APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

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APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by APOD Robot » Sun Apr 26, 2020 4:05 am

Image Edwin Hubble Discovers the Universe

Explanation: How big is our universe? This very question, among others, was debated by two leading astronomers 100 years ago today in what has become known as astronomy's Great Debate. Many astronomers then believed that our Milky Way Galaxy was the entire universe. Many others, though, believed that our galaxy was just one of many. In the Great Debate, each argument was detailed, but no consensus was reached. The answer came over three years later with the detected variation of single spot in the Andromeda Nebula, as shown on the original glass discovery plate digitally reproduced here. When Edwin Hubble compared images, he noticed that this spot varied, and so wrote "VAR!" on the plate. The best explanation, Hubble knew, was that this spot was the image of a variable star that was very far away. So M31 was really the Andromeda Galaxy -- a galaxy possibly similar to our own. The featured image may not be pretty, but the variable spot on it opened a door through which humanity gazed knowingly, for the first time, into a surprisingly vast cosmos.

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by jks » Sun Apr 26, 2020 4:26 am

Hi,

Very nice APOD today, but the astronomy's Great Debate link appears to be broken as it gives the dreaded "404 Not Found" response.

guenthert

Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by guenthert » Sun Apr 26, 2020 5:42 am

How could one hope to resolve an individual star, variable or not, in a galaxy 2 million ly away? Isn't the fact that one can resolve the star an indication that it belongs to the local galaxy?

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by Ann » Sun Apr 26, 2020 7:30 am

guenthert wrote: Sun Apr 26, 2020 5:42 am How could one hope to resolve an individual star, variable or not, in a galaxy 2 million ly away? Isn't the fact that one can resolve the star an indication that it belongs to the local galaxy?
A star's variability can sometimes tell us how far away it really is.
Wikipedia wrote:

Henrietta Swan Leavitt (/ˈlɛvɪt/; July 4, 1868 – December 12, 1921) was an American astronomer. A graduate of Radcliffe College, she worked at the Harvard College Observatory as a "computer", tasked with examining photographic plates in order to measure and catalog the brightness of stars. This work led her to discover the relation between the luminosity and the period of Cepheid variables. Leavitt's discovery provided astronomers with the first "standard candle" with which to measure the distance to faraway galaxies. After her death, Edwin Hubble used Leavitt's period-luminosity relation, together with the galactic spectral shifts first measured by Vesto Slipher at Lowell Observatory, in order to establish that the universe is expanding (see Hubble's law).
Wikipedia wrote:

A Cepheid variable (/ˈsɛfiːɪd, ˈsiːfiːɪd/) is a type of star that pulsates radially, varying in both diameter and temperature and producing changes in brightness with a well-defined stable period and amplitude.

A strong direct relationship between a Cepheid variable's luminosity and pulsation period established Cepheids as important indicators of cosmic benchmarks for scaling galactic and extragalactic distances. This robust characteristic of classical Cepheids was discovered in 1908 by Henrietta Swan Leavitt after studying thousands of variable stars in the Magellanic Clouds. This discovery allows one to know the true luminosity of a Cepheid by simply observing its pulsation period. This in turn allows one to determine the distance to the star, by comparing its known luminosity to its observed brightness.
When Hubble discovered a variable star in the Andromeda Galaxy, he could see that it is similar to other, brighter stars that are variable, and since this star looks so faint, we can be certain that it is located outside the boundaries of the Milky Way.

The Andromeda Galaxy was long believed to be just a nebula of gas, not entirely unlike other nebulas in the sky like the Orion Nebula except for its spiral shape. The Andromeda Galaxy was called the Andromeda Nebula.


















