What Is a Star?

A star is a giant ball of hot, glowing gas held together by its own gravity. Deep inside the core of every star, atoms of hydrogen are smashed together so hard that they fuse into helium, a process called nuclear fusion. Fusion releases huge amounts of energy as light and heat, which is what makes a star shine. Our Sun is a star, an absolutely average one, and on a clear, dark night you can see a few thousand more of them with just your eyes. The Milky Way alone holds somewhere between 100 billion and 400 billion stars.

  • What stars are made ofapprox. 70% hydrogen, 28% heliumPlus a tiny bit of heavier elements
  • Star types7O, B, A, F, G, K, M from hottest to coolest
  • Sun's surfaceapprox. 5,500 °CA yellow-white G-type star
  • Hottest stars40,000+ °CBlue O-type, very rare and very massive
  • Coolest starsapprox. 2,500 °CRed M-type, very small (red dwarfs)
  • Stars in the Milky Wayapprox. 100 to 400 billionMost are smaller and cooler than the Sun

What stars are made of

Almost every star in the universe has roughly the same recipe: about 70% hydrogen, 28% helium and just 2% of everything else (oxygen, carbon, iron and so on). The reason is that hydrogen and helium are the simplest, lightest elements, and they were forged in huge amounts in the first few minutes after the Big Bang.

A star is enormous, so even though most of it is just the two simplest gases in the universe, the sheer pressure at the centre crushes the atoms together hard enough for nuclear reactions to happen. That makes stars completely different from any gas you would meet on Earth.

How stars produce light

Every star shines because of nuclear fusion in its core. Gravity squeezes the centre of the star so tightly that the temperature reaches over 10 million degrees C. At that temperature, four hydrogen atoms can be forced together to become one helium atom. The new helium atom weighs slightly less than the four hydrogens it came from, and the missing mass turns into energy according to Einstein's famous equation E = mc².

That energy starts deep inside the star and slowly works its way out, taking thousands of years for a single photon of light to travel from the core to the surface. Then in just over 8 minutes, that photon can cross all 150 million km from the Sun to your eye.

Why do stars have different colours?

A star's colour tells you how hot its surface is. The hotter a star burns, the bluer its light. The cooler the star, the redder the light. Astronomers sort stars into seven main types based on temperature and colour, in a system that runs from hottest to coolest: O, B, A, F, G, K, M. (A traditional way to remember the order is the sentence "Oh Be A Fine Guy, Kiss Me".)

  • O: blue, over 30,000 °C, very rare and very massive
  • B: blue-white, 10,000 to 30,000 °C
  • A: white, 7,500 to 10,000 °C (Sirius, Vega)
  • F: yellow-white, 6,000 to 7,500 °C
  • G: yellow, 5,200 to 6,000 °C (the Sun)
  • K: orange, 3,700 to 5,200 °C
  • M: red, under 3,700 °C (red dwarfs, most common)
Fact Although blue stars look the most exciting, around three quarters of all stars in the universe are tiny, cool, red M-type stars called red dwarfs. They are so dim that you cannot see a single one with the naked eye, even the closest one (Proxima Centauri) is invisible without binoculars or a telescope. The bright stars you see at night are an unusual minority of the brightest, hottest types.

The size range of stars

Stars come in an extraordinary range of sizes. The smallest stars are red dwarfs, often less than a tenth the diameter of the Sun. At the top end are red supergiants like Betelgeuse or UY Scuti, which can be more than a thousand times wider than the Sun. If a red supergiant replaced the Sun in our solar system, it would swallow all the inner planets and stretch out past the orbit of Jupiter.

Mass is a different story. Most stars (including all the supergiants and white dwarfs) fall between roughly 0.1 and 100 times the mass of the Sun. The biggest red supergiants are big because their outer layers are puffed up and very thin, not because they contain enormous amounts of matter.

How do we measure stars?

Stars are so far away that we can never visit them, but astronomers have clever tools for measuring them anyway. The light from a star can tell us:

  • Its temperature, from the colour of the light.
  • What it is made of, by splitting the light into a rainbow with a spectroscope. Different elements leave their own dark bands in the spectrum.
  • Its distance, by watching how it shifts against background stars as Earth moves around the Sun (a method called parallax).
  • Its true brightness, by combining its distance with how bright it looks from here.
  • Its motion, by measuring how the colour of the light is shifted slightly to red or blue (the Doppler effect).

Together those measurements let astronomers work out the size, age and mass of stars hundreds of light years away.

Did you know? Almost every atom heavier than helium that exists today was forged inside an old star and blasted out into space when the star died. The iron in your blood, the calcium in your bones, the carbon in your DNA, all of it was made in stars long before our Solar System even formed. The astronomer Carl Sagan put it nicely: "We are made of star-stuff".
Deeper dive: the Hertzsprung-Russell diagram

One of the most important tools in modern astronomy is the Hertzsprung-Russell diagram (or "HR diagram" for short). Around 1910, the Danish astronomer Ejnar Hertzsprung and the American astronomer Henry Norris Russell independently noticed that if you plot a large number of stars on a graph with temperature on one axis and true brightness on the other, the dots are not scattered everywhere. They fall into clear groups.

The main group is a diagonal line across the graph, called the main sequence. Stars on the main sequence (including the Sun) are in their stable, fuel-burning middle age. There are also two big groups off the main sequence: a clump of cool but bright stars in the top right (the red giants and supergiants), and a clump of hot but faint stars in the bottom left (the white dwarfs). Astronomers worked out that stars start on the main sequence and then drift up to the giants and finally end up as white dwarfs, leaving "tracks" on the HR diagram as they age.

By looking at where a star sits on the HR diagram, astronomers can tell you almost everything about it: its rough temperature, size, mass and age. The HR diagram has been called the most important graph in the history of astronomy.

Want to follow a star through its life? See the life cycle of a star. Or jump to the specific types: red dwarfs, white dwarfs, supergiants and neutron stars.