Massive means big, less massive means small, right? It's not that simple when it comes to stars and their sizes. If we compare the planet Earth with the Sun, it turns out that it is possible to place 109 of our planets on top of one another, just to pave the way from one end of the star to the other. But there are stars smaller than the Earth and much, much larger than the Earth's orbit around the Sun. How is this possible? What determines the size of a star? Why are “suns” so different?
The question is not easy, because we hardly see the size of a star.
A deep telescopic view of stars in the night sky clearly shows stars of different sizes and brightness, but all stars are shown as dots. The difference in size is an optical illusion associated with saturation of observation cameras
Even in a telescope, most stars look like simple points of light due to the gigantic distances to us. Their differences in color and brightness are easy to see, but the size is quite the opposite. An object of a certain size at a certain distance will have a so-called angular diameter: the apparent size that an object occupies in the sky. The closest star to the Sun, Alpha Centauri A, is only 4.3 light years away and 22% larger than the Sun in radius.
Two sun-like stars, Alpha Centauri A and B, are located just 4.37 light years from us and orbiting each other at a distance between Saturn and Neptune. Even in this Hubble image, they appear as simply oversaturated point sources; no disk visible
Nevertheless, it seems to us that its angular diameter is only 0.007 ”, or seconds of arc. 60 seconds of arc consists of one minute of arc; 60 minutes of arc is 1 degree, and 360 degrees is a full circle. Even a telescope like Hubble can only see 0.05“; there are very few stars in the Universe that a telescope can actually "see" in a decent resolution. Typically, these are giant stars nearby, like Betelgeuse or R Doradus - the largest stars in the entire sky in terms of angular diameter.
A radio image of the very, very large star Betelgeuse. One of the few stars that we see as more than a point source from Earth
Fortunately, there are indirect measurements that allow us to calculate the physical size of a star, and they are incredibly hopeful. If you have a spherical object that becomes so hot that it emits radiation, the total amount of radiation emitted by a star is determined by two parameters: the object's temperature and its physical size. The reason for this is that the only place that emits light in the Universe is the surface of a star, and the surface area of a sphere is always calculated using the same formula: 4πr2, where r is the radius of the sphere. If you can measure the distance to this star, its temperature and brightness, you know its radius, and therefore its size, simply because these are the laws of physics.
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Close-up shot of the red giant UY Scuti, processed with the Rutherford Observatory telescope. This bright star may be just a "dot" for most telescopes, but it is actually the largest star known to mankind.
When we make observations, we see that some stars are only a few tens of kilometers in size, while others are 1,500 times the size of the Sun. Among supergiant stars, the largest is UY Scuti with a diameter of 2.4 billion kilometers, which is larger than Jupiter's orbit around the Sun. Of course, these incredible examples of stars cannot be judged on the majority. The most common type of stars are main sequence stars like our Sun: a star that is made of hydrogen and gets its energy by fusing hydrogen into helium in its core. And they come in many different sizes, depending on the mass of the star itself.
A young star-forming region in our own Milky Way. As gas clouds are compacted by gravity, the protostars heat up and become denser until fusion finally begins in their cores.
When you form a star, gravitational contraction converts potential energy (gravitational potential energy) into kinetic (heat / motion) particles in the core of the star. If there is enough mass, the temperature will become high enough to ignite nuclear fusion in the innermost regions, where hydrogen nuclei are converted to helium in a chain reaction. In a low-mass star, only a tiny fraction of the center itself will reach the threshold of 4,000,000 degrees and fusion will begin and proceed slowly. On the other hand, the largest stars can be hundreds of times more massive than the Sun and reach core temperatures of several tens of millions of degrees, fusing hydrogen into helium at a rate millions of times faster than our Sun's.
The current Morgan-Keenan spectral classification system with the temperature range of each star class shown above in Kelvin. The vast majority of stars (75%) are M-class stars, of which only 1 in 800 are massive enough to become supernova
The smallest stars have the smallest external flux and radiation pressure, and the most massive ones have the largest. This external radiation and energy keeps the star from gravitational collapse, but it may surprise you that the range is relatively narrow. The smallest stars, red dwarfs like Proxima Centauri and VB 10, account for only 10% of the Sun's size, slightly larger than Jupiter. But the largest blue giant, R136a1, is 250 times the mass of the Sun, but only 30 times larger in diameter. If you synthesize hydrogen into helium, the star will not change much in size.
But not every star synthesizes hydrogen into helium. The smallest stars do not synthesize anything at all, and the largest are at a much more energetic stage in their lives. We can break down stars into types by size and highlight five general classes
Neutron stars: Supernova remnants containing a mass of one to three suns, but compressed into one giant atomic nucleus. They still emit radiation, but in small amounts due to their size. An ordinary neutron star is 20-100 kilometers in size.
White dwarf stars: Formed when a sun-like star burns the last helium fuel in its core, and the outer layers swell as the inner layers contract. Usually a white dwarf star has from 0.5 to 1.4 times the mass of the Sun, but in physical volume it is close to the Earth: about 10,000 kilometers across, consisting of highly compressed atoms.
Main Sequence Stars: These include red dwarfs, solar-like stars, and the blue giants we mentioned earlier. Their sizes are very different, from 100,000 kilometers to 30,000,000 kilometers. But even the largest of these stars, if put in the place of the Sun, will not swallow Mercury.
Red Giants: Shows what happens when the core runs out of hydrogen. Unless you are a red dwarf (in which case you will simply become a white dwarf), gravitational contraction will heat up your core enough to start fusing helium into carbon. Fusion of helium to carbon emits much more energy than fusion of hydrogen to helium, so the star is expanding greatly. The physics is that the outgoing force (radiation) at the edge of the star must balance the incoming force (gravity) in order for the star to be stable, and the greater the force that tends outward, the larger the star will be. Red giants are usually 100-150,000,000 kilometers in diameter. That's enough to swallow Mercury, Venus, and possibly Earth.
Supergiant stars: The most massive stars that end up fusing helium and begin fusing even heavier elements in their cores: carbon, oxygen, silicon and sulfur. These stars are doomed to become supernovae or black holes, but before that they will swell to billions of kilometers or more. Among them are the largest stars like Betelgeuse, and if we put such a star in place of our Sun, it would swallow all our solid planets, the asteroid belt and even Jupiter.
The sun is still relatively small compared to the giants, but grows to the size of Arcturus in its red giant phase
For the smallest stars of all, such as neutron stars and white dwarfs, the rule is that trapped energy can only escape through a tiny surface area that keeps them bright for a long time. But for all other stars, the size is determined by a simple balance: the strength of the outgoing radiation on the surface should be equal to the inward gravitational attraction. Large radiation forces mean the star swells to a large size, with the largest stars swelling to billions of kilometers.
The Earth, if the calculations are correct, will not be swallowed up by the Sun in the red giant phase. But the planet itself will get very, very hot
As the sun ages, its core heats up, expands and gets hotter over time. In one to two billion years, it will be hot enough to boil Earth's oceans if we don't put the planet into a safer orbit. In a few hundred million years, the Sun will be big and bright. But let's face it: no matter how big our Sun becomes, it will never become more massive than neutron stars and the largest supergiants, even if it is larger.
ILYA KHEL