One hypothesis about the "spiral nebulas" was that they might be solar systems in the making. Therefore, it was surmised by some people that the "Andromeda Nebula" was a swirling gas cloud well inside the borders of the Milky Way, and that the center of this gas cloud might suddenly light up with a newborn star. So when the supernova SN 1885A appeared near the center of Andromeda in 1885, some people took this to mean that Andromeda was indeed a "protoplanetary disk" (even though that word was not used back then) and that the central star that some people waited for had indeed appeared. So, conceivably, a tiny pinprick of brightness inside what looked like a gas cloud inside the Milky Way might not necessarily mean that this object was a star.

But if the tiny pinprick of light was variable, then it really was a star. And then Andromeda became a whole new "island universe", separate from our own galaxy. And all those other mysterious "spiral nebulas" (what about M33, for example?) were surely galaxies too, right?

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Last edited by Ann on Sun Apr 26, 2020 9:37 am, edited 2 times in total.
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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by Ann » Sun Apr 26, 2020 8:03 am

jks wrote: Sun Apr 26, 2020 4:26 am Hi,

Very nice APOD today, but the astronomy's Great Debate link appears to be broken as it gives the dreaded "404 Not Found" response.
Check out this nice summary of the Great Debate in Astronomy Magazine.

To give you a brief summary, though, Harlow Shapley had demonstrated the huge size of the Milky Way by explaining that the globular clusters of the Milky Way are symmetrically distributed around the center of the Milky Way, which is located in the constellation of Sagittarius. From our point of view, the globulars seem to be asymmetrically distributed in the sky, for the simple reason that our Solar system is located some 26,000 light-years away from our galaxy's center.

In short, though, Harlow Shapley had demonstrated that the Milky Way was larger than astronomers had believed. This made him believe that the Milky Way was the entirety of the Universe.

Heber Curtis had spent a decade studying "the spiral nebulae", and he thought that they may well be galaxies in their own right. Curtis was right, of course, but The Great Debate didn't resolve the question of the nature of "the spiral nebulae". It was left to Hubble to prove that Heber Curtis was the one that had been right.

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by Fref » Sun Apr 26, 2020 8:35 am

I am confused. The glass plate shows proof of other galaxies existence in form of the andromeda galaxy. What did Hubble think it was if not a galaxy?

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by JohnD » Sun Apr 26, 2020 8:55 am

Fref, read Anne's useful, account above.

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by heehaw » Sun Apr 26, 2020 8:59 am

Vast universe? I call it a half-vast universe, myself!

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by Ann » Sun Apr 26, 2020 9:28 am

heehaw wrote: Sun Apr 26, 2020 8:59 am Vast universe? I call it a half-vast universe, myself!
This video suggests that the minimum size of the Universe is 250 Hubble spheres. A Hubble sphere is the size of the observable Universe, which is about 93 billion light-years in diameter.

250 Hubble spheres is pretty decently big, I think.

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by neufer » Sun Apr 26, 2020 11:04 am

Ann wrote: Sun Apr 26, 2020 9:28 am
heehaw wrote: Sun Apr 26, 2020 8:59 am
Vast universe? I call it a half-vast universe, myself!
This video suggests that the minimum size of the Universe is 250 Hubble spheres. A Hubble sphere is the size of the observable Universe, which is about 93 billion light-years in diameter.

250 Hubble spheres is pretty decently big, I think.
How many multiverse Hubble bubble spheres does it take to produce one bad joke?
https://www.merriam-webster.com/dictionary/half-assed#h1 wrote:
half-assed: adjective [circa 1932]

1 slang, often vulgar : lacking significance, adequacy, or completeness

… a half-assed attempt at generative semantics.— James D. McCawley

Note how they want and get full credit for every little half-ass act of generosity they perform.— George Garrett

Several jets were indeed clogged, and it took about five minutes to do a half-assed job of cleaning them out, good enough to get them through until tomorrow and a more thorough cleaning.— Richard Russo

2 slang, often vulgar : lacking intelligence, character, or effectiveness

… said that the Administration had no intention of allowing "a bunch of half-assed people to send foreigners into combat."— Seymour M. Hersh

… had been beaten up by every half-assed bully all through his miserable childhood…— Joseph Wambaugh
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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by Ann » Sun Apr 26, 2020 11:37 am

neufer wrote: Sun Apr 26, 2020 11:04 am
How many multiverse Hubble bubble spheres does it take to produce one bad joke?
Hmmm... one Hubble sphere?

