What a star is

Hot gas held by gravity

A star is mostly hydrogen and helium, squeezed by its own weight.

NASA describes stars (opens in a new tab) as giant balls of hot gas, mostly hydrogen, with some helium and small amounts of other elements. Our Milky Way alone holds more than 100 billion of them, and astronomers estimate the universe may contain up to a septillion, a one followed by 24 zeros.

What holds a star together is its own gravity, and what stops it collapsing is the heat and pressure inside. NASA's guide to star types (opens in a new tab) explains the balance: energy released in the core heats the star and creates pressure that pushes outward against gravity's pull inward.

Sources for this section NASA Science, Star Basics (opens in a new tab) · ; NASA Science, Star Types (opens in a new tab)

Fusion, not fire

Stars shine by joining atomic nuclei together, not by burning in the everyday sense.

In a star's core, immense pressures and temperatures (opens in a new tab) squeeze hydrogen nuclei together to form helium, a process called nuclear fusion, and the energy it releases keeps the star from collapsing. NASA puts it simply (opens in a new tab): two protons, the nuclei of hydrogen atoms, merge into one helium nucleus.

NASA's own pages often say stars “burn” their fuel, but that is shorthand. A fire is a chemical reaction; fusion changes the nuclei themselves (opens in a new tab), starting only when a core reaches millions of degrees.

A star that is steadily fusing hydrogen into helium is called a main-sequence star (opens in a new tab), and this is the longest stage of its life. Around 90 percent (opens in a new tab) of the universe's stars are in it.

Sources for this section NASA Science, Star Basics (opens in a new tab) · ; NASA Science, Star Types (opens in a new tab)

The Sun is one

The Sun is an ordinary star, roughly halfway through its life.

NASA classes the Sun (opens in a new tab) as a G-type yellow dwarf on the main sequence. It is about 4.6 billion years old (opens in a new tab), and scientists expect it to stay on the main sequence for about another 5 billion years before it swells into a red giant.

NASA calls it (opens in a new tab) our most well-studied star, roughly midway through its main-sequence stage. The stars you see at night are the same kind of object, only very much farther away.

How to see it The nearest star after the Sun that you can see with the unaided eye is Alpha Centauri, which NASA names (opens in a new tab) as the closest main-sequence star visible to the unaided eye; it is a Southern Hemisphere star.

Keep in mind Not everything star-like is a star. Brown dwarfs (opens in a new tab), with 13 to 80 times Jupiter's mass, never become massive enough to fuse hydrogen like a star, and the brightest “stars” in the evening are sometimes planets.

Sources for this section NASA Science, Stars in an Exoplanet World (opens in a new tab) · ; NASA Science, Star Types (opens in a new tab) · ; NASA Science, Star Basics (opens in a new tab)