What is the largest blue star in the universe. What is the largest star in the universe? Huge deposits of water
Today you will learn about the most unusual stars. It is estimated that there are about 100 billion galaxies in the universe and about 100 billion stars in each galaxy. Given so many stars, there must be strange ones among them. Many of the sparkling, burning balls of gas are quite similar to each other, but some stand out for their odd size, weight, and behavior. Using modern telescopes, scientists continue to study these stars to better understand them and the universe, but mysteries still remain. Curious about the strangest stars? Here are 25 of the most unusual stars in the universe.
25. UY Scuti
Considered a supergiant star, UY Scuti is large enough to swallow up our star, half of our neighboring planets, and virtually our entire solar system. Its radius is about 1700 times the radius of the Sun.
24. Star of Methuselah
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The star of Methuselah, also called HD 140283, really lives up to its name. Some believe it is 16 billion years old, which is a problem since the Big Bang only happened 13.8 billion years ago. Astronomers have tried to use better methods of age determination to better date the star, but still believe it to be at least 14 billion years old.
23. Thorn-Zhitkov object
Photo: Wikipedia Commons.com
Initially, the existence of this object was proposed theoretically by Kip Thorne (Kip Thorne) and Anna Zhitkova (Anna Zytkow), it represents two stars, a neutron and a red supergiant, combined into one star. A potential candidate for the role of this object has been named HV 2112.
22. R136a1 
Photo: flickr
Although UY Scuti is the largest star known to man, R136a1 is definitely one of the heaviest in the universe. Its mass is 265 times greater than the mass of our Sun. What makes her weird is that we don't know exactly how she was formed. The main theory is that it was formed by the merger of several stars.
21.PSR B1257+12
Photo: en.wikipedia.org
Most of the exoplanets in the solar system PSR B1257+12 are dead and bathed in deadly radiation from their old star. A surprising fact about their star is that the zombie star or pulsar has died, but the core still remains. The radiation emanating from it makes this solar system a no man's land.
20. SAO 206462
Photo: flickr
Consisting of two spiral arms spanning 14 million miles across, SAO 206462 is certainly the strangest and most unique star in the universe. While some galaxies are known to have arms, stars usually don't. Scientists believe that this star is in the process of creating planets.
19. 2MASS J0523-1403
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2MASS J0523-1403 is arguably the smallest known star in the universe and is only 40 light years away. Due to its small size and mass, scientists believe that its age may be 12 trillion years.
18. Heavy metal subdwarfs
Photo: ommons.wikimedia.org
Astronomers recently discovered a pair of stars with a lot of lead in their atmospheres, which creates thick and heavy clouds around the star. They are called HE 2359-2844 and HE 1256-2738 and are located 800 and 1000 light years away respectively, but you can just call them heavy metal subdwarfs. Scientists are still not sure how they form.
17. RX J1856.5-3754
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From the moment of their birth, neutron stars begin to ceaselessly lose energy and cool down. Thus, it is unusual that a 100,000-year-old neutron star such as RX J1856.5-3754 could be so hot and not show any signs of activity. Scientists believe that interstellar material is held together by the star's strong gravitational field, resulting in enough energy to heat the star.
16. KIC 8462852
Photo: Wikipedia Commons.com
The star system KIC 8462852 has received a lot of attention and interest from SETI and astronomers for its unusual behavior of late. Sometimes it dims by 20 percent, which may mean that something is orbiting around it. Of course, this prompted some to conclude that these were aliens, but another explanation is the wreckage of a comet that entered the same orbit with the star.
15. Vega
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Vega is the fifth brightest star in the night sky, but that doesn't make it weird at all. The high rotation speed of 960,600 km per hour gives it the shape of an egg, and not spherical, like our Sun. There are also temperature variations, with colder temperatures at the equator.
14.SGR 0418+5729
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A magnet located 6,500 light-years from Earth, SGR 0418+5729 has the strongest magnetic field in the universe. The strange thing about it is that it does not fit the image of traditional magnetars with a surface magnetic field, as in ordinary neutron stars.
