The universe is a very big place, and there is no way in which we can know which star is the biggest. But what is the biggest star we know of?
Before we get to the answer, let's look at our own Sun for scale. Our mighty star measures 1.4 million km across. This is such a huge distance that it is difficult to put it to scale. The Sun makes up 99.9% of all matter in our Solar System. In fact, there are one million planet Earths inside the Sun.
Astronomers use the terms "solar radius" and "solar mass" to compare larger and smaller stars, so we'll do the same. The solar radius is 690,000 km, one solar mass is 2 x 10 30 kilograms. This amounts to 2,000,000,000,000,000,000,000,000,000,000 kg.
One huge known star in our galaxy is Eta Carinae, located 7,500 light years from the Sun, weighing 120 solar masses. It is a million times brighter than the Sun. Most stars lose their mass over time, much like the solar wind. But Eta Carinae is so large that every year it throws off a mass equal to 500 Earth masses. With so much mass lost, it is very difficult for astronomers to accurately measure where a star ends and its stellar wind begins.
So the best answer from astronomers right now is that the radius of Eta Carinae is 250 times the size of the Sun.
And one interesting note: Eta Carinae is set to explode soon, making it one of the most spectacular supernovae humans have ever seen.
But the most massive star in the Universe is considered to be R136a1, located in the Large Magellanic Cloud. There are disputes, but its mass may be more than 265 solar masses. And this is a mystery to astronomers, because theoretically the largest stars were thought to be about 150 solar masses, formed in the early Universe, when stars formed from hydrogen and helium left over from the Big Bang. The answer to this controversy is that R136a1 may have been formed when several large stars merged together. Needless to say, R136a1 could explode into a hypernova any day now.
In terms of big stars, let's look at a familiar star in the constellation Orion - Betelgeuse. This red supergiant has a radius 950 to 1200 times the size of the Sun, and would span the orbit of Jupiter if placed in our Solar System.
But this is nothing. Largest known star VY Canis Major. A red hypergiant star in the constellation Canis Major, located approximately 5,000 light-years from Earth. Professor Robert Humphreys of the University of Minnesota recently calculated its upper size to be greater than 1,540 times the size of the Sun. If VY Canis Majoris was placed in our system, its surface would extend beyond the orbit of Saturn.
It's the largest star we know of, but the Milky Way likely has dozens of stars that further obscure the clouds of gas and dust so we can't see them.
But let's see if we can answer original question, what is the largest star in the Universe? Obviously, it is virtually impossible for us to find it, the Universe is a very big place, and there is no way in which we could peer into every corner.
Pistol is another star, which is considered one of the largest.
The largest stars will be cool supergiants, theorists say. For example, the temperature of VY Canis Majoris is only 3500 K. A really large star would be even colder. A cool supergiant with a temperature of 3000 K would be 2,600 times the size of the sun.
And finally, here's a great video that shows the size of various objects in space, from our tiny planet to VV Cepheus. VY Canis Majoris is not included in the animation, probably because they didn't have new information by this star.
Myriads of stars dot the night sky. And to a person from Earth they seem exactly the same. Well, in some parts of the sky, for example, in the Milky Way region, stars merge into luminous streams.
This is because the universe is incredible great amount stars
In fact, there are so many of them that even the knowledge of modern researchers, which was obtained using the latest equipment (by the way, it allows you to look into the territory of space 9 billion light years away) is not enough.
There are currently approximately 50 billion stars in the depths of space. And every day the figure is only growing, because scientists do not get tired of exploring space and making new discoveries.
Brighter than the Sun
All stars in the Universe have different diameters. And even our Sun is not the largest star, nor is it small. It has a diameter of 1,391,000 kilometers. There are heavier stars in the Universe; they are called hypergiants. For quite a long time, VY, which is located in the constellation Canis Major, was considered the largest star. Not so long ago, the radius of the star was clarified - and approximately ranges from 1300 to 1540 solar radii. The diameter of this supergiant is about 2 billion kilometers. VY is located 5 thousand light years from the Solar System.
Scientists have calculated, to imagine how gigantic the size is, one revolution around a hypergiant star will take 1200 years, and then if you fly at a speed of 800 kilometers per hour. Or, if you reduce the Earth to 1 centimeter and also proportionally reduce VY, then the size of the latter will be 2.2 kilometers.
