Twinkle, twinkle little star. You're not always what you are.
The "stars" you see at night may not be stars. Some of them are planets. Some of them are galaxies. But most of them are stars - and they're all completely unique.
But the human race does like to put things into boxes, so here are all the categories of early-life stars*:
A protostar is what you have before a star forms. It's the swirling collection of gas that's collapsing down into a star. It's getting hotter and denser - but nuclear fusion reactions haven't started yet.
T Tauri Star
Just before a star moves onto the main sequence, they're classified as a T Tauri. They look like a main sequence star and have the same temperature, but they're brighter because they're larger. The key difference is that a T Tauri generates its energy by gravitational pressure - not nuclear fusion.
Main Sequence Star
The majority of the stars in our galaxy are main sequence stars. They're in a state called hydrostatic equilibrium. This means the gravity of the star that's pulling everything together is balanced by the light pressure from the nuclear fusion reactions pushing everything outwards. The result is that the star is spherical (or nearly spherical) in shape.
Main sequence stars are a pretty diverse bunch and can change in colour, size, brightness and mass but they all do the same thing: convert hydrogen to helium in their cores, a process called nuclear fusion, which releases a tremendous amount of energy.
Main sequence stars are classified by their luminosity (aka their brightness) into seven categories: O, B, A, F, G, K and M. O are the hottest and blue in colour, M are the coolest and read in colour.
Main sequence stars can be as small as 8% the mass of our Sun and can, theoretically, go up to 100 times the mass of the Sun.
Smaller bodies (less than 8% the mass of our Sun) do not have enough mass for nuclear fusion reactions to start. These brown dwarfs are too small to be stars and too big to be planets - and they never twinkle.
Next week, I'll look at what happens at the end of a star's life...
Extra reading and watching
This is a pretty thorough description of the different classifications of main sequence star. Here's some more information on brown dwarfs.
This post goes into a little more depth around main sequence stars and here's some more information on the nuclear fusion reactions in stars that keep them shining.
* By early-life I mean those stars that are about to, may never, or have achieved the nuclear fusion of hydrogen to helium.
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