13. Kepler-47
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In the constellation Cygnus, 4,900 light-years from Earth, astronomers have first discovered a pair of planets orbiting two stars. Known as the Kelper-47 system, the orbiting stars outshine each other every 7.5 days. One star is roughly the size of our Sun, but only 84 percent as bright. The discovery proves that more than one planet can exist in a stressful orbit of a binary star system.
12. La Superba
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La Superba is another massive star located 800 light years away. It is about 3 times heavier than our Sun and four astronomical units in size. It is so bright that it can be seen from Earth with the naked eye.
11. MY Camelopardalis
Photo: commons.wikimedia.org
MY Camelopardalis was thought to be a single bright star, but the two stars were later found to be so close that they practically touch each other. Two stars slowly fuse together to form one star. No one knows when they will fully merge.
10.PSR J1719-1438b
Photo: Wikipedia Commons.com
Technically, PSR J1719-1438b is not a star, but it was once. When it was still a star, its outer layers were sucked out by another star, turning it into a small planet. What's even more amazing about this former star is that it's now a giant diamond planet five times the size of Earth.
9. OGLE TR-122b
Photo: Photo: commons.wikimedia.org
Usually, against the background of an average star, the rest of the planets resemble pebbles, but OGLE TR-122b is about the same size as Jupiter. That's right, it's the smallest star in the universe. Scientists believe it originated as a stellar dwarf billions of years ago, the first time a star comparable in size to a planet has been discovered.
8. L1448 IRS3B
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Astronomers discovered the three-star system L1448 IRS3B as it began to form. Using the ALMA telescope in Chile, they observed two young stars orbiting a much older star. They believe that these two young stars appeared as a result of a nuclear reaction with gas rotating around the star.
Photo: Wikipedia Commons.com
Mira, also known as Omicron Ceti, is 420 light-years away and is quite strange due to its constantly fluctuating brightness. Scientists consider it a dying star, located in the last years of its life. Even more amazing is that it travels through space at 130 kilometers per second and has a tail that spans several light years.
6. Fomalhaut-C
Photo: Wikipedia Commons.com
If you think the two-star system was cool, then you might want to see Fomalhaut-C. It is a system with three stars only 25 light years from Earth. Although triple star systems are not entirely unique, this one is because the arrangement of the stars far away rather than close together is an anomaly. The star Fomalhaut-C is especially far away from A and B.
5. Swift J1644+57
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The appetite of a black hole is not picky. In the case of Swift J1644+57, a dormant black hole woke up and engulfed the star. Scientists made this discovery in 2011 using X-ray and radio waves. It took 3.9 billion light years for light to reach Earth.
4.PSR J1841-0500
Photo: Wikipedia Commons.com
Known for their regular and constantly pulsating glow, they are rapidly rotating stars that rarely "turn off". But PSR J1841-0500 surprised scientists by only doing it for 580 days. Scientists believe that studying this star will help them understand how pulsars work.
3.PSR J1748-2446
Photo: Wikipedia Commons.com
The strangest thing about PSR J1748-2446 is that it is the fastest rotating object in the universe. It has a density 50 trillion times that of lead. To top it off, its magnetic field is a trillion times stronger than that of our Sun. In short, this is an insanely hyperactive star.
2. SDSS J090745.0+024507
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SDSS J090745.0+024507 is a ridiculously long name for a runaway star. With the help of a supermassive black hole, the star has been blown out of its orbit and is moving fast enough to exit the Milky Way. Let's hope that none of these stars will rush in our direction.
1. Magnetar SGR 1806-20
Photo: Wikipedia Commons.com
Magnetar SGR 1806-20 is a terrifying force that exists in our universe. Astronomers detected a bright flash at a distance of 50,000 light-years, and it was so powerful that it reflected off the Moon and illuminated the Earth's atmosphere for ten seconds. The solar flare raised questions among scientists about whether such a flare could lead to the extinction of all life on Earth.
Astronomers never cease to delight us with new discoveries, finding more and more stars in the Universe. Some of them can be seen at night with the naked eye, just by looking at the night sky. In order to see others, the most powerful telescopes are required. What is the largest star in the universe? Where is it located and how is it different from its neighbors? We invite you to familiarize yourself with the rating of the largest stars that have already been discovered by astronomers in the universe.
