – and a humble farm boy, Niulang, represented by the star Altair. Separated in the night sky by the Milky Way, or Celestial River, the two lovers may meet only once a year. It’s said that their meeting comes on the 7th night of the 7th moon, when a bridge of magpies forms across the Celestial River, and the two lovers briefly reunite.

Their reunion marks the time of the Qixi Festival.

More star lore

In Japan, the Tanabata festival features Orihime, a celestial princess or goddess, represented by Vega, who falls in love with a mortal, Hikoboshi, represented by the star Altair. But when Orihime’s father finds out, he is enraged and forbids her to see this mere mortal. Then … you know the story. The gods place the two lovers in the sky, separated by the Celestial River or Milky Way. Yet the sky gods in kindness let them reunite on the 7th night of the 7th moon each year. Sometimes Hikoboshi’s annual trip across the Celestial River is treacherous, though, and he doesn’t make it. In that case, Orihime’s tears form raindrops that fall over Japan.

Many Japanese celebrations of Tanabata occur in July, but sometimes they take place in August. Sometimes the Perseid meteor shower represents Orihime’s tears in myth.

For observation, Vega’s position is RA: 18h 36m 56.3s, dec: +38° 47′ 1.3″.

Bottom line: The star Vega in the constellation Lyra is one of the sky’s most beloved stars, for people around the world.

Our Summer Triangle series includes:

Vega is bright and blue-white

Deneb is distant and very luminous

Altair spins fast!

The post Summer Triangle star: Vega is bright and blue-white first appeared on EarthSky.

10 cool things about stars

Every star you see in the night sky is bigger and brighter than our sun

You can’t see millions of stars on a dark night

Red hot and cool ice blue – NOT!

Stars are black bodies

There are no green stars

Our sun is a green star

Our sun is a dwarf star

Stars don’t twinkle

You can see 20 quadrillion miles, at least

Black holes don’t suck

1. Every star you see in the night sky is bigger and brighter than our sun

Of the 5,000 or so stars brighter than magnitude 6 (that is, bright enough to see with the eye), only a handful of very faint stars are approximately the same size and brightness of our sun. And the rest are all bigger and brighter. Of the 500 or so that are brighter than 4th magnitude (which includes essentially every star visible to the unaided eye from an urban location), all are intrinsically bigger and brighter than our sun, many by a large percentage. Of the brightest 50 stars visible to the human eye from Earth, the least intrinsically bright is Alpha Centauri, the closest star system to Earth at 4.2 light-years away. And Alpha Centauri is still more than 1.5 times more luminous than our sun (plus, because it’s so far south on the sky’s dome, it can’t be easily seen from most of the Northern Hemisphere).

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2. You can’t see millions of stars on a dark night

Despite what you may hear in TV commercials, poems and songs, you cannot see a million stars … anywhere. There simply are not enough stars close enough and bright enough to equal a million. On a really exceptional night, with no moon and far from any source of lights, a person with very good eyesight may be able to see 2,000-2,500 stars at any one time, although counting even this small number still would be difficult. So the next time you hear someone claim to have seen a million stars in the sky, just appreciate it as artistic license or exuberant exaggeration. It isn’t true and can’t be true!

3. Red hot and cool ice blue – NOT!

We are accustomed to referring to things that are red as hot and those that are blue as cool. This is not entirely unreasonable, since a red, glowing fireplace poker is hot and ice, especially in glaciers and polar regions, can have a bluish cast. But we say that only because our everyday experience is limited. In fact, heated objects change color as their temperature changes. And red represents the lowest temperature at which a heated object can glow in visible light. As it gets hotter, the color changes to white and ultimately to blue. So the red stars you see in the sky are the “coolest” (least hot), and the blue stars are the hottest!

4. Stars are black bodies

A black body is an object that absorbs 100% of all electromagnetic radiation (that is, light, radio waves and so on) that falls on it. A common image here is that of a brick oven with the interior painted black and the only opening a small window. All light that shines through the window is absorbed by the interior of the oven and none is reflected outside the oven. It is a perfect absorber. As it turns out, this definition of being perfect absorbers suits stars very well! But this just says that a black body absorbs all the radiant energy that hits it. And it does not forbid the blackbody from re-emitting the energy. In the case of a star, it absorbs all radiation that falls on it, but it also radiates back into space much more than it absorbs. Thus a star is a black body that glows with great brilliance! An even more perfect black body is a black hole. But, unlike stars, a black hole appears truly black, and radiates no light.

