The Sun

For those of us in the United States and other northern latitudes, the Sun makes itself scarce at this time of year. In fact, the shortest day of the year — the winter solstice — is coming up on Friday.

Despite the colder, shorter days, the Sun still warms our planet just as it’s done for the past four-and-a-half billion years. That warmth is the result of nuclear reactions deep in the Sun’s heart, known as the core. The core accounts for about a quarter of the Sun’s diameter, so it’s hundreds of thousands of miles below the surface.

The Sun’s powerful gravity squeezes the core so tightly that it heats it to millions of degrees. At those temperatures, the hydrogen that makes up most of the core consists not of atoms, but of individual electrons and protons — the building blocks of atoms.

As these particles zip around, some of the protons fuse together through nuclear reactions to form the nucleus of a helium atom. There’s so much material in the core that this process adds up. Every