AH Scorpio
This is a real red giant, which is located in the region of the constellation Scorpio at a distance of 12 thousand light years relative to our planet. Its radius exceeds the radius of the Sun by 1.5 thousand times.
KY Swan
This star, which is located in the constellation Cygnus, will have to fly from the Earth as much as 5 thousand light years. Comparing the radius of the planet with the Sun, we can say that its radius is equal to 1420 solar radii. But the mass of the planet is not so large - it is only 25 times heavier than our star. It could illuminate much more than the Sun, since the brightness of KY Cygnus exceeds the solar one by many millions of times, so it can quite win in the “Brightest” nomination.
VV Cephei A
This double is located in the constellation of the same name, the distance to which is about 5000 light years. It is recognized as one of the largest in its galaxy, second only to VY Canis Major. Estimating the radius along the equator of this star, we can say that it is equal to 1900 equatorial radii of our star.
VY Big Dog
If we consider the Milky Way, then it was this star that became its champion, with a radius exceeding the size of the Sun by more than 1540 times. According to astronomers, this star is very unstable and there is an assumption that over the next 100,000 years it will certainly explode, resulting in a gamma-ray burst that can destroy all life that is within 1-2 light years. As for the planet Earth, only a huge distance from our planet to VY Canis Major, which is about 4000 light years, can save it. Therefore, earthlings can be calm.
VX Sagittarius
Scientists note the pulsation of this variable star, as studies have proven a periodic change in its temperature and volume. And its pulsation can be compared with the beating of a human heart. The equatorial radius of VX Sagittarius is equal to 1520 solar. The star is located in the constellation of the same name, from which it got its name.
Westerland 1-26
The numerical value of the radius of this giant exceeds the solar one by 1540 times. From Earth to Westerland 1-26 is about 11,500 light years.
WOH G64
The star WOH G64 is called the red star. It can be found by exploring the constellation, which has the name Golden Fish, which is located in a galaxy called the Large Magellanic Cloud. Our solar system is about 163,000 light-years away. Its radius is 1730 times greater than that of the Sun. According to research, the star will cease to exist, becoming a supernova. However, this will happen no earlier than in 10-20 thousand years. Although during this time a lot of things can change.
RW Cephei
This giant star has a red color and is located at a distance of more than 2700 light years from Earth. Its radius along the equator is 1636 times greater than the radius of the Sun.
NML Cygnus
The star acquired its name based on the name of the constellation, where it was discovered by astronomers. Its radius exceeds the solar one by 1650 times. A distance of 5300 light years separates us from NML Cygnus. Exploring the structure of the planet, scientists found sulfur oxide, hydrogen sulfide and other substances in it.
UY Shield
Scientists agreed that UY Shield is the largest in the entire universe. The record holder is located in the constellation with the same name at a distance of approximately 9.5 thousand light years from us. The star is very bright, but this is prevented by a huge amount of dust and gas around the planet.
One of the popular ways of presenting information today is to compile 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 the stars.
From school lessons, we know well that our Sun, which gives our planet heat 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.
"Star" life cycle
Before looking for the largest 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 become denser, increase in mass and, under the influence of their own gravity, are compressed more and more. The temperature inside the cluster gradually increases, while the diameter decreases.
The phase, which indicates that an 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 luminary eventually begins to cool down and at the same time expand in volume, turning into a "red giant", the diameter of which exceeds the original star by tens or even hundreds of times. 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 are compressed, 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 quite a lot of "red giants", which can be called the largest stars in the observable part of the universe. Since this type of star is subject to pulsation, in different years the leaders in magnitude were considered:
- KY Cygnus - the mass exceeds the mass of the Sun by 25 times, and the diameter is 1450 solar;
- VV Cephei - with a diameter of about 1200 solar;
- VY Canis Major - is considered the largest in our Galaxy, its diameter is about 1540 solar diameters;
- VX Sagittarius - the diameter in the maximum phase of the pulsation reaches 1520 solar;
- WOH G64 - a star from the nearest neighboring galaxy to us, the diameter of which reaches, according to various estimates, 1500-1700 solar; 
- RW Cephei - with a diameter of 1630 solar diameters;
- NML Cygnus - "red giant", in a circle exceeding 1650 diameters of the Sun;
- Shield UV - 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
Another champion star should be mentioned, which is designated by astronomers as R136a1 and is located in one of the galaxies of the Large Magellanic Cloud. Its diameter is not too impressive yet, but the 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, in accordance with astronomical calculations, R136a1 often 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 exceeds the brightness of the Sun ten million times. 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.