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by orin stepanek » Sun Apr 26, 2020 12:34 pm

Orin

Smile today; tomorrow's another day!

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by Iksarfighter » Sun Apr 26, 2020 12:44 pm

A picture from Hubble so.

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by Chris Peterson » Sun Apr 26, 2020 1:23 pm

Fref wrote: Sun Apr 26, 2020 8:35 am I am confused. The glass plate shows proof of other galaxies existence in form of the andromeda galaxy. What did Hubble think it was if not a galaxy?
If you don't know how far away it is, how do you know it isn't a nebulous area inside our galaxy? How do you know that the galaxies seen with telescopes are the same thing as what we live in?
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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by Chris Peterson » Sun Apr 26, 2020 1:26 pm

guenthert wrote: Sun Apr 26, 2020 5:42 am How could one hope to resolve an individual star, variable or not, in a galaxy 2 million ly away? Isn't the fact that one can resolve the star an indication that it belongs to the local galaxy?
"Resolve" in this context simply means separating it from its background, not resolving it as anything other than a point. All that requires is that it be bright enough with respect to that background. Your eyes resolve thousands of stars in the nighttime sky- each one an optical point source. Every supernova we observe in other galaxies is readily detected, but not optically resolved.
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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by ta152h0 » Sun Apr 26, 2020 9:22 pm

I bought a large amount of diodes and transistors as a replacement for the ones that keep burning up in my head as I keep thinking about this particular endeavor. pass the ice cold one
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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by scorpion136 » Sun Apr 26, 2020 9:29 pm

Yes, it’s quite difficult to tell whether a star is bright and far away or dim and close, since the two appearances would be similar

Henrietta Leavitt was the first to solve this problem. She was studying variable stars in the Large Magellanic Cloud, a satellite galaxy of our Milky Way.

Every star tells you how bright it looks (you just look at it and you know!). This is called its apparent brightness (“apparent” for how it “appears”). However as mentioned it might be a bright star far away that looks dim, or a dim star that is close to us which therefore looks bright. So appearances can be deceiving.

Henrietta Leavitt was studying variable stars in the LMC. Most of them varied in the same way, rather like a sine wave. However she noticed that a very few (16 out of ~1000) of the variable stars in the LMC had an interesting property. They got bright really fast but then took a longer time to dim, and (this is the cool part) she noticed that the brighter a star became at its brightest, the longer it took to dim back to it’s dimmest point. This relationship between the star's period and its peak luminosity turned out to be the key to unlock the size of the universe.

Let’s do a little thought experiment. You have a box of 100 flashlights of 5 different brightnesses. Each flashlight has a number on it from 1 to 5 corresponding to its brightness, and the brightest one is 5 times brighter than the dimmest one.

You give a bunch of kids each a random flashlight and you send them out into a field at night at different distances from you. They all turn on their flashlights and point them at you. You can see the apparent brightness of each flashlight by just looking, but you can’t tell if one that looks bright to you is just a dim one close up. What is the actual brightness (its brightness number from 1 to 5) of each flashlight? You cannot tell from just watching the light from the flashlights.

However, let’s imagine that the kids know which number flashlight they got. You tell them that once each minute they should quickly blink their light one time if they have a “1” (dimmest) flashlight, blink it twice if they have a “2”, up to 5 times if they have one of the brightest ones.

Voila! Now you can tell the difference between a dim flashlight close to you and a bright one far away - you just count the blinks when they come.

