Universe

The Universe consists of everything that exists: stars, planets, rocks, people, and so on. It even includes empty space. Nearly all the visible matter in the Universe is contained...

Big Bang

Scientists believe that the Universe began in a single momentous event. This was a sudden burst of energy called the Big Bang which took place about 13.8 billion years ago. It is...

Expansion of the Universe

It was only about 100 years ago that scientists first realised that the Universe extended far beyond our own Milky Way Galaxy and contained many other galaxies besides our own. US...

Galaxies

Stars do not exist in isolation but are clustered together in vast "cities" in space known as galaxies. Galaxies consist of stars, planets and vast clouds of gas and dust, all held...

Milky Way Galaxy

Every star you see in the night sky is part of the Milky Way Galaxy (although on a clear night, you can glimpse one or two galaxies that lie beyond the Milky Way). The Milky Way...

Stars

Stars are giant spinning balls of hot gases. They produce vast amounts of energy, including heat and light, which they radiate across space as they shine. They vary enormously in...

Binaries, multiples, variables and clusters

More than half of stars (60%) in the Milky Way Galaxy exist as systems of two or more stars. Solitary stars—like our Sun, for example—are therefore on the unusual side. Double...

Interstellar objects

An interstellar object is an object such as an asteroid, a comet or rogue planet—but not a star—in interstellar space, the space between the stars in a galaxy. An interstellar...

Black holes

Black holes are tiny regions of space with a force of gravity so strong that nothing, not even light, can escape from them. All bodies in space have a gravitational pull, the force...

Dark matter

Scientists think that a large part of the total mass of the Universe—about 85% of it—cannot be accounted for by the mass of objects we can see or detect. Only the small...

Gravitational waves

In February 2016, a team of US scientists announced that they had detected gravitational waves. These are the ripples in “space-time” that were predicted a century ago by the...

Stars A-Z

Galaxies A-Z

Universe A-Z

What is the difference between a star and a planet?

The main difference between stars and planets is that stars have high temperatures compared to planets. Stars undergo nuclear reactions—usually they burn hydrogen in their cores...