Stars in general are variable on a multitude of scales. The most popularly known cycle is the recurrance of sun spots, see e.g. the wikipedia page [1].

However, stars in general have their own 'life' from young to old. If you take a star like our sun, which has -- surprise, surprise -- one sun mass, it burns hydrogen to helium in a fusion process. Over billions of years it will deplete its hydrogen fuel and a core of helium will form. This leads the hydrogen fusion to move to the outer layers of the star which make it blow up. During this stage it will blow up like a balloon and have a cooler but very thin transparent surface. Hence the name of this stage: red giant.

During this stage, the sun's radius will become so large that it swallows the orbit of the earth. However, since its density is very low at this stage, the earth will keep orbiting inside the sun (though it's a bit warm then). Luckily we still have another 4 billion years or so until then.

When the sun depletes all of its fuel, it will collapse, and maybe (don't know about the current stage of research on that) a Helium Flash will occur where Helium fusion will occur for a brief moment.

During the collapse of the red giant the dense core will bounce back the outer layers and all what's left is the white dwarf.

It get's even more interesting once you take a look at massive stars which immediately start burning Helium (or even heavier elements). Their lifetime is much shorter than the sun's and they end up in different kinds of catastrophic events called supernovae, which leave high density cores behind. Those are either neutron stars, i.e. blobs of tens of kilometers diameter consisting ONLY of neutrons, or black holes.

If you want to model these lifetimes of stars in a crude way, I would start looking at Hertzsprung-Russel diagrams to figure out the paths which stars of different masses take [2]. From the main sequence to the terminal age main sequence, all the way to the red giant stage, possibly a helium burning stage, maybe a second red giant stage, and what have you...

Once you have a few paths figured out, you can toss a die for the initial mass of the star and then toss another one to find out its age. Then lookup the color, brightness, opacity and size.

[1] https://en.wikipedia.org/wiki/Solar_cycle [2] https://en.wikipedia.org/wiki/Main_sequence

And now I feel old because that was my first computer too.