The universe is a very large place, and there is no way we can find out 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 is 1.4 million km across. This is such a huge distance that it is difficult to scale it. The sun makes up 99.9% of all matter in our solar system. In fact, there are one million Earth planets inside the Sun.
Astronomers use the terms "solar radius" and "solar mass" to compare larger and smaller stars, we will 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, Eta Carinae, is located 7,500 light-years from the Sun and weighs 120 solar masses. It is a million times brighter than the Sun. Most stars lose their mass over time, like the solar wind. But Eta Carinae is so big 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.
Thus, 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: This Carina is about to explode, it will be one of the most spectacular supernovae that humans have ever seen.
But the most massive star in the Universe is R136a1, located in the Large Magellanic Cloud. There is controversy, 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 the stars were 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.
In terms of large stars, let's look at a familiar star in the constellation of Orion, Betelgeuse. This red supergiant has a radius of 950 to 1200 times the size of the Sun, and would span the orbit of Jupiter if it were located in our solar system.
But this is nothing. The largest known star is VY Canis Major. A red hypergiant in the constellation Canis Major, located about 5,000 light-years from Earth. Professor Robert Humphreys of the University of Minnesota recently calculated its top size to be greater than 1,540 times the size of the Sun. If VY of Canis Major were placed in our system, then its surface would extend beyond the orbit of Saturn.
It is the largest star we know of, but the Milky Way probably has dozens of stars that further obscure the clouds of gas and dust, so we cannot see them.
But let's see if we can answer the 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 large place, and there is no way for us to peer into every corner.
The pistol is another star that is considered one of the largest.
The largest stars will be cold supergiants, according to theorists. For example, the temperature VY of Canis Major is only 3500 K. A really large star would be even colder. A cold supergiant with a temperature of 3000 K, would be 2,600 solar in size.
Finally, here's a great video that shows the size of various objects in space, from our tiny planet to VV Cepheus. VY Big Dog is not included in the animation, probably because they did not have new information on this star.
A myriad 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 there is an incredible number of stars in the universe.
In fact, there are so many of them that even the knowledge of modern researchers, which was obtained with the help of the latest equipment (by the way, it allows you to look into the territory of space for 9 billion light years) is not enough.
There are now about 50 billion stars in the bowels 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, however, and not small. It has 1,391,000 kilometers in diameter. There are more weighty stars in the Universe, they are called hypergiants. For a long time, VY was considered the largest star, which is located in the constellation Canis Major. Not so long ago, the radius of the star was refined - 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 it 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 we reduce the Earth to 1 centimeter and also proportionally decrease 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 gases inside it is incredibly low. Well, the brightness of the star can only be admired. It shines 500 thousand times stronger than our heavenly body.
The first VY observations that were recorded are in the star catalog of Joseph Jerome de Lalande. The information is dated 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 has at least six discrete components, so it is likely a multiple star. But now it turned out that the discrete components are nothing more than bright areas of the nebula that surrounds the hypergiant. In 1957, visual observations and quality images from 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 the star's fuel dries up, it is likely to explode in a supernova and turn into a neutron star or black hole. The star has been observed to lose its brightness since 1850.
Lost Leadership
However, scientists do not leave the study of the Universe 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 in the late summer of 2010.
The researchers studied the Large Magellanic Cloud and found the star R136a1. NASA's Hubble Space Telescope helped make an incredible discovery.
The giant is 256 times more massive than our Sun. But the brightness of R136a1 exceeds the celestial body by ten million times. Such fantastic numbers 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 continuing to explore the clusters of stars in the Large Magellanic Cloud, experts have found several more stars that have exceeded this limit. Well, R136a1 turned out to be a real record holder. The most interesting thing is that throughout their entire 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 the calculations of experts, if such a star is imagined in our Galaxy, it would be brighter than the Sun as much as the Sun is brighter than the Moon.
Record stars
But the brightest in the visible sky are the stars Rigel and Deneb from the constellations Orion and Cygnus, respectively. Each shines brighter than the Sun 55 thousand times and 72.5 thousand times. These stars are removed from us at 1600 and 820 light years.
Another bright star from the constellation Orion is the star Betelgeuse. It has the third highest luminosity. It is 22 thousand times brighter than sunlight in terms of light intensity. By the way, most of the bright stars are collected in Orion, although their brightness periodically changes.
But the brightest among the stars closest to Earth is Sirius from the constellation Canis Major. It shines brighter than our Sun by only 23.5 times. And the distance to this star is 8.6 light years. In the same constellation there is another bright star - Adara. This star shines just like 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 the Ursa Minor and is 780 light-years distant from Earth.
If instead of the Sun there were other stars and planets
It is noteworthy that astronomers distinguish the zodiacal constellation Taurus from the total mass. It contains an unusual star, which is distinguished by a supergiant density and a rather small spherical magnitude. According to astrophysicists, it mainly consists of fast neutrons that scatter to the sides. It was once the brightest star in the universe.
