There’s no formula for predicting how long a person will live. Genetics, nutrition, exercise, and other factors all play a role, but you can’t just plug in a couple of numbers and come up with an exact lifespan.
But that’s not the case with the stars. A single number determines how long a star will live: its mass.
An example of how it works is in good view on winter nights: Pollux, the brightest star of Gemini. It’s in the east at nightfall, below its slightly fainter “twin,” Castor.
Pollux is almost twice as massive as the Sun. And that number makes all the difference in its lifespan. Pollux is roughly half as old as the Sun. But it’s already finished its “normal” lifetime — the phase the Sun is in right now.
During that normal lifetime, a star “burns” the hydrogen in its core to make helium. When the hydrogen is gone, the core gets smaller and hotter, so it begins to burn the helium to make carbon and oxygen. At the same time, its outer layers puff up like a balloon, making the star much brighter. That also makes the star’s surface much cooler, so it glows reddish-orange, as Pollux does now.
Over time, Pollux will get even bigger and brighter. Then it’ll expel its outer layers, briefly surrounding itself with a colorful bubble of gas. That bubble will quickly fade, though, leaving only the star’s small dead core — a hot ember known as a white dwarf.
The same fate awaits the Sun — several billion years in the future.
More about Gemini tomorrow.
Script by Damond Benningfield, Copyright 2012
For more skywatching tips, astronomy news, and much more, read StarDate magazine.