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, color 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, however, due to their high internal radiation pressure. The maximum luminosity a star can achieve before shedding mass is known as the Eddington limit.
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.
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 know by catalogue numbers, coordinated by the International Astronomical Union (the IAU).