The mass of this star is not that impressive. VY is only 40 times heavier than the Sun. This happened because the density of the gases inside it is incredibly low. Well, one can only admire the brightness of the star. It shines 500 thousand times stronger than our heavenly body.
The first observations of VY that were recorded are in the star catalog of Joseph Jérôme de Lalande. The information dates back to March 7, 1801. Scientists have indicated that VY is a seventh magnitude star.
But in 1847, information appeared that VY has a crimson hue. In the nineteenth century, researchers discovered that the star, according to at least, six discrete components, so it is likely a multiple star. But it has now become clear that the discrete components are nothing more than bright areas of the nebula that surrounds the hypergiant. Visual observations in 1957 and high-quality images in 1998 showed that VY lacks a companion star.
However, by our time, the largest star in the universe has already lost more than half of its mass. That is, the star is aging and its hydrogen fuel is already running out. The outer part of VY has become larger due to the fact that gravity can no longer prevent weight loss. Scientists say that when a star runs out of fuel, it will likely explode into a supernova and become a neutron star or black hole. According to observations, the star has been losing its brightness since 1850.
Lost Leadership
However, scientists do not stop studying the Universe even for a minute. Therefore, this record was broken. Astronomers have found an even larger star in the vastness of space. The discovery was made by a group of British scientists led by Paul Crowther at the end of the summer of 2010.
Researchers studied the Large Magellanic Cloud and found the star R136a1. NASA's Hubble Space Telescope helped make this incredible discovery.
The giant is 256 times more massive than our Sun. But R136a1 is ten million times brighter than the celestial body. Such fantastic figures became a revelation for scientists, because it was believed that stars that exceed the mass of the Sun by more than 150 times did not exist.
And while continuing to explore clusters of stars in the Large Magellanic Cloud, experts have found several more stars that have exceeded this threshold. Well, R136a1 turned out to be a real record holder. The most interesting thing is that throughout their existence, stars lose their mass. At least, such statements are made by scientists. And R136a1 has now lost one fifth of its original mass. According to calculations, it was equal to 320 solar masses.
By the way, according to experts’ calculations, if such a star were imagined in our Galaxy, it would be brighter than the Sun as much as the Sun is brighter than the Moon.
Record-breaking stars
But the brightest stars in the visible sky are Rigel and Deneb from the constellations Orion and Cygnus, respectively. Each shines 55 thousand times and 72.5 thousand times brighter than the Sun. These luminaries are 1600 and 820 light years away from us.
Another bright star from the Orion constellation is the star Betelgeuse. It is the third most luminous. She's brighter sunlight the intensity of light emission is 22 thousand times. By the way, the most bright stars are collected in Orion, although their brightness changes periodically.
But the brightest among the stars closest to Earth is Sirius from the constellation Canis Major. It shines only 23.5 times brighter than our Sun. And the distance to this star is 8.6 light years. In the same constellation there is another bright star - Adara. This star is as luminous as 8,700 Suns combined at a distance of 650 light years. Well, the North Star, which many incorrectly consider the brightest visible star, shines 6 thousand times brighter than the Sun. Polaris is located at the tip of Ursa Minor and is 780 light years away from Earth.
If instead of the Sun there were other stars and planets
It is noteworthy that astronomers distinguish from the general mass and zodiac constellation Taurus. It contains an unusual star, which is distinguished by its supergiant density and rather small spherical size. According to astrophysicists, it mainly consists of fast neutrons that fly apart. It was once the brightest star in the Universe.
Star R136a1 and the Sun
Scientists say blue stars have great luminosity. The brightest known is UW SMa. It is 860 thousand times brighter than our heavenly body. But this figure drops rapidly as the brightness of stars changes over time. For example, according to the chronicle, which is dated July 4, 1054, the brightest star was in the constellation Taurus; it could be seen in the sky with the naked eye even in the middle of the day. But over time, the star began to dim and after a while disappeared altogether. And in the place where it shone, a nebula formed that looked like a crab. This is how the name Crab Nebula came about. It appeared after a supernova explosion. By the way, modern scientists in the center of this nebula have found a powerful source of radio emission, in other words, a pulsar. This is the remnant of that bright supernova that was described in the ancient chronicle.
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One of the popular ways of presenting information today is compiling ratings - finding out the tallest person in the world, the most long river, the oldest tree, etc. There are such ratings in the world of astronomy - the science of stars.