Star R136a1 and the Sun
Blue stars have a high luminosity, scientists say. The brightest known is UW CMa. It is 860 thousand times brighter than our heavenly body. But this figure is falling rapidly, as the brightness of the stars changes over time. For example, according to the chronicle, which is dated July 4, 1054, in the constellation Taurus there was the brightest star, 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 fade and after a while it disappeared altogether. And in the place where it shone, a nebula formed that looked like a crab. This is how the name Crab Nebula appeared. She 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 most popular ways of presenting information today is the compilation of ratings - finding out the tallest person in the world, the longest 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, many much larger and more spectacular astronomical objects can 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 you know, stars are formed from giant clouds of interstellar dust and gas, which gradually thicken, increase their mass and, under the influence of their own gravity, shrink more and more. The temperature inside the cluster gradually rises and the diameter decreases.
The phase, indicating that the astronomical object has become a full-fledged star, lasts 7-8 billion years. Depending on the temperature, stars can be blue, yellow, red, etc. in this phase. The color is determined by the mass of the star and the physical and chemical processes taking place in it. 
But any star eventually begins to cool down and simultaneously expand in volume, turning into a “red giant”, tens or even hundreds of times larger in diameter than the original star. At this time, the star can pulsate, expanding, then 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", neutron star or "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.
The biggest star
To date, astronomers know a lot of "red giants", which can be called the largest stars in the observable part of the Universe. Since this type of stars is subject to pulsation, in 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 Major - 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 - a star from the nearest neighboring galaxy, the diameter of which, according to various estimates, reaches 1500-1700 solar; 
- RW Cephei - with a diameter of 1630 solar diameters;
- NML Cygnus - "red giant", in circumference exceeding 1650 diameters of the Sun;
- UV Shield - today it 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
One more champion star should be mentioned, which is designated by astronomers as R136a1 and is located in one of the galaxies in the Large Magellanic Cloud. Its diameter is not yet very impressive, but its mass is 256 times the mass of our Sun. This star violates one of the main astrophysical theories, which claims 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, in accordance with astronomical calculations, R136a1 has 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 we move R136a1 to our galaxy, it will eclipse the Sun with the same brightness with which the Sun is now eclipsing 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 (constellation Orion) and the red Deneb (constellation Cygnus) have. 
The third brightest is the red Betelgeuse, which, together with Rigel, makes up the famous Orion Belt.
Seemingly inconspicuous Shield UY
In terms of stars, modern astrophysics seems to be reliving its infancy. Observing stars gives more questions than answers. Therefore, when asking about which star is the largest in the Universe, you need to be immediately ready for answering questions. Are you asking about the largest star known to science, or what limits science limits a star to? As is usually the case, in both cases, you will not get a definite answer. The most likely candidate for the largest star shares the palm with his “neighbors” on an equal footing. As to how much smaller he can be than the real "king of the star" also remains open.
Comparison of the sizes of the Sun and the star UY Shield. The Sun is an almost invisible pixel to the left of the UY Shield.
The supergiant UY Shield, with some reservations, can be called the largest star observed today. Why "with a reservation" will be discussed below. Shield UY is 9,500 light-years distant and is seen as a faint variable asterisk visible through a small telescope. According to astronomers, its radius exceeds 1700 solar radii, and during the pulsation period this size can increase to as much as 2000.
It turns out, to place such a star in the place of the Sun, the current orbits of the terrestrial planet would be in the bowels of the supergiant, and the boundaries of its photosphere would at times rest against 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.
It will take as much as 7-8 hours to fly around such a star at the speed of light. Let's remember that the light emitted by the Sun reaches our planet in just 8 minutes. If you fly at the same speed with which it makes one revolution around the Earth in an hour and a half, then the flight around the UY Shield will last almost five years. Now let's imagine these scales, given that the ISS flies 20 times faster than a bullet and dozens of times faster than passenger airliners.
Mass and luminosity of the UY Shield
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 that surrounds 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 the layer of the atmosphere located at an altitude of about one hundred kilometers above sea level. This layer, also called the Karman line, is a conditional border between the earth's atmosphere and space. It turns out that the density of the UY Shield is only slightly below the cosmic vacuum!
Also UY Shield is not the brightest. With its own luminosity of 340,000 solar, it is ten 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 as many times more massive than the UY Shield, despite the fact that it is 50 times smaller than the giant.
Physical parameters of the UY Shield
In general, UY Shita is a pulsating variable red supergiant of spectral type M4Ia. That is, on the Hertzsprung-Russell spectrum-luminosity diagram, the UY Shield is located in the upper right corner.
At the moment, the star is approaching the final stages of its evolution. Like all supergiants, it began to actively burn helium and some other heavier elements. According to modern models, after a few millions of years, the UY of the Shield will successively transform into a yellow supergiant, then into a bright blue variable or a Wolf-Rayet star. The final stages of its evolution will be a supernova explosion, during which the star will throw off its shell, most likely leaving behind a neutron star.
