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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 about 36 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 the 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.

At the 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 current 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 UY Scuti 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 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 us 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 a dozen known largest stars, it is impossible to identify a specific leader and arrange them in order of increasing size. In this case, UY Shield was 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. It will not help in the search for a “stellar leader” that the largest stars can stand out for their luminosity. Whatever their brightness, it will fade when observing distant galaxies. Moreover, as noted earlier, the brightest stars are not the largest (for 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 largest star is practically impossible to find, 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. The modern theoretical model of evolution and physics of stars does 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.

Stars are huge balls of hot plasma. The size of some of them will amaze even the most unimpressive reader. So, are you ready to be surprised?
Below is a list of the ten largest (in diameter) stars in the Universe. Let us immediately make a reservation that this ten is made up of those stars that we already know. With a high degree of probability, in the vastness of our vast Universe, there are luminaries with an even larger diameter. It is also worth noting that some of the presented celestial bodies belong to the class of variable stars, i.e. they periodically expand and contract. And finally, we emphasize that in astronomy all measurements have some error, so the figures given here may differ to an insignificant degree for such a scale from the actual sizes of stars.

1. VY Canis Majoris
This red hypergiant has left all its competitors far behind. The radius of the star, according to various estimates, exceeds the solar one by 1800-2100 times. If VY Canis Majoris was the center of our Solar System, its edge would be very close to the orbit. This star is located about 4.9 thousand light years in the constellation Canis Major.

2. VV Cephei A
The star is located in the constellation Cepheus at a distance of about 2.4 thousand light years. This red hypergiant is 1600-1900 times larger than ours.

3. Mu Cephei
Located in the same constellation. This red supergiant is 1650 times larger than the Sun. In addition, Mu Cephei is one of the brightest stars. It is more than 38,000 times brighter than our star.

4. V838 Unicorn
This red variable star is located in the constellation Monoceros at a distance of 20 thousand light years from Earth. Perhaps this star was even larger than VV Cephei A and Mu Cephei, but the huge distance separating the star from our planet does not allow more accurate calculations at the moment. Therefore, it is usually assigned from 1170 to 1970 solar radii.

5. WHO G64
It was previously thought that this red hypergiant could rival VY Canis Majoris in size. However, it was recently discovered that this star from the constellation Doradus is only 1540 times larger than the Sun. The star is located outside the Milky Way in the dwarf galaxy Large Magellanic Cloud.

6. V354 Cephei
This red hypergiant is quite a bit smaller than WHO G64: it is 1520 times larger than the Sun. The star is relatively close, only 9 thousand light years from Earth in the constellation Cepheus.

7. KY Swan
This star is at least 1420 times larger than the Sun. But, according to some calculations, it could even top the list: the argument is serious - 2850 solar radii. However, the real size of the celestial body is most likely close to the lower limit, which brought the star to the seventh line of our rating. The star is located 5 thousand light years from Earth in the constellation Cygnus.

8. KW Sagittarius
Located in the constellation Sagittarius, the red supergiant is 1460 times the radius of the Sun.

9. RW Cepheus
There is still controversy over the dimensions of the fourth representative of the Cepheus constellation. Its dimensions are about 1260-1650 solar radii.

10. Betelgeuse
This red supergiant is located just 640 light-years from our planet in the constellation Orion. Its size is 1180 solar radii. Scientists believe that Betelgeuse can be reborn at any moment, and we will be able to observe this interesting process almost “from the front row.”

The comparative sizes of stars can be estimated from this video:

For many centuries, millions of human eyes, with the onset of night, direct their gaze upward - towards the mysterious lights in the sky - stars of our Universe. Ancient people saw various figures of animals and people in clusters of stars, and each of them created its own history. Later, such clusters began to be called constellations. Today, astronomers identify 88 constellations that divide the starry sky into certain areas by which one can navigate and determine the location of stars. In our Universe, the most numerous objects accessible to the human eye are stars. They represent a source of light and energy for the entire solar system. They also create the heavy elements necessary for the origin of life. And without the stars of the Universe there would be no life, because the Sun gives its energy to almost all living beings on Earth. It warms the surface of our planet, thereby creating a warm oasis full of life among the permafrost of space. The degree of brightness of a star in the Universe is determined by its size.