With the addition of the blinks, each kid is sending you information about the flashlight in two ways. One is how bright it looks to you, but you know you can’t trust that because apparent brightness varies with distance.

However they are also sending you information about the actual brightness of flashlight with the blinks, and (here’s the key point) the information carried by the blinks does not change with distance! So you can trust the information carried by the blinks to tell you how bright the flashlight really is, whereas you cannot trust the apparent brightness to tell you how bright the flashlight really is (i.e. what number is on it).

The variable stars that Henrietta Leavitt was studying were also sending her information in two ways. One way was their apparent brightness, which we already know she could not trust to tell her which one was actually bright and which was actually dim.

However they were also sending her information in the amount of time it took the variable star to dim from it’s brightest to its dimmest value. This information did not change with distance, so she could trust it to tell her which stars were actually brightest (the ones that took the longest time to dim) and which stars were actually the dimmest (the ones that took the shortest time to dim).

She assumed that all the stars were approximately the same distance away (since they were all in the same star cloud). This gave her a way to measure the distance to other variable stars in other parts of our galaxy (and in other galaxies) in units of the distance to the LMC. She did not know the actual distance to the LMC, but that could come later. For now she had unlocked the secret to measuring the size of the universe. You can read about this story in the book “Miss Leavitt’s Stars” by George Johnson.

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by guenthert » Mon Apr 27, 2020 4:38 am

This assumes that one can spatially resolve those flashlight, err, stars. Afaik, the Andromeda galaxy is thought to be approximately as large as the local one and contains about as many stars, i.e. about 10e8. It spans about the same area as the moon on the night sky, hence with current technology, never mind the one available to Hubble, it's impossible to resolve a single star. Or a dozen clustered together or a hundred. What's the resolution limit? 1000 stars, a million? So if there is a variable star changing its luminescence periodically by a generous 1000%, in a cluster of 1000 stars, the total brightness will change only 1%. Hubble was able to detect that on photo plates and able to distinguish it from dimming caused by turbulence in the air?

Thanks for all the answers, but there's something I'm still missing.

guenthert

Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by guenthert » Mon Apr 27, 2020 4:44 am

I meant 1e11 starts in the galaxy of course, not the erroneously stated 10e8. "billions with a b :oops:

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by guenthert » Mon Apr 27, 2020 4:52 am

Chris Peterson wrote: Sun Apr 26, 2020 1:26 pm [..] Every supernova we observe in other galaxies is readily detected, but not optically resolved.
With supernovae briefly outshining all other stars in that galaxy, it's not necessary to resolve them spatially. Cepheids come not even close, not by a large margin. I'd rather think their variability is lost in the noise at the distance of Andromeda or rather they blend in with the background.

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by Ann » Mon Apr 27, 2020 5:58 am

guenthert wrote: Mon Apr 27, 2020 4:38 am This assumes that one can spatially resolve those flashlight, err, stars. Afaik, the Andromeda galaxy is thought to be approximately as large as the local one and contains about as many stars, i.e. about 10e8. It spans about the same area as the moon on the night sky, hence with current technology, never mind the one available to Hubble, it's impossible to resolve a single star. Or a dozen clustered together or a hundred. What's the resolution limit? 1000 stars, a million? So if there is a variable star changing its luminescence periodically by a generous 1000%, in a cluster of 1000 stars, the total brightness will change only 1%. Hubble was able to detect that on photo plates and able to distinguish it from dimming caused by turbulence in the air?

Thanks for all the answers, but there's something I'm still missing.
You are very wrong indeed if you think it is impossible for the Hubble Space Telescope to resolve individual stars in the Andromeda Galaxy. Here you can see a tiny field in Andromeda that Hubble has resolved down to stars certainly no brighter than Sirius, and possibly down to stars even fainter than the Sun.

