How Sun Burns

Stupid Question ™
Jan. 6, 2000
By John Ruch
© 2000

Q: Since fire needs oxygen to burn, how does the Sun burn in the vacuum of space? And why doesn’t it shrink as it burns?
—anonymous


A: Quite simply, the superheated hydrogen in the Sun isn’t undergoing the familiar oxidation reaction of normal fire. It’s undergoing nuclear fusion, which requires no oxygen. (Good thing, too: 19th century scientists estimated that even if the Sun was made of pure coal, it would burn only 10,000 years.)

And while the Sun eventually will shrink, its size is currently stable because the outward thrust of the nuclear explosions (exactly the same as our much smaller hydrogen bombs) equals the inward pull of gravity on the gases.

Stars form when a cloud of gas and dust starts collapsing under its own gravity. Extreme pressure and heat (millions of degrees) at the center overcome the electrostatic forces that usually keep atoms separate and cause the atoms to fuse. In the Sun, the gas is hydrogen, and when its nuclei fuse, they create a helium atom (while also releasing the enormous energy that kept the original atoms together—partly as the electromagnetic energy of sunlight).

This continues in a chain reaction—each initially unstable helium atom setting off more fusion reactions—as long as hydrogen’s around. (It is currently being consumed at a rate of 400 million to 700 million tons per second.) This stable chain reaction results in a continuous outward explosive thrust that balances gravity’s inward pull. It’s been going on for 4.5 billion years, and will continue for about another 5 billion years. Thus, the Sun’s size will not change during our lifetimes, and has not changed during the entire history of life on Earth.

When the Sun’s size does change, it will at first get bigger.

Fusion begins in the core, which eventually runs out of hydrogen. So the locus of fusion progresses outward from the core for the hydrogen there. This outward movement will eventually (6 billion years from now) cause the Sun’s surface to expand to perhaps 500 times its current size. The expanded surface will cool more rapidly, resulting in a reddish cast. (This is the “red giant” phase of solar life.) The Earth will be fried and perhaps enveloped by the Sun. Finally, the outer layers will be too cool for hydrogen fusion to continue.

Meanwhile, the helium-rich core, no longer exploding and fighting gravity, will contract. Eventually it will shrink and heat up enough for helium fusion to begin. When the helium runs low, gravity will again collapse the core, enough that fusion can begin yet again. The Sun will pulse every few thousand years in this way for some time, hurling off up to half its current mass.

Finally, the helium will completely run out. Gravity will take over, squishing the remaining carbon-oxygen core until it’s smaller than Earth. (Today, the Sun could contain 1.3 million Earths.) This core will retain heat for billions, perhaps even a trillion, years, feebly flickering until it finally cools into a single, dense crystal-cinder.