Stars in their Skies

My intention had been to write another gravity post, this time about tides, tidal locking and Saturn's rings, but unfortunately I've been too busy to fully cover the scope I wanted to. Instead of a half-hearted post on the above, I bring you: stars! Stay tuned for tidal forces next week.

It's full of stars!

There are many different types of stars, as you may recall from the HR diagrams I've discussed previously (I've also drawn a rather crude, annotated HR diagram below). Stars come in a whole spectrum of colours, based on their (surface) temperatures, and these are loosely correlated with their masses. In general (for the main sequence), small stars are red and hence cool (only 3700 Kelvin) and large stars are blue and hence very hot (more than 33 000 Kelvin).

These large blue stars are sometimes called blue giants and small red stars are often called red dwarfs. In terms of mass, these stars can range from over 100 times the mass of the sun for blue giants down to about a tenth of the mass of the sun for red dwarfs. The sun, by the way, is on the cooler and smaller end of the middle of the main sequence.

Aside from descriptive words, stars are also separated into lettered classes based on their temperature. The hottest stars are designated O type, then it goes B, A, F, G (the sun is a G-type star), K and the coolest red dwarfs are M type. I'm not terribly fond of the mnemonic I use to remember them, so if you can think of a good one, please leave it in the comments. ;-)

Main sequence stars generate energy by fusing hydrogen and helium in their cores. They need to generate enough energy to over come the pressure of gravity trying to pull them into their central point. This means that bigger stars burn their hydrogen faster because they need to produce more energy to prevent gravitational collapse. Well, technically it's increased gravitational pressure that drives faster fusion in the core. Smaller stars don't have as much gravitational pressure acting on them, so the hydrogen nuclei in their cores are pushed as close together, meaning that their fusion reactions happen much more slowly and it takes longer for them to burn through all their fuel. So big stars burn bright and fast and die young. Small stars burn more conservatively and lead much longer lives. For reference, the lifetime for the hottest blue stars is around a million years whereas red dwarfs can expect to live a lengthy fifty billion years or so (the universe is currently only 13.4 billion years old). Our sun has been around for four and a half billion years and can expect to keep going for another five billion or so.

A very crude representation of where different types of stars fall on the HR diagram.

Stellar life

Talking about how long stars last is all well and good, but where do they come from and where do they go?

Nebulae (singular: nebula) are giant clouds of dust and gas in space. Even though the particles in a nebula are fairly spread out, over enough time gravity pulls them together into clumps. When these clumps get big enough that the gravitational pressure holding them together is great enough for fusion to start in the core, they "turn on" and switch from warm balls of gas into blazing young stars. The rest of the gas and dust that didn't make it into the protostar before it turned into a star proper either get blasted away by the new stellar wind unless they already clumped together enough to form planets.

After that, the star lands on the main sequence, based on its mass. What happens when it uses up all its fuel and reaches the end of its main sequence life varies depending on its mass. Smaller stars, like our sun, will throw off their outer layers and swell up into a red giant. The star will eventually eject all its outer layers and all that will be left is the star's exposed core; a white dwarf. A white dwarf no longer fuses hydrogen or helium or anything else. Instead it just slowly radiates away all of its heat until it eventually (over trillions of years) cools. White dwarfs are basically just spheres of carbon or oxygen or a mix of the two, depending on the initial star. Those news stories you might have seen about the "largest diamond in the universe"? Those are talking about carbon white dwarfs.

If we start with a larger star—big enough that after the star has ejected all its outer layers the core that's left behind is more than 1.4 times the mass of the sun—then the core will be too massive to remain merely a white dwarf. Instead it will collapse in a giant explosion known as a supernova. In the immense pressures exerted in the supernova, the protons in the old stellar core combine with the electrons to form neutrons (atoms are usually made of protons, neutrons and electrons). This type of star, composed entirely of neutrons and not found on an HR diagram, is called a neutron star. A really massive star can, after a supernova, collapse into a black hole, an even denser object (previously mentioned here).

Life elsewhere

Since the sun is a G type star and has a life-bearing planet, of course G type stars elsewhere could have life-bearing planets too. There is also a reasonable chance that F and K type stars, which are fairly similar to our sun, could also harbour life. The biggest problem is when we look at very massive stars. When we're talking about stellar lifetimes in the millions of years, then there probably isn't enough time for life to form. It took four or so billion years for life on Earth to get to the stage it is now. If the sun had only lasted for ten million years before exploding, then we wouldn't be here.

What about red dwarf stars then? They live for a very, very long time, so there's certainly ample time for life to develop. However, because the habitable zone is so close in to the star (because the star is so dim and cool), there are two issues:

1. The planet will probably be tidally locked, meaning that one side is always facing its sun while the other never gets any direct heat or light. It's likely in this case that both the day side and the night side will be permanently too hot or cold. The ring of the terminator (the boundary between the day side and the night side) would probably be the best bet for life developing, temperature-wise.
2. Small stars seem to have more flares than larger stars. Flares aren't terribly conducive to life, particularly at such close proximity. This is still an active area of research, however.

But, theoretically, in optimal conditions life could develop on a planet orbiting a red dwarf. Also, both of those issues are things I will probably blog about in the future.

As I mentioned last week, just because life can't develop there, doesn't mean humans can't try to colonise planets around different and interesting stars. Of course, colonies with a five or so million year time-limit might be a bit sort sighted, but if there are planets in suitable places, it could be good for a laugh.