Most stars, around 90% of the total, are red dwarfs. We don’t see them in our night skies, because they are too faint, but they make up the vast majority of the Milky Way Galaxy. They burn slowly and coolly (with surface temperatures of around 3000C), and are considerably smaller than our Sun, from about 0.075 solar masses to around 50 percent of a solar mass. These unnoticed stalwarts of the Milky Way can burn for trillions of years before running out of hydrogen fuel, far longer than any other type of star. It’s thought unlikely that red dwarf stars would make good sites for life – a planet would have to be very close indeed to be in any kind of habitable zone, where gravitational fields might be somewhat stormy.
Some stars are yellow or white and around the same size as our Sun. These, like red dwarfs, are ordinary – or “main sequence” stars – which burn hydrogen fuel and have a lifespan of around 10 billion years altogether. These stars make up around 5-10 percent of the Milky Way galaxy and are very good candidates for supporting life – well, our own star does, so why shouldn’t any similar stars?
The “main sequence” stars are stars which are happily burning hydrogen fuel in their cores. Smaller stars do it for longer, more massive ones for shorter timeframes – for the most massive stars perhaps less than 10 million years. Sooner or later all stars will begin to run out of hydrogen fuel and will leave the main sequence, when some strange things seem to happen.
Many of the stars we can see in the sky are bloated, luminous stars towards the end of their lives, or otherwise very massive main sequence stars. Stars like the Sun will expand and become red giants, swelling to several hundred times their current size, as they stop burning hydrogen and instead burn helium. Eventually they will puff away their outer layers and their cores will contract into white dwarfs – hot, dense but dead stars. There are millions of these in our Milky Way Galaxy – difficult to see unless they have a companion, but prevalent nonetheless.
More massive stars will do something similar but become red supergiants, thousands of times larger than the Sun. A good example is Betelgeuse, Orion’s right shoulder, which is visible to the naked eye as a yellowy reddish star. It is so large that if it were in place of the Sun, it would extend out to between Mars and Jupiter. Betelgeuse is already off the main sequence, despite being less than 10 million years old, and will eventually become one of the more exotic starlike things in the galaxy.
Stars as massive as Betelgeuse will eventually explode in a violent eruption called a supernova and will, for a time, provide the brightest objects in the Milky Way Galaxy. Supernovae (of which there are different types) have been seen periodically in the skies for thousands of years. A famous example was the explosion in 1054 which caused the Crab Nebula, and was carefully observed by Chinese astronomers. Betelgeuse, it is thought, will do this in less than a thousand years. When it does, it will appear comparable to the moon in the night sky.
This leaves us with two more exotic star types in the Milky Way. Some supernova explosions will leave a remnant called a neutron star – an unimaginably dense ball of matter, very hot and often spinning ferociously. Neutron star matter is so dense that a teaspoonful would weigh more than the Earth itself. If however, the supernova star is larger than around 5 solar masses, the core will not be able to stabilise at this point and will collapse further until it disappears from view behind an impenetrable wall of gravity – a black hole. There are thought to be many of these in the galaxy, especially from the early days of the Milky Way, when there would have been more massive stars forming from giant clouds of gas. There is commonly held to be a massive black hole at the centre of the Milky Way.
These main star types make up a great deal of our Milky Way Galaxy, and there may be other things out there as well. Many star-like objects, such as quark stars, are at the moment, still hypothetical.
