What Is a Star?

A star is an enormous ball of hot plasma — mostly hydrogen and helium — held together by its own gravity and powered by nuclear fusion in its core. That fusion is what makes a star shine with its own light.

What a star is made of:

• Roughly 71% hydrogen and 27% helium, with small traces of heavier elements.
• The material is plasma, not ordinary gas — it's so hot that atoms are stripped of their electrons.

How a star works: In the core, temperatures reach around 15 million °C (in the Sun's case) and pressure is immense. Under those conditions hydrogen fuses into helium, releasing vast amounts of energy that eventually escapes as light and heat.

A star stays stable because of a balance called hydrostatic equilibrium: gravity pulls everything inward while the outward pressure from fusion pushes back. When a star finally runs out of fuel, that balance breaks — and the star dies.

Is a star a planet? No. The key difference is that a star produces its own light through fusion, while a planet only reflects light and orbits a star. Planets are also far smaller and never get hot or massive enough to sustain fusion.

Is every star a sun? Essentially, yes — the Sun is an ordinary star that just happens to be extremely close to us. Other stars look like faint points of light only because of their enormous distance.

The closest star to Earth is the Sun (about 93 million miles away). The nearest star beyond it is Proxima Centauri, roughly 4.24 light-years away.

Types and colours of stars: Astronomers classify stars by temperature using the sequence O, B, A, F, G, K, M:

• Blue stars (O, B) are the hottest and most massive.
• White and yellow stars (A, F, G) are in the middle — our Sun is a G-type.
• Orange and red stars (K, M) are the coolest and longest-lived. Red dwarfs are the most common type of star in the universe.

The life of a star: Stars are born inside cold clouds of gas and dust called nebulae, where gravity pulls material together until fusion ignites. What happens at the end depends almost entirely on mass — average stars like the Sun swell into red giants and fade into white dwarfs, while the most massive stars explode as supernovae, leaving behind a neutron star or a black hole.

How many are there? Astronomers estimate the observable universe could hold up to a septillion stars (a 1 followed by 24 zeros), with hundreds of billions in our Milky Way alone.