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, the stars merge into luminous streams.
This is because there is an incredibly huge number of stars in the universe.
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 space at 9 billion light years) is not enough.
Now there are about 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, however, not a small one either. She has 1,391,000 kilometers in diameter. There are more significant stars in the Universe, they are called hypergiants. For 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 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 the 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 reduce VY, then the size of the latter will be 2.2 kilometers.
The mass of this star is not so 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 observations of VY that were recorded are in the star catalog of Joseph Jérôme de Lalande. The information is dated March 7, 1801. Scientists pointed out that VY is a star of the seventh magnitude.
But in 1847, information appeared that VY had 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 patches of the nebula that surrounds the hypergiant. In 1957, visual observations and high-quality images from 1998 showed that VY was missing a companion star.
However, by our time, the largest star in the universe has already managed to lose 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 most likely explode in a supernova and turn into a neutron star or a black hole. According to observations, the star has been losing 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 bigger 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.
The researchers studied the Large Magellanic Cloud and found the star R136a1. NASA's Hubble Space Telescope helped make an incredible discovery.
The giant in its mass is 256 times larger than our Sun. But in terms of brightness, R136a1 exceeds the celestial body ten million times. Such fantastic figures were a revelation for scientists, because it was believed that stars that exceed the mass of the Sun by more than 150 times do 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 milestone. 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, if such a star is presented 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 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 luminaries are removed from us by 1600 and 820 light years.
Another bright star from the constellation Orion is the star Betelgeuse. It is the third largest luminosity. It is brighter than sunlight by the strength of light emission by 22 thousand times. By the way, most of the brightest 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 brighter than our Sun 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 like 8700 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. The North Star 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 single out the zodiac 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 fly apart. It was once the brightest star in the universe.
Star R136a1 and the Sun
Big luminosity, scientists say, have blue stars. The brightest known is UW CMa. It is 860 thousand times brighter than our heavenly body. But this figure is rapidly falling, as the brightness of the 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 fade and after a while it disappeared altogether. And in the place where she shone, a nebula formed, which looked like a crab. Hence the name Crab Nebula. It appeared after a supernova explosion. By the way, modern scientists have found a powerful source of radio emission in the center of this nebula, in other words, a pulsar. This is the remnant of that bright supernova, which was described in the old chronicle.
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Seemingly inconspicuous UY Shield
Modern astrophysics in terms of stars seems to be re-experiencing its infancy. Observations of the stars give more questions than answers. Therefore, when asking which star is the largest in the Universe, you need to be immediately ready for answers. Are you asking about the largest star known to science, or about what limits science limits a star to? As is usually the case, in both cases you will not get a definitive answer. The most likely candidate for the largest star quite equally shares the palm with his "neighbors". As for how much it can be less 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 Shield.
The supergiant UY Scutum, with some reservation, can be called the largest star observed today. Why "with reservation" will be said below. UY Scuti is 9500 light-years away and is seen as a dim variable star visible through a small telescope. According to astronomers, its radius exceeds 1700 radii of the Sun, and during the pulsation period this size can increase to as much as 2000.
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 bowels of a supergiant, and the boundaries of its photosphere would sometimes 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 television tower.
To fly around such a star at the speed of light will take as much as 7-8 hours. Recall 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 about 36 years. Now imagine these scales, given that the ISS flies 20 times faster than a bullet and tens of times faster than passenger airliners.
Mass and Luminosity of 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 surrounding us! For comparison, the density of the Sun is one and a half times the density of water, and a grain of matter even “weighs” millions of tons. Roughly speaking, the average 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 boundary between the earth's atmosphere and space. It turns out that the density of the UY Shield is only a little short of the vacuum of space!