Do you know the biggest star in the entire Universe?

The star VY Canis Majoris, located in the constellation Canis Major, is the largest representative of the stellar world. At the moment it is the largest star in the Universe. The star is located 5 thousand light years from the solar system. The diameter of the star is 2.9 billion km.

But not all stars in the Universe are so huge. There are also so-called dwarf stars.

Comparative sizes of stars

Astronomers rate the size of stars on a scale according to which the brighter the star, the lower its number. Each subsequent number corresponds to a star ten times less bright than the previous one. The brightest star in the night sky in the Universe is Sirius. Its apparent magnitude is -1.46, meaning it is 15 times brighter than a star with magnitude zero. Stars whose magnitude is 8 or more cannot be seen with the naked eye. Stars are also classified by color into spectral classes, indicating their temperature. There are the following classes of stars in the Universe: O, B, A, F, G, K, and M. Class O corresponds to the hottest stars in the Universe - blue. The coolest stars belong to class M, their color is red.

Contrary to popular belief, it is worth noting that the stars of the Universe do not actually twinkle. This is just an optical illusion - the result of atmospheric interference. A similar effect can be observed on a hot summer day, looking at hot asphalt or concrete. Hot air rises, and it seems as if you are looking through shaking glass. The same process causes the illusion of starry twinkling. The closer a star is to Earth, the more it will “twinkle” because its light passes through denser layers of the atmosphere.

Nuclear Hearth of the Universe Stars

A star in the Universe is a giant nuclear center. The nuclear reaction inside it converts hydrogen into helium, thanks to the process of fusion, which is how the star acquires its energy. Hydrogen nuclei with one proton combine to form helium atoms with two protons. The nucleus of an ordinary hydrogen atom has only one proton. Two isotopes of hydrogen also contain one proton, but also have neutrons. Deuterium has one neutron, while tritium has two. Deep inside the star, a deuterium atom combines with a tritium atom to form a helium atom and a free neutron. As a result of this long process, enormous amounts of energy are released.

For main sequence stars, the main source of energy is nuclear reactions involving hydrogen: the proton-proton cycle, characteristic of stars with masses around the Sun, and the CNO cycle, which occurs only in massive stars and only if they contain carbon. At later stages of a star’s life, nuclear reactions can occur with heavier elements, up to iron.

When a star's hydrogen supply is exhausted, it begins to convert helium into oxygen and carbon. If the star is massive enough, the conversion process will continue until carbon and oxygen form neon, sodium, magnesium, sulfur and silicon. Eventually, these elements are converted into calcium, iron, nickel, chromium and copper until the core is composed entirely of metal. Once this happens, the nuclear reaction will stop because the melting point of iron is too high. The internal gravitational pressure becomes higher than the external pressure of the nuclear reaction and, eventually, the star collapses. The further development of events depends on the initial mass of the star.

Types of stars in the Universe

The main sequence is the period of existence of stars in the Universe, during which a nuclear reaction takes place inside it, which is the longest period of a star’s life. Our Sun is currently in this period. During this time, the star undergoes slight fluctuations in brightness and temperature. The duration of this period depends on the mass of the star. In large massive stars it is shorter, and in small ones it is longer. Very large stars have internal fuel that lasts for several hundred thousand years, while small stars like the Sun will shine for billions of years. The largest stars turn into blue giants during the main sequence.

