Stars, Galaxies and the Universe

Think of a stove element: as it heats up it glows dull red, then orange, then yellow-white. Stars work the same way. A star's colour shows its surface temperature: red stars are the coolest and blue stars are the hottest. (This is the opposite of the colours on a tap, where red means hot.) A star's brightness is trickier. How bright it looks from Earth (its apparent brightness) depends on how much light it really gives off, called its luminosity, and on how far away it is. A dim-looking star might be a powerful star very far away, or a weak star close by.

More than a century ago (around 1911 to 1913), astronomers Ejnar Hertzsprung and Henry Norris Russell each made a graph of many stars, with temperature along the bottom (hottest on the left, by tradition) and luminosity up the side. The stars did not land randomly: they fell into groups. This Hertzsprung–Russell (H-R) diagram is still one of astronomy's most useful tools.

• Main sequence: a band running from hot, bright, blue stars at the top left to cool, dim, red stars at the bottom right. About 90% of stars, including the Sun, are here. They are fusing hydrogen into helium in their cores.
• Giants and supergiants (top right): cool but very luminous. They are so enormous that their huge surfaces give off lots of light even at low temperatures.
• White dwarfs (bottom left): hot but dim, because each one is only about the size of Earth.

1. A nebula is a cloud of gas (mostly hydrogen) and dust. Gravity pulls a clump of it together.
2. The clump shrinks and heats up into a protostar.
3. When the core reaches about 10 million °C, hydrogen fusion begins. It is now a main-sequence star, and it stays this way for most of its life.
4. When the core runs low on hydrogen, the star swells. A Sun-like star becomes a red giant, then gently puffs its outer layers into space as a glowing planetary nebula (nothing to do with planets; they looked like planets in old telescopes). The leftover core is a white dwarf, which slowly cools.
5. A star more than about 8 times the Sun's mass becomes a red supergiant, then its core collapses and it explodes in a supernova, briefly outshining a whole galaxy. What remains is a neutron star (a city-sized ball of neutrons) or, for the most massive stars, a black hole, whose gravity is so strong that not even light can escape.

A galaxy is a huge system of stars, gas, dust and dark matter held together by gravity. We live in the Milky Way, a barred spiral galaxy about 100 000 light-years across containing roughly 100 to 400 billion stars. The Sun sits in one of the spiral arms about 26 000 light-years from the centre, and takes about 230 million years to orbit the centre once. At the very centre is a supermassive black hole called Sagittarius A-star (written with an asterisk after the A), about 4 million times the Sun's mass; the Event Horizon Telescope released the first image of its surroundings in 2022. On a dark night, the hazy band of light called the Milky Way is our edge-on view of the galaxy's disc from inside it. Beyond our galaxy, the observable universe holds at least hundreds of billions of other galaxies.