star
A star is a colossal ball of hot gas — mostly hydrogen — held together by its own gravity, glowing because of a furnace buried at its heart. Deep in the core, the crush of gravity squeezes hydrogen so hard that atomic nuclei smash together and fuse into helium, and each fusion releases a flicker of energy. Pile up countless such flickers, and you get the steady light and warmth of a star. Our Sun is one ordinary example; on a dark night, nearly every twinkling point you see is another sun, almost unimaginably far away — though the steadiest, brightest "stars" are often planets in our own Solar System, which shine by reflected light and barely twinkle at all.
A star is really a tug-of-war held in perfect balance. Gravity is forever trying to crush it inward, while the heat and pressure from the core push relentlessly outward — and for most of a star's life the two are matched exactly, so it sits there shining, neither collapsing nor flying apart. That truce is what makes a star so steady; our Sun has kept it for about 4.6 billion years and has billions more to go.
It's tempting to picture a star as something "burning" like a campfire, but nothing is on fire there — there's no oxygen, and ordinary flames could never last billions of years. A star shines by fusion, not combustion: it is squeezing tiny nuclei together, not burning fuel in air. And much of you was forged inside stars long ago: the iron in your blood, and most of the heavier elements in your body, were cooked up in stellar cores and supernovae. The hydrogen in your body's water is older still — it was made in the Big Bang, not in any star.
In the Sun's core, four hydrogen nuclei fuse into one helium nucleus, and the tiny lost mass becomes energy: 4 ¹H → ⁴He + light.
The fusion-powered picture of stars is surprisingly recent. In 1920 Arthur Eddington argued that converting hydrogen to helium could release enough energy to keep the Sun shining — but he did not work out how. It took until 1938–39 for Hans Bethe to derive the actual nuclear reactions that do it (the proton-proton chain and the CNO cycle). The English word "star" shares an ancient root with Latin stella and Greek astēr, the source of "astronomy" and "astronaut."