More importantly, though, we don't need Hubble in order to resolve bright stars in Andromeda. Take a look at the picture at right, taken by amateur Jason Guenzel, also known as u/TheVastReaches. On this page, he said this about his picture,
My extreme close-up portrait of the Andromeda Galaxy. Even amateur gear can reveal individual blue supergiant stars that are 2.5 million light years away.

Edwin Hubble resolved a likely Cepheid variable in the Andromeda Galaxy back in 1923. Cepheids are bright, much brighter than the Sun. Delta Cephei, the best-known Cepheid and the star that gave this class of stars its name, is probably about ~2,000 times brighter than the Sun, and Cepheids may possibly be as bright as 100,000 times solar. Such stars would certainly have been within reach for Edwin Hubble and the 100-inch (2.5 meter) Hooker Telescope at Mount Wilson Observatory that he used to observe the Andromeda Galaxy.






As for the Hubble Space Telescope, one of its first missions was to search for Cepheids in galaxy Messier 100 (M100) in the Virgo Cluster in order to determine the distance to M100. The distance to M100 based on the luminosity of its Cepheids was determined to be around 56 million light-years, some ~25 times farther away than the Andromeda Galaxy. You can read about it here.

In the long accompanying text, it is said that the Hubble constant, the rate of the expansion of the Universe, is 80, which would imply that the Universe is expanding at the rate of 80 km/sec per megaparsec. This expansion rate has now been revised down to ~70 km/sec per megaparsec. Note that the Hubble Telescope has not only been serviced but also upgraded since its pictures of Cepheids in M100 were taken in the 1990s.

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by Ann » Mon Apr 27, 2020 7:54 am

guenthert wrote: Mon Apr 27, 2020 4:38 am This assumes that one can spatially resolve those flashlight, err, stars. Afaik, the Andromeda galaxy is thought to be approximately as large as the local one and contains about as many stars, i.e. about 10e8. It spans about the same area as the moon on the night sky, hence with current technology, never mind the one available to Hubble, it's impossible to resolve a single star. Or a dozen clustered together or a hundred. What's the resolution limit? 1000 stars, a million? So if there is a variable star changing its luminescence periodically by a generous 1000%, in a cluster of 1000 stars, the total brightness will change only 1%. Hubble was able to detect that on photo plates and able to distinguish it from dimming caused by turbulence in the air?

























Here you can see the pictures taken when the Hubble Space Telescope visited Edwin Hubble's V1 star in Andromeda. Edwin Hubble's image is from 1923 and the Hubble Space Telescope's pictures are from 2010 and 2011.

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by Ann » Mon Apr 27, 2020 9:59 am



























As you can see from the HST pictures at left, the brightness variations of Edwin Hubble's variable star V1 in the Andromeda Galaxy don't seem to be that great. Would Hubble really have detected this variability back in 1923?

Yes, he would indeed have been able to detect it, and the reason for that is the color changes of the star V1 as it goes through its variability cycle, as well as the equipment that Edwin Hubble would have used to observe it.

As you can see, the star V1 is bluer when it is brighter and yellower when it is fainter. And Edwin Hubble would have taken his pictures of star V1 in Andromeda using a blue-sensitive emulsion on photographic plates.

Consider how the relative brightness of stars changes if you photograph them with blue-sensitive equipment. Take a look at the picture of the Orion Constellation at right, which was taken in the 1950s. Note how very faint orange Betelgeuse appears to be. Note, on the other hand, how brilliantly bright the blue stars of Orion's Belt appear to be.

So when Edwin Hubble observed variable star V1 in Andromeda with blue-sensitive equipment, the star would appear to grow even brighter at its maximum due to its bluer color than it appears to be to us, and it would appear to be even fainter at its minimum due to its redder color than it appears to be to us.