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. At the same time, the star is only 36 times larger than the Sun. It turns out that R136 is 25 times brighter and about the same times more massive than UY Shield, despite the fact that it is 50 times smaller than the giant.
Physical parameters of the UY Shield
In general, UY Scuti is a pulsating variable red supergiant of spectral type M4Ia. That is, on the Hertzsprung-Russell spectrum-luminosity diagram, UY Scutum is located in the upper right corner.
At the moment, the star is approaching the final stages of its evolution. Like all supergiants, she began to actively burn helium and some other heavier elements. According to modern models, in a matter of millions of years UY Scutum 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 shed its shell, most likely leaving behind a neutron star.
Already now UY Scutum shows its activity in the form of semi-regular variability with an approximate pulsation period of 740 days. Given that a star can change its radius from 1700 to 2000 solar radii, the rate of its expansion and contraction is comparable to the speed of spaceships! Its mass loss is an impressive rate of 58 millionth solar masses per year (or 19 Earth masses per year). This is almost one and a half earth masses per month. So, being on the main sequence millions of years ago, UY Scutum could have had a mass of 25 to 40 solar masses.
Giants among the stars
Returning to the reservation mentioned above, we note that the primacy of UY Shield as the largest known star cannot be called unequivocal. The fact is that astronomers still cannot determine the distance to most stars with a sufficient degree of accuracy, and therefore estimate their size. In addition, large stars tend to be very unstable (recall the UY Scutum pulsation). Similarly, they have a rather blurry structure. They may have a rather extended atmosphere, opaque gas and dust shells, disks, or a large companion star (an example is VV Cephei, see below). It is impossible to say exactly where the boundary of such stars passes. In the end, the well-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 Cepheus A, VY Canis Major, 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 in a matter of millions, or even 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 in UY Shield, this feature is the previously discussed variability. WOH G64 has a toroidal gas and 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 centers of these stars are located from each other in some 17-34 . Considering that the VV radius of 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 with great 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 one talk about the size of a star if its atmosphere flows into another star, or smoothly passes 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, you can be sure that a neutron star is now pulsating in its place or a black hole is warping space, surrounded by the remnants of a supernova explosion. If such a star is even thousands of light years away from us, one cannot be completely sure that it still exists or has remained the same giant.
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 to lead the Big Ten. This does not mean at all that its leadership is undeniable and that, for example, NML Cygnus or VY Canis Major cannot be larger than her. Therefore, different sources can answer the question about the largest known star in different ways. This speaks rather not about their incompetence, but about 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 discovered stars, how can we say 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 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 part of them. The fact that the largest stars can be distinguished by their luminosity will not help in the search for a “stellar leader”. Whatever their brightness is, it will fade when observing distant galaxies. Moreover, as noted earlier, the brightest stars are not the largest (an example is R136).
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, its searches will be simply meaningless.
Hypergiants
If the largest star is impossible to find practically, 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. Even here, however, modern science faces a problem. The current theoretical model of the evolution and physics of stars does not explain much of what actually exists and is observed in telescopes. An example of this is the 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 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 hydrostatic equilibrium. The light pressure of the 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 to them and using powerful computers. So the modern theoretical limit for the mass of stars is 150 solar masses. Now 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 the fingers. Such stars are called hypergiants. Note that R136a1 is much smaller than the stars that, it would seem, should be below it in class - for example, the supergiant UY Shield. This is because hypergiants are called 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 bar for the mass of a hypergiant has not been established, but, as a rule, their mass exceeds 100 solar masses. None of the biggest stars of the "Big Ten" falls short of these limits.
Theoretical impasse
Modern science cannot explain the nature of the existence of stars whose mass exceeds 150 solar masses. This raises the question of how a theoretical limit to the size of stars can be determined if the radius of a star, unlike 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, metallicity of stars can lead to radical changes in their structure. Astrophysicists have only to comprehend the surprises that will be presented to them by further observations and theoretical research. 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 farthest corners of our Universe.
