The sun is hot because of nuclear fusion in its core, which releases tremendous amounts of energy. The sun is basically a gigantic thermonuclear bomb held together by its own gravity.

The sun is bright because it is hot. The hotter something is, the more brightly it will glow.

Astronomers use a variety of instruments, mostly telescopes of one sort or another, to study the sun. Images, spectra (graphs of the relative energy of different colors of light), and so on are the kinds of data that these instruments collect.

Solar flares can break off from the sun and become solar wind, which flows throughout the solar system. Normally, when we talk about a solar flare, though, we are talking about several times the size of the Earth.