The Sun is getting bigger, hotter, and brighter. The change isn’t enough to notice during a human lifetime – or even a thousand lifetimes. It plays out over hundreds of millions of years. And it’s all the result of changes deep inside our star.

Like all stars in the prime phase of life, the Sun is “fusing” atoms of hydrogen in its core to make helium. That generates the energy that makes the Sun shine. As the amount of helium builds up, the core gets denser, so gravity squeezes it more tightly. That speeds up the fusion reactions, making the core even hotter.

Radiation from the hotter core pushes on the Sun’s outer layers, making the Sun bigger. It also makes its surface hotter. The combination of bigger and hotter makes the Sun brighter. So over its four-and-a-half-billion-year lifetime, our star has grown about 15 percent wider, and perhaps 40 percent brighter.

That should mean that the young Earth would have been an iceball. But studies suggest the atmosphere was much thicker when Earth was young. The atmosphere also contained much more carbon dioxide and other greenhouse gases. They trapped more heat, keeping Earth from freezing over.

The Sun’s bigger-hotter-brighter trend will continue. In perhaps a billion to two billion years, it’ll be so hot and bright that Earth’s air and oceans will boil away. That will reduce our planet to a bare cinder.

Script by Damond Benningfield