From school lessons we know well that our Sun, which gives our planet warmth and light, is very small on the scale of the Universe. Stars of this type are called yellow dwarfs, and among the countless millions of stars there are many much larger and more spectacular astronomical objects to be found.
"Stellar" life cycle
Before looking for the biggest star, let's remember how stars live and what stages they go through in their development cycle.
As is known, stars are formed from giant clouds of interstellar dust and gas, which gradually become denser, increase in mass and, under the influence of their own gravity, compress more and more. The temperature inside the cluster gradually increases, and the diameter decreases.
The phase indicating that an astronomical object has become a full-fledged star lasts 7-8 billion years. Depending on the temperature, stars in this phase can be blue, yellow, red, etc. The color is determined by the mass of the star and the physical and chemical processes occurring in it. 
But any star eventually begins to cool down and at the same time expand in volume, turning into a “red giant”, with a diameter tens or even hundreds of times greater than the original star. At this time, the star can pulsate, either expanding or contracting in diameter.
This period lasts several hundred million years and ends with an explosion, after which the remnants of the star collapse, forming a dim " white dwarf", a neutron star or a "black hole".
So, if we are looking for the largest star in the Universe, then it will most likely be a “red giant” - a star in the aging phase.
Biggest star
Today, astronomers know quite a lot of “red giants”, which can be called the most big stars in the observable part of the Universe. Since this type of star is subject to pulsation, then different years The leaders in size were considered:
- KY Cygnus - the mass exceeds the mass of the Sun by 25 times, and the diameter is 1450 solar;
- VV Cepheus - with a diameter of about 1200 solar;
- VY Canis Majoris - considered the largest in our Galaxy, its diameter is about 1540 solar diameters;
— VX Sagittarius – the diameter at the maximum pulsation phase reaches 1520 solar;
— WOH G64 is a star from the closest neighboring galaxy to us, the diameter of which reaches, according to different estimates, 1500-1700 solar; 
— RW Cepheus – with a diameter of 1630 times the diameter of the Sun;
— NML Cygnus is a “red giant” with a circumference exceeding 1650 solar diameters;
- UV Scutum - today is considered the largest in the observable part of the Universe, with a diameter of about 1700 diameters of our Sun.
The heaviest star in the Universe
It is worth mentioning another champion star, which is designated by astronomers as R136a1 and is located in one of the galaxies of the Large Magellanic Cloud. Its diameter is not very impressive yet, but its mass is 256 times the mass of our Sun. This star violates one of the main astrophysical theories, which states that the existence of stars with a mass of more than 150 solar masses is impossible due to the instability of internal processes.
By the way, according to astronomical calculations, R136a1 lost a fifth of its mass - initially this figure was within 310 solar masses. It is believed that the giant was formed as a result of the merger of several ordinary stars, so it is not stable and can explode at any moment, turning into a Supernova.
Even today it is ten million times brighter than the Sun. If you move R136a1 into our galaxy, it will eclipse the Sun with the same brightness with which the Sun now eclipses the Moon.
The brightest stars in the sky
Of those stars that we can see with the naked eye in the sky, the blue giant Rigel (Orion constellation) and the red Deneb (Swan constellation) have. 
The third brightest is the red Betelgeuse, which together with Rigel makes up the famous Belt of Orion.
The seemingly inconspicuous UY Shield
In terms of stars, modern astrophysics seems to be reliving its infancy. Star observations provide more questions than answers. Therefore, when asking which star is the largest in the Universe, you need to be immediately prepared for answering questions. Are you asking about the largest star known to science, or about what limits science limits a star? As is usually the case, in both cases you will not get a clear answer. The most likely candidate for the biggest star quite equally shares the palm with its “neighbors.” How much smaller it may be than the real “king of the star” also remains open.
Comparison of the sizes of the Sun and the star UY Scuti. The Sun is an almost invisible pixel to the left of UY Scutum.
With some reservations, the supergiant UY Scuti can be called the largest star observed today. Why “with reservation” will be stated below. UY Scuti is 9,500 light-years away from us and is observed as a faint variable star, visible in a small telescope. According to astronomers, its radius exceeds 1,700 solar radii, and during the pulsation period this size can increase to as much as 2,000.