Already, the UY Shield is showing its activity in the form of semi-regular variability with an approximate pulsation period of 740 days. Considering that a 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! The loss of its mass amounts to 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. So, being millions of years ago on the main sequence, the UY of the Shield could have a mass from 25 to 40 solar masses.
Giants among the stars
Returning to the disclaimer mentioned above, we note that the primacy of UY Shield as the largest known star 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 the UY Shield). Likewise, they have a rather blurry structure. They can have a rather extended atmosphere, opaque gas and dust envelopes, disks, or a large companion star (for example, VV Cephei, see below). It is impossible to say exactly where the border of such stars lies. In the end, the well-established concept of the boundary of stars as the radius of their photosphere is already extremely conditional.
Therefore, this number can include about a dozen stars, which include NML Cygnus, VV Cepheus A, VY Canis Major, WOH G64 and some others. All these stars are located in the vicinity of our galaxy (counting 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 hypergiants). Each of them in a matter of millions, if not thousands of years, will turn into a supernova. They are also similar in size, ranging from 1400-2000 solar.
Each of these stars has its own peculiarity. So for UY Shield, this feature is the previously mentioned variability. WOH G64 has a toroidal gas and dust shell. The double eclipsing variable star VV Cephei is extremely interesting. It is a close system of two stars, consisting of a red hypergiant VV Cephei A and a blue main sequence star VV Cephei B. The cents of these stars are located some 17-34 apart from each other. Considering that the VV radius of Cepheus B can reach 9 AU. (1900 solar radii), the stars are located at an "arm's length" from each other. Their tandem is so cramped that whole pieces of the hypergiant are flowing at high speeds to 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 you talk about the size of a star if its atmosphere flows into another star, or smoothly transforms into a gas and dust disk? This is despite the fact that the star itself consists of a 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, observing a giant star in another galaxy, one can be sure that a neutron star is pulsating in its place or a black hole surrounded by supernova remnants is curving space. Even if such a star is thousands of light years away, one cannot be completely sure that it still exists or remains the same giant.
We add to this the imperfection of modern methods for determining the distance to stars and a number of unspecified problems. It turns out that even among the ten largest known stars, it is impossible to single out a certain leader and arrange them in ascending order of size. In this case, Shield's UY was cited as the most likely candidate for leadership among the Big Ten. This does not mean at all that his leadership is undeniable and that, for example, NML Swan or VY Big Dog cannot be larger than her. Therefore, different sources can answer the question about the largest known star in different ways. This speaks rather not of their incompetence, but of the fact that science cannot give unambiguous answers even to such direct questions.
The largest in the universe
If science does not undertake to single out the largest among the open stars, how can we talk about which star is the largest in the Universe? According to scientists, the number of stars, even within the boundaries of the observable Universe, is ten times 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 small fraction of them. The fact that the largest stars can stand out for their luminosity will not help in the search for a "star leader". Whatever their brightness, it will fade when observing distant galaxies. Moreover, as noted earlier, the brightest stars are not the largest (for example, R136).
Also remember that 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, its search will be simply pointless.
Hypergiants
If the largest star is practically impossible to find, maybe it is 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. The current theoretical model of evolution and stellar physics does not explain much of what actually exists and is observed through telescopes. Hypergiants are an example of this.
Astronomers have repeatedly had to raise the stellar mass limit. This limit was first introduced in 1924 by the English astrophysicist Arthur Eddington. Having received the 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 sooner or later this will lead to a violation of the hydrostatic equilibrium. The light pressure of increasing brightness will literally blow off the outer layers of the star. The limit calculated by Eddington was 65 solar masses. Subsequently, astrophysicists refined his calculations, adding unaccounted for components and using powerful computers. So the current theoretical mass limit for stars is 150 solar masses. Now let's remember that the mass of R136a1 is 265 solar masses, which is almost twice the theoretical limit!
R136a1 is the most massive star known today. In addition to it, several more 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 much smaller than the stars, which, it would seem, should be lower in class - for example, the supergiant UY Shield. This is because he calls hypergiants not the largest, but the most massive stars. 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 the hypergiant has not been established, but, as a rule, their mass exceeds 100 solar. None of the biggest stars in the Big 10 fall within these limits.
Theoretical impasse
Modern science cannot explain the nature of the existence of stars, the mass of which exceeds 150 solar masses. This raises the question of how the theoretical limit of the size of stars can be determined if the radius of the star, as opposed to the mass, is itself a vague concept.
Let's take into account the fact that it is not known exactly what the stars of the first generation were, and what they will be in the course of the further evolution of the Universe. Changes in the composition and metallicity of stars can lead to radical changes in their structure. An astrophysicist only has to comprehend the surprises that further observations and theoretical research will present them. It is quite possible that UY Shield may turn out to be a real crumb against the background of a hypothetical "king-star" that shines somewhere or will shine in the most distant corners of our Universe.
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