Types of stars in the Universe
Red giant- This is a large star of a reddish or orange color. It represents the late stage of the cycle when hydrogen supplies are running low and helium begins to be converted into other elements. An increase in the internal temperature of the core leads to the collapse of the star. The outer surface of the star expands and cools, causing the star to turn red. Red giants are very large. Their size is a hundred times larger than ordinary stars. The largest of the giants turn into red supergiants. A star called Betelgeuse in the constellation Orion is the brightest example of a red supergiant.
White dwarf- this is what remains of an ordinary star after it goes through the red giant stage. When a star has no more fuel left, it can release some of its matter into space, forming a planetary nebula. What remains is a dead core. A nuclear reaction is not possible in it. It shines due to its remaining energy, but sooner or later it runs out, and then the core cools down, turning into a black dwarf. White dwarfs are very dense. They are no larger in size than the Earth, but their mass can be compared to the mass of the Sun. These are incredibly hot stars, with temperatures reaching 100,000 degrees or more.
Brown dwarf also called a substar. During their life cycle, some protostars never reach a critical mass to begin nuclear processes. If the mass of a protostar is only 1/10 the mass of the Sun, its radiance will be short-lived, after which it will quickly fade. What remains is a brown dwarf. It's a massive ball of gas, too big to be a planet and too small to be a star. It is smaller than the Sun, but several times larger than Jupiter. Brown dwarfs emit neither light nor heat. This is just a dark clot of matter existing in the vastness of the Universe.
Cepheid is a star with variable luminosity, the pulsation cycle of which ranges from a few seconds to several years, depending on the type of variable star. Cepheids typically change their luminosity at the beginning of their lives and at the end of their lives. They are internal (changing luminosity due to processes inside the star) and external, changing brightness due to external factors, such as the influence of the orbit of a nearby star. This is also called a dual system.
Many stars in the Universe are part of large star systems. Double stars is a system of two stars gravitationally bound to each other. They rotate in closed orbits around one center of mass. It has been proven that half of all the stars in our galaxy have a pair. Visually, paired stars look like two separate stars. They can be determined by the shift of spectrum lines (Doppler effect). In eclipsing binary systems, stars periodically eclipse each other because their orbits are located at a small angle to the line of sight.

Life Cycle of Stars in the Universe

A star in the Universe begins its life as a cloud of dust and gas called a nebula. The gravity of a nearby star or the blast wave from a supernova can cause the nebula to shrink. Elements of the gas cloud coalesce into a dense region called a protostar. As a result of subsequent compression, the protostar heats up. Eventually, it reaches critical mass and the nuclear process begins; gradually the star goes through all the phases of its existence. The first (nuclear) stage of a star's life is the longest and most stable. The lifespan of a star depends on its size. Large stars use up their vital fuel faster. Their life cycle can last no more than several hundred thousand years. But small stars live for many billions of years, as they spend their energy more slowly.

But, be that as it may, sooner or later, the stellar fuel runs out, and then the small star turns into a red giant, and the large star into a red supergiant. This phase will last until the fuel is completely used up. At this critical moment, the internal pressure of the nuclear reaction will weaken and can no longer balance the force of gravity, and as a result, the star will collapse. Small stars in the universe then typically develop into a planetary nebula with a bright, shining core called a white dwarf. Over time, it cools down, turning into a dark clot of matter - a black dwarf.

For big stars, things happen a little differently. During the collapse, they release incredible amounts of energy, and a powerful explosion gives birth to a supernova. If its magnitude is 1.4 solar magnitudes, then, unfortunately, the core will not be able to maintain its existence and, after the next collapse, the supernova will become neutron. The internal matter of the star will compress to such an extent that the atoms form a dense shell consisting of neutrons. If the stellar magnitude is three times the solar magnitude, then the collapse will simply destroy it, erase it from the face of the Universe. All that will remain of it is an area of ​​​​strong gravity, nicknamed a black hole.

The nebula left behind by a star in the Universe can expand over millions of years. In the end, it will be affected by the gravity of a neighboring star or the blast wave of a supernova and everything will happen again. This process will occur throughout the Universe - an endless cycle of life, death and rebirth. The result of this stellar evolution is the formation of heavy elements necessary for life. Our solar system originated from the second or third generation of the nebula, and due to this, there are heavy elements on Earth and other planets. This means that there are pieces of stars in each of us. All the atoms of our body were born in an atomic source or as a result of a destructive supernova explosion
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