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Going Noisily Into That Good Night
The Universe >
The recent passing of Marlon Brando reminds us that some really big stars can live really long lives. Not so in the world of astronomy. In fact, it is quite the opposite up there in the starry heavens; the big guys absolutely do not go gentle into that good night.
So what determines how long a star will live?
Stars may not come in all shapes --- they are all just big spheres --- but they come in all sizes. And it is the size, or more accurately how much stuff the star is made of, that determines how long it will live. And how much stuff or mass a star ends up with depends a lot on where and how it was born.
Galaxies, like our own Milky Way spiral wonder, are filled with giant pockets of hydrogen and helium, like raisins on a raisin pizza. (Go with it.) These massive, tenuous, invisible clouds of gas, light years across, can just hang around for eons doing nothing much but taking up space. Until, that is, something catastrophic happens.
One kind of catastrophe that will change life dramatically for a huge haze of hydrogen and helium is a nearby supernova explosion. The shock waves from that exploded powder keg will pass through our lifeless cloud and cause some parts of it to squish together, to condense.
Now it's time for the thinking cap. Little local areas in our big cloud where the gases got pushed together may now dense enough that their own collective --- but still wimpy --- gravity will perpetuate a collapse. This continued collapse makes the gas there even more dense, which increases the local gravity, which pulls the whole little area closer together, which makes it more dense, etc.
Large, dense neighborhoods in the big cloud will collapse rather quickly. The more tenuous areas take longer. One way or another, all these gassy areas balling up and looking like grapes on the vine aren't stars just yet.
When gases condense they heat up. As a mini-cloud --- one of the "grapes" on our vine --- gets smaller and smaller it gets hotter and hotter, and soon a new phenomenon, fusion, enters the scene.
Nuclear fusion happens when small particles like hydrogen and helium are so hot --- that is, they are moving so fast --- that they actually fuse together and make something new when they strike each other, rather than bounce off and float away. And when this happens a truckload of energy is released. Think nuclear fusion weapons, like the hydrogen bomb, where this same phenomenon occurs.
When our little spherical cloud finally experiences fusion at its core (where it is hottest) it is officially a star. But how long can this last?
The big guys, the stars that can get up to a hundred times the mass of our sun, burn for only millions of years then --- kaboom zaloom! --- they die a violent death. It isn't difficult to see why they are here and gone so quickly.
The huge weight of gas pressing on the core raises the temperatures below to frantic heights, and fusion takes place at a breakneck pace. It takes just a short while for these monsters to run out of fusible material. When the fuel runs out, the whole core collapses and then, in a complicated process called a supernova, the entire star blows to bits. End of story.
Tiny stars, however, down to about a tenth of the size of the sun, have barely enough mass to keep them burning at all, but burn they do. Once tiny newborn stars finally start fusing at their core they are in no hurry to finish. These types will last over a hundred billion years before they run out of fuel! The tiniest stars formed at the earliest epochs of the universe are still quietly burning today.
Stars the size of our sun have enough material at the core and enough weight above them to keep fusion going nicely for about 10 billion years, which is exactly what we want for life on a planet with human beings on it --- a star not too big, not too small, not too hot, not too cool. Just one boring lone star will do nicely, thank You.
When you look out at the sky on any night, all the stars you see are the big, bright, live-fast-die-young stars. Their days are numbered. Come back just millions of years from now and there will be a new suite of constellations up there.
Where in the skies are the little long-lived stars? They are all over the place, but too dim to see with the naked eye. They don't live the life of the big stars; they are content being bit players, out of the limelight. But they are on stage long after the big stars have left the scene.
Mark Ritter teaches astronomy at Temecula Valley High School and can be reached at mritter@firstlightastro.com.
Posted by Administrator at 2004.07.10 12:26 PM | Comments (0)
