“The Largest Stars in the Universe: Exploring Red Giants, Hypergiants, and More”

At the outset, let us note that the largest star in terms of size does not have to be the most massive, i.e. the “heaviest” at the same time – the mentioned quantities are only very loosely related. While the mass of a star changes only slowly during its life and in the vast majority of cases it decreases due to so-called mass loss, its size, on the contrary, changes considerably and is closely related to the development stage: Generally speaking, the star is the largest in the “pre-retirement” phase of the so-called red giant.

It is not entirely clear what the highest value is that limits the size of stars. The evolution of massive stars that in the stage red giants they take on gigantic proportions, namely still shrouded in a light mist of mystery. Scientists do not even agree on the upper limit of the mass of a star that can still be stable. In the case of massive stars, dynamic effects (pulsations or strong stellar wind) also play a major role, which have a fundamental, but difficult to estimate, effect even on the maximum size of the star.

Red champions

However, scientists agree that stars reach their largest sizes in the red giant phase, i.e. after leaving the main sequence, or on the asymptotic giant branch. In the first case, we find helium in the core, and the main source of heat is a burning hydrogen shell gradually moving away from the core. The second case is similar, only heavier elements accumulate in the core and the surrounding shell of helium burns.

He is currently the biggest known star the red supergiant Stephenson 2-18 from in the constellation of the Shield, which is part of the open star cluster Stephenson 2. It is approximately 6,000 parsecs (20,000 light-years) from Earth. Compared to the Sun, it has a 2,150x larger radius.

Another great is UY Scuti, a red hypergiant 1,700 times the size of the Sun. If UY Scuti were the central body of the Solar System, its photosphere would extend beyond the current orbit of Jupiter (and it is clear that even Jupiter, let alone any closer planets, would not exist in the system). Currently, astronomers know about 20 stars with dimensions more than a thousand times larger than the size of the Sun. Among the biggest known constants is the red giant, for example WOH G64 from the Large Magellanic Cloud and VY Canis Mayoris from the constellation Canis Major.

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One giant star can also be seen with the naked eye: it is located in the constellation Cepheus and has the designation μ Cephei. Already in a small telescope, even an inexperienced observer will notice its distinctive red coloration, to which the body owes its name Garnet star; μ Cep is a red star with a radius of 650–1,420 solar radii and a mass of twenty Suns, and lies about 6,000 light-years away. Other well-known red warthogs include Betelgeuse in Orion or Antares in Scorpio.