It turns out that if such a star were placed in the place of the Sun, the current orbits of a terrestrial planet would be in the depths of a supergiant, and the boundaries of its photosphere would at times abut the orbit. If we imagine our Earth as a grain of buckwheat, and the Sun as a watermelon, then the diameter of the UY Shield will be comparable to the height of the Ostankino TV tower.
To fly around such a star at the speed of light it will take as much as 7-8 hours. Let us remember that the light emitted by the Sun reaches our planet in just 8 minutes. If you fly at the same speed as one revolution around the Earth takes one and a half hours, then the flight around UY Scuti will last almost five years. Now let’s imagine these scales, taking into account that the ISS flies 20 times faster than a bullet and tens of times faster than passenger airliners.
Mass and luminosity of UY Scuti
It is worth noting that such a monstrous size of the UY Shield is completely incomparable with its other parameters. This star is “only” 7-10 times more massive than the Sun. It turns out that the average density of this supergiant is almost a million times lower than the density of the air around us! For comparison, the density of the Sun is one and a half times higher than the density of water, and a grain of matter even “weighs” millions of tons. Roughly speaking, the averaged matter of such a star is similar in density to a layer of atmosphere located at an altitude of about one hundred kilometers above sea level. This layer, also called the Karman line, is the conventional boundary between earth's atmosphere and space. It turns out that the density of the UY Shield is only slightly short of the vacuum of space!
Also UY Scutum is not the brightest. With its own luminosity of 340,000 solar, it is tens of times dimmer than the brightest stars. A good example is the star R136, which, being the most massive star known today (265 solar masses), is almost nine million times brighter than the Sun. Moreover, the star is only 36 times larger than the Sun. It turns out that R136 is 25 times brighter and about the same number of times more massive than UY Scuti, despite the fact that it is 50 times smaller than the giant.
Physical parameters of UY Shield
Overall, UY Scuti is a pulsating variable red supergiant of spectral class M4Ia. That is, on the Hertzsprung-Russell spectrum-luminosity diagram, UY Scuti is located in the upper right corner.
On this moment the star is approaching the final stages of its evolution. Like all supergiants, it began actively burning helium and some other heavier elements. According to modern models, in a matter of millions of years, UY Scuti will successively transform into a yellow supergiant, then into a bright blue variable or Wolf-Rayet star. The final stages of its evolution will be a supernova explosion, during which the star will shed its shell, most likely leaving behind a neutron star.
Already now, UY Scuti is showing its activity in the form of semi-regular variability with an approximate pulsation period of 740 days. Considering that the star can change its radius from 1700 to 2000 solar radii, the speed of its expansion and contraction is comparable to the speed of spaceships! Its mass loss is at an impressive rate of 58 million solar masses per year (or 19 Earth masses per year). This is almost one and a half Earth masses per month. Thus, being on the main sequence millions of years ago, UY Scuti could have had a mass of 25 to 40 solar masses.
Giants among the stars
Returning to the disclaimer stated above, we note that the primacy of the UY Shield as the largest of famous stars cannot be called unambiguous. The fact is that astronomers still cannot determine the distance to most stars with a sufficient degree of accuracy, and therefore estimate their sizes. In addition, large stars are usually very unstable (remember the pulsation of UY Scuti). Likewise, they have a rather blurry structure. They may have a fairly extensive atmosphere, opaque shells of gas and dust, disks, or a large companion star (for example, VV Cephei, see below). It is impossible to say exactly where the boundary of such stars lies. After all, the established concept of the boundary of stars as the radius of their photosphere is already extremely arbitrary.
Therefore, this number can include about a dozen stars, which include NML Cygnus, VV Cephei A, VY Canis Majoris, WOH G64 and some others. All these stars are located in the vicinity of our galaxy (including its satellites) and are in many ways similar to each other. All of them are red supergiants or hypergiants (see below for the difference between super and hyper). Each of them will turn into a supernova in a few millions, or even thousands of years. They are also similar in size, lying in the range of 1400-2000 solar.
Each of these stars has its own peculiarity. So in UY Scutum this feature is the previously mentioned variability. WOH G64 has a toroidal gas-dust envelope. Extremely interesting is the double eclipsing variable star VV Cephei. It is a close system of two stars, consisting of the red hypergiant VV Cephei A and the blue main sequence star VV Cephei B. The centra of these stars are located from each other at some 17-34 . Considering that the radius of VV Cepheus B can reach 9 AU. (1900 solar radii), the stars are located at “arm’s length” from each other. Their tandem is so close that whole pieces of the hypergiant flow at enormous speeds onto the “little neighbor”, which is almost 200 times smaller than it.
