Ask the Dreadlocked Science Geek

[dreadgeek]

Originally Posted by June

Dear Anti-Matter Specialist,

Some people worry about losing their jobs, but one of the things I worry about the most is the Sun going out. "They" say it will be a few billion years before that happens, but how do they really know? Isn't it entirely possible that it could break into a zillion flaming pieces hurtling outward at any time? What exactly is holding it together, and what do they base the "Billions" theory on?

Is it *possible* that the Sun could break up? Yes, but it would take a truly extraordinary set of circumstances for that to happen. Anything that would cause that (and I can think of only two off the top of my head--a black hole or another very massive star wandering into the neighborhood) would also make life on Earth very interesting and intense--for a very short period of time.

So how do we know that the Sun has a few billion (5 or 6) years left? Largely because of the mass of the Sun. To understand how this relates, we have to digress and talk about stars generally.

A star is simply a ball of plasma (matter in a very energized state) held together by gravity. The energy is provided by the fusing of hydrogen into helium. At the heart of a star, there is a wrestling match--gravity wants to collapse all of the mass of the star into the smallest possible space while heat wants to expand the star. Stars on what astronomers call the 'main sequence' are happily fusing hydrogen into helium. However, in ANY process there is is loss due to inefficiency. So as the star burns it begins to lose mass. Remember that mass is what is creating the gravity so as the star loses mass, pressure begins to win.

Because our Sun is a very ordinary star (it is a G-type dwarf star, the second or third most common type star in the universe) we have a lot of observational data from different stars like ours at different stages of life. Given a particular burn rate (and we know the burn rate of the star by the spectral lines--the light we see from the Sun is only part of the EMF spectrum being put out by it) we can determine at what rate the Sun is losing mass.

The end-game for a star is determined by its mass. For an ordinary dwarf star like ours, the end-game looks like this:

Around 5 or 6 billion years the Sun will have lost enough mass that pressure will, temporarily, have the upper hand. The outer shell of the Sun will then expand out to 1 AU (Astronomical unit which is 93 million miles). This is inconveniently the orbit that Earth occupies. It will then be a red giant star. Over the course of another billion years or so, it will burn off the rest of the helium and slowly collapse back into a white dwarf. This will basically be only the core of the Sun and will be about the size of Earth (although MUCH more massive than Earth is). Over the next few billion years, it will cool down through a brown-dwarf phase until it is a black-dwarf.

Within a reasonable margin of error (say 1% either way) we're pretty certain when the Sun will begin its end-game because of its present mass and heat.

Just because it is SO cool, I'll take you through the end-game of a much more massive star than ours.

REALLY massive stars (like Betelgeuse) have a much more interesting life cycle. They still stay on the main sequence H --> He but once they reach the Helium stage (where that's the only fuel that is left) it will begin fusing Helium into Carbon. This transformation keeps happening until the core becomes Iron. At that point, there's no place else to go. No natural force and fuse Iron into a heavier element and gravity gets the upper hand. The core collapses into itself and the resulting energy release is called a supernova. The star *literally* blows itself apart. If the star has sufficient mass, after the cataclysm of the supernova a black hole or a neutron star will result. A black hole results if the remaining core has sufficient mass to continue collapsing. Otherwise all that is left is a superdense core of neutrons known as a neutron star. These completely exotic objects are some of the strangest things in a very strange universe. They are so dense that a single teaspoon of the stuff would weigh as much as the Earth!

Cheers
Aj