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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by Chris Peterson » Mon Apr 27, 2020 1:34 pm

guenthert wrote: Mon Apr 27, 2020 4:38 am This assumes that one can spatially resolve those flashlight, err, stars. Afaik, the Andromeda galaxy is thought to be approximately as large as the local one and contains about as many stars, i.e. about 10e8. It spans about the same area as the moon on the night sky, hence with current technology, never mind the one available to Hubble, it's impossible to resolve a single star. Or a dozen clustered together or a hundred. What's the resolution limit? 1000 stars, a million? So if there is a variable star changing its luminescence periodically by a generous 1000%, in a cluster of 1000 stars, the total brightness will change only 1%. Hubble was able to detect that on photo plates and able to distinguish it from dimming caused by turbulence in the air?

Thanks for all the answers, but there's something I'm still missing.
Even amateur imagers readily resolve individual stars in Andromeda. And the variability of Cepheids is easily detected, and much greater than seeing noise. A typical Cepheid might have a variability of a half magnitude or more; I routinely perform photometric measurements on the order of 10 millimag with a 300 mm aperture and poor seeing.
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Re: APOD: Edwin Hubble Discovers the Universe (2020 Apr 26)

Post by neufer » Mon Apr 27, 2020 3:09 pm

  • Mention should be made of the relatively bright nova shown in today's APOD:
https://en.wikipedia.org/wiki/Andromeda_Galaxy wrote:
<<In 1917, Heber Curtis observed a nova within Andromeda. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred elsewhere in the sky. As a result, he was able to come up with a distance estimate of 500,000 ly. He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were actually independent galaxies. Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of Andromeda. These were made using the 2.5-metre Hooker telescope, and they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our own galaxy, but an entirely separate galaxy located a significant distance from the Milky Way.

A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star, that slowly fades over several weeks or many months. All observed novae involve a white dwarf in a close binary system. The main sub-classes of novae are classical novae, recurrent novae (RNe), and dwarf novae. Novae have some promise for use as standard candle measurements of distances. For instance, the distribution of their absolute magnitude is bimodal, with a main peak at magnitude −8.8, and a lesser one at −7.5. Novae also have roughly the same absolute magnitude 15 days after their peak (−5.5). Comparisons of nova-based distance estimates to various nearby galaxies and galaxy clusters with those measured with Cepheid variable stars [absolute magnitude -2.5 to -6.5], have shown them to be of comparable accuracy.

Recurrent novae (RNe) are objects that have been seen to experience multiple nova eruptions. As of 2009, there are ten known galactic recurrent novae, as well as several extragalactic ones (in the Andromeda Galaxy (M31) and the Large Magellanic Cloud). One of these extragalactic novae, M31N 2008-12a, erupts as frequently as once every 12 months. The recurrent nova typically brightens by about 8.6 magnitudes, whereas a classic nova may brighten by more than 12 magnitudes.

Astronomers estimate that the Milky Way experiences roughly 30 to 60 novae per year, but a recent examination has found the likely improved rate of about 50±27. The number of novae discovered in the Milky Way each year is much lower, about 10, probably due to distant novae being obscured by gas and dust absorption. Roughly 25 novae brighter than about the twentieth magnitude are discovered in the Andromeda Galaxy each year and smaller numbers are seen in other nearby galaxies.

Classical nova eruptions are the most common type of nova. They are likely created in a close binary star system consisting of a white dwarf and either a main sequence, subgiant, or red giant star. When the orbital period falls in the range of several days to one day, the white dwarf is close enough to its companion star to start drawing accreted matter onto the surface of the white dwarf, which creates a dense but shallow atmosphere. This atmosphere, mostly consisting of hydrogen, is thermally heated by the hot white dwarf and eventually reaches a critical temperature causing ignition of rapid runaway fusion.

The sudden increase in energy expels the atmosphere into interstellar space creating the envelope seen as visible light during the nova event and previously mistaken as a "new" star. A few novae produce short-lived nova remnants, lasting for perhaps several centuries. Recurrent nova processes are the same as the classical nova, except that the fusion ignition may be repetitive because the companion star can again feed the dense atmosphere of the white dwarf.>>
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