How do we Know What Will Happen to the Sun?

I have heard that the sun is a star and, like other stars, will expand and blow up, gobbling the 4 inner planets. How do scientists know when the Sun will blow up? What sort of math do they do?-Amadeo Caligiuri, 12 almost 13
- Amadeo Caligiuri (age 12)
Souderton, PA, USA

The math of stars is all about balance: is one force greater than another force, or are they equal?

In the center of a star, nuclear reactions release energy as heat and light. (A lot of the energy also goes into tiny, hard-to-detect particles called neutrinos.) The escaping light and the hot, fast-moving particles inside the star push outwards, causing a force that would make the star expand. At the same time, the force of gravity pulls everything towards the center of the star, trying to make it contract.

When these two forces are equal, the star is stable and stays about the same size. This is what's happening in our sun now: it's been about the same size for the last five billion years, and it won't change much for five billion more. Right now, its energy comes from fusing hydrogen into helium in its core, where the temperature and pressure are highest. When all the hydrogen in the core is used up, the sun will start to change. First it will start to fuse hydogen outside the core. Eventually it will get hot enough to start fusing helium into carbon. During these changes the balance between outward pressure and gravity will shift, and the sun will get bigger and brighter, but its surface will get cooler, making it look red. (It won't really "blow up," though—that usually only happens to stars much bigger than the sun. It will just slowly expand over millions of years.)

How do scientists know all this? No one has ever watched a star over ten billion years. But the galaxy is full of stars of various ages—young, old, and in between—and we can observe many different stars to learn about what happens as they age. We can measure things like how bright these stars are, what color they are, and even what they're made of. It's a little like trying to figure out what the life of a human is like, if all you have is snapshot of every person on earth at one moment in time. Astrophysicists also have mathematical models and computer simulations of the physical processes happening inside stars. The predictions of those models mostly match what we observe in the universe, so we think they are mostly correct.

Rebecca H.

(published on 03/23/2015)