The term star was originally associated with the visible stars we recognise from the night sky. Stellar means “star-like”.

As our knowledge of the Universe increased, it was soon realised that our Sun was a fairly normal star, just close enough to be very bright due to the effect of the inverse square law.

The scientific use of the telescope brought many stars into view for the first time, and astronomers now believe there are some ~1022 stars in the observable Universe.

Stars have a wide range of masses, and their luminosity varies by many orders of magnitude. As stars increase in mass their lifetimes become dramatically shorter, with stars 10 times that of the Sun living for only about 0.1% of the time, albeit at much greater luminosity (about 10,000 times brighter). Astronomers refer to how stars live and die as stellar evolution even though it has nothing to do with Darwin’s theories.

Stars like our Sun live for about 10 Billion years before they exhaust their primary source of fuel, the simplest element, hydrogen. After this occurs they swell up dramatically becoming red giants before losing their outer layers and resembling a planetary nebula. Once the outer layers peel off, the star becomes known as a white dwarf. White dwarfs are still referred to as stars. Whilst burning hydrogen in their cores, stars are said to be “on the main sequence” of the Hertzsprung Russell diagram.

Stars originally more massive than about 6-8 times the mass of the Sun can burn elements more massive than Hydrogen, and ultimately create cores that collapse catastrophically, creating neutron stars or black holes in a supernova explosion. Neutron stars and black holes are frequently referred to as stars, even though they are frequently invisible at optical wavelengths. The exact mass at which a star ceases to form a neutron star and starts creating a black hole is not known, but thought to be around 20 solar masses. Neutron stars manifest themselves in various ways, among them pulsars and magnetars.

Stars less massive than about 0.8 solar masses have not had sufficient time to exhaust their hydrogen since the Big Bang, and are still on the main sequence.

Very low-mass stars with masses less than about 0.08 solar masses, cannot burn Hydrogen at all in their cores and are often called “brown dwarfs”.

Stars are not formed individually, but in massive groups and are usually associated with galaxies (collections of billions of stars) or globular clusters.

Shooting stars have nothing to do with stars whatsoever, and are small particles striking the Earth’s atmosphere.

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