It is estimated there are up to 400 billion stars in our own galaxy, the Milky Way, and over 170 billion galaxies in the observable universe. This means there could be as many as a septillion stars – that’s 1024 or a 1 followed by twenty-four zeros – in the observable universe, with possibly many more beyond this observational limit.
The Sun is our nearest star and hence the easiest for us to study, although stars vary greatly in their size, composition, colour and other properties.
The least massive known stars (brown dwarfs) are around three times the mass of Jupiter – much less massive than our Sun, which is over one thousand Jupiter masses. A star’s mass cannot be much less than this, as it wouldn’t be able to sustain thermonuclear fusion in its core.
The largest known stars are over 200 times the mass of the Sun, and can be over 1,700 times the Sun’s radius. Stars above around 150 solar masses tend to push themselves apart or shed mass, due to their high internal radiation pressure. The maximum luminosity a star can achieve before shedding mass is known as the Eddington limit.
In terms of mass, Westerhout 49-2 is believed to be the most massive known star. Westerhout 49-2 is thought to be around 250 times the mass of the Sun. This is much higher than the theoretical mass limit, although there is a large uncertainty of ±120 solar masses in this measurement, and Westerhout 49-2 might even turn out to be a binary star. Westerhout 49-2 is around 36,200 light-years from Earth in the constellation Aquila and is classified as an evolved slash star, with a spectral type of O2-3.5If*.
In terms of size, one of the largest known stars is Stephenson 2-18, a red supergiant or possible extreme red hypergiant star around 19,000 light-years away in the constellation of Scutum. The radius of Stephenson 2-18 is estimated to be around 2,150 times the radius of the sun. This is around 1.50×109 km or around 10 times the radius of the Earth’s orbit around the Sun. This corresponds to a volume nearly 10 billion times that of the Sun.
The brightest known star is BAT99-98 M, which lies around 165,000 light-years away in the Tarantula Nebula of the Large Megallanic Cloud. BAT99-98 M is a Wolf-Rayet star of spectral type WN6. This star is around five million times brighter than the Sun, although much of its light output is in the ultraviolet part of the spectrum giving is at absolute visible magnitude of -8.11 and an absolute bolometric magnitude is -12.0. in comparison, the Sun has an absolute visible magnitude of +4.83. As BAT99-98 M is so distant from Earth it only has a visual magnitude is +13.38.
The brightest star in the night sky is Sirius, which is about 8.6 light-years away in the constellation Canis Minor. Sirius is a main sequence star of spectral type A0mA1 Va, DA2, with an apparent magnitude of -1.46 and an absolute magnitude of +1.43.
Despite their size, stars appear as points of light through all but the most powerful telescopes, due to their great distances from Earth. Our nearest star other than the Sun, is Proxima Centauri, which is around 4.24 light-years away.
The oldest known stars are believed to be around 13.7 to 13.8 billion years old – close to the age of the universe. The first stars formed around 370,000 years after Big Bang during the reionisation phase transition that ended the “dark ages” of the Universe.
An example of one of the oldest known stars is HD 140283, which is about 200 light years away in the constellation of Libra. This star is a metal-poor subgiant around four fifths the mass of the Sun with an absolute magnitude of +3.377 and an apparent magnitude of 7.2, making it visible through a small telescope. HD 140283 is also known as the Methuselah star due to its extreme age.
Around 300 to 350 of the few thousand stars visible in the night sky to the unaided eye have traditionally recognised names, mostly derived from Arabic and Latin, sometimes dating back thousands of years.
Nowadays, stars are given catalogue numbers, coordinated by the International Astronomical Union (the IAU).