5. There are no green stars

There are scattered claims for stars that appear green, including Beta Librae (Zubeneschamali). But most observers do not see green in any stars except as an optical effect from their telescopes, or else an idiosyncratic quirk of personal vision and contrast. Stars emit a spectrum (“rainbow”) of colors, including green. But the human eye-brain connection mixes the colors together in a manner that rarely if ever comes out green. One color can dominate the radiation, but within the range of wavelengths and intensities found in stars, greens get mixed with other colors. And in that case the star appears white. For stars, the general colors are, from lower to higher temperatures, red, orange, yellow, white and blue. So as far as the human eye can tell, there are no green stars.

6. Our sun is a green star

What was said just above notwithstanding, our sun is a “green” star, or more specifically, a green-blue star, whose peak wavelength lies clearly in the transition area on the spectrum between blue and green. This isn’t just an idle fact, but is important because the temperature of a star is related to the color of its most predominate wavelength of emission (whew!). In the sun’s case, the surface temperature is about 5,800 kelvin (about 5500 Celsius or 10000 Fahrenheit), or 500 nanometers, a green-blue. However, as indicated above, when the human eye factors in the other colors around it, the sun’s apparent color comes out a white or even a yellowish white.

7. Our sun is a dwarf star

We are accustomed to think of the sun as a “normal” star, and in many respects, it is. But did you know that it is a “dwarf” star? You may have heard of a “white dwarf,” but that is not a regular star at all, but the corpse of a dead star. Technically, as far as “normal” stars go (that is, astronomical objects that produce their own energy through sustained and stable hydrogen fusion), there are only “dwarfs,” “giants” and “supergiants.” The giants and supergiants represent the terminal (old age) stages of stars. But the vast majority of stars – those in the long, mature stage of evolution (called the main sequence by astronomers) – are all called “dwarfs.” There is quite a bit of range in size here, but they are all much smaller than the giants and supergiants. So technically, the sun is a dwarf star, sometimes called “yellow dwarf” in contradiction to the entry above!

8. Stars don’t twinkle

Stars appear to twinkle (“scintillate”), especially when they are near the horizon. The brightest star, Sirius, twinkles, sparkles and flashes so much some times that people actually report it as a UFO. Is twinkling a property of the stars then? No. It’s a property of Earth’s turbulent atmosphere. As the light from a star passes through the atmosphere, especially when the star appears near the horizon, it must pass through many layers of often rapidly differing density. This has the effect of deflecting the light slightly like a ball in a pinball machine. The light eventually gets to your eyes, but every deflection causes it to change slightly in color and intensity. The result is “twinkling.” Above the Earth’s atmosphere, stars do not twinkle.

9. You can see 20 quadrillion miles, at least

On a good night, you can see about 19,000,000,000,000,000 miles, easily. That’s 19 quadrillion miles, the approximate distance to the bright star Deneb in Cygnus, which is prominent in the evening skies of Fall and Winter. Deneb is bright enough to be seen virtually anywhere in the Northern Hemisphere and, in fact, from almost anywhere in the inhabited world. There is another star, Eta Carinae, that is a little more than twice as far away, or about 44 quadrillion miles. But Eta Carinae is faint, and not well placed for observers in most of the Northern Hemisphere. Those are stars, but both the Andromeda Galaxy and the Triangulum Galaxy are also visible under certain conditions, and are roughly 15 and 18 quintillion miles away! (One quintillion is 10^18!)

10. Black holes don’t suck

Many writers frequently describe black holes as “sucking” in everything around them. And it is a common worry among the ill-informed that the so-far hypothetical “mini” black holes that may be produced by the Large Hadron Collider (LHC) would suck in everything around them in an ever increasing vortex that would consume the Earth! “Say it ain’t so, Joe!” Well, I am not Shoeless Joe Jackson, but it ain’t so. In the case of the LHC, it isn’t true for a number of reasons, but black holes in general do not “suck.”

This not just a semantic distinction, but one of process and consequence as well. The word “suck” via suction, as in the way vacuum cleaners work, is not how black holes attract matter. In a vacuum cleaner, the fan produces a partial vacuum (really, just a slightly lower pressure) at the floor end of the vacuum, and regular air pressure outside, being greater, pushes the air into it, carrying along loose dirt and dust.

In the case of black holes, there is no suction involved. Instead, matter is pulled into the black hole by a very strong gravitational attraction. In one way of visualizing it, it really is a bit like falling into a hole, but not like being hoovered into it. Gravity is a fundamental force of Nature, and all matter has it. When something is pulled into a black hole, the process is more like a fish being reeled in by an angler, rather than being pushed along like a rafter inexorably being dragged over a waterfall.

The difference may seem trivial, but from a physical standpoint it is fundamental.

So black holes don’t suck, but they are very cool. Actually, they are cold. Very, very cold. But that’s a story for another time.

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Bottom line: Here’s a collection of 10 cool things about stars that you probably didn’t know. Big stars, green stars, black holes, stars by the millions, and more!

The post Top 10 cool things about stars that you probably didn’t know first appeared on EarthSky.