Looking for a leader
Under such conditions, estimating the size of stars is already problematic. How can we talk about the size of a star if its atmosphere flows into another star, or smoothly turns into a disk of gas and dust? This is despite the fact that the star itself consists of very rarefied gas.
Moreover, all the largest stars are extremely unstable and short-lived. Such stars can live for a few millions, or even hundreds of thousands of years. Therefore, when observing a giant star in another galaxy, you can be sure that a neutron star is now pulsating in its place or a black hole is bending space, surrounded by the remnants of a supernova explosion. Even if such a star is thousands of light years away from us, one cannot be completely sure that it still exists or remains the same giant.
Let's add to this imperfection modern methods determining the distance to the stars and a number of unspecified problems. It turns out that even among a dozen known largest stars, it is impossible to identify a specific leader and arrange them in order of increasing size. IN in this case UY Shield has been cited as the most likely candidate to lead the Big Ten. This does not mean at all that his leadership is undeniable and that, for example, NML Cygnus or VY Canis Majoris cannot be greater than her. Therefore, different sources may answer the question about the largest known star in different ways. This speaks less of their incompetence than of the fact that science cannot give unambiguous answers even to such direct questions.
Largest in the Universe
If science does not undertake to single out the largest among the discovered stars, how can we talk about which star is the largest in the Universe? Scientists estimate that the number of stars, even within the observable Universe, is ten times greater than the number of grains of sand on all the beaches of the world. Of course, even the most powerful modern telescopes can see an unimaginably smaller portion of them. In search " star leader“It also doesn’t help that the largest stars can stand out with their luminosity. Whatever their brightness, it will fade when observing distant galaxies. Moreover, as noted earlier, the most bright stars are not the largest (example - R136).
Let us also remember that when observing a large star in a distant galaxy, we will actually see its “ghost”. Therefore, it is not easy to find the largest star in the Universe; searching for it will simply be pointless.
Hypergiants
If the biggest star It’s impossible to find practically, maybe it’s worth developing it theoretically? That is, to find a certain limit after which the existence of a star can no longer be a star. However, even here modern science faces a problem. Modern theoretical model The evolution and physics of stars do not explain much of what actually exists and is observed in telescopes. An example of this is hypergiants.
Astronomers have repeatedly had to raise the bar for the limit of stellar mass. This limit was first introduced in 1924 by the English astrophysicist Arthur Eddington. Having obtained a cubic dependence of the luminosity of stars on their mass. Eddington realized that a star cannot accumulate mass indefinitely. The brightness increases faster than the mass, and this will sooner or later lead to a violation of hydrostatic equilibrium. The light pressure of increasing brightness will literally blow away the outer layers of the star. The limit calculated by Eddington was 65 solar masses. Subsequently, astrophysicists refined his calculations by adding unaccounted components and using powerful computers. So the current theoretical limit for the mass of stars is 150 solar masses. Now remember that R136a1 has a mass of 265 solar masses, almost twice the theoretical limit!
R136a1 is the most massive star currently known. In addition to it, several other stars have significant masses, the number of which in our galaxy can be counted on one hand. Such stars were called hypergiants. Note that R136a1 is significantly smaller than stars that, it would seem, should be lower in class - for example, the supergiant UY Scuti. This is because it is not the largest stars that are called hypergiants, but the most massive ones. For such stars, a separate class was created on the spectrum-luminosity diagram (O), located above the class of supergiants (Ia). The exact initial mass of a hypergiant has not been established, but, as a rule, their mass exceeds 100 solar masses. None of the Big Ten's biggest stars measure up to those limits.
Theoretical dead end
Modern science cannot explain the nature of the existence of stars whose mass exceeds 150 solar masses. This raises the question of how one can determine the theoretical limit on the size of stars if the radius of a star, unlike mass, is itself a vague concept.
Let us take into account the fact that it is not known exactly what the stars of the first generation were like, and what they will be like during the further evolution of the Universe. Changes in the composition and metallicity of stars can lead to radical changes in their structure. Astrophysicists have yet to comprehend the surprises that further observations and theoretical research will present to them. It is quite possible that UY Scuti may turn out to be a real crumb against the background of a hypothetical “king star” that shines somewhere or will shine in the farthest corners of our Universe.
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