Earth to the Edge of the Universe

A light-year is the distance light travels in one year in empty space: about 9.46 trillion kilometres (9.46 × 10¹² km). It is used to measure distances to stars and galaxies. It is a unit of distance, not time.

Nobody knows the size of the whole universe; it may be infinite. The part we can see, the observable universe, is about 93 billion light-years across. It's bigger than 13.8 billion light-years (the age of the universe in years) because space has been expanding while the light travelled.

The Milky Way's disc of stars is about 1,00,000 light-years across and contains roughly 100 to 400 billion stars. The Sun is about 26,000 light-years from the centre and takes about 230 million years to go round it once.

The Sun is the nearest star to Earth. After the Sun, the nearest is Proxima Centauri, about 4.24 light-years away, part of the Alpha Centauri system. It is too faint to see without a telescope.

Astronomers estimate there are hundreds of billions of galaxies in the observable universe, and possibly more. Our nearest big neighbour is the Andromeda galaxy, about 25 lakh (2.5 million) light-years away, which you can see as a faint smudge from a dark site.

The Local Group is the small family of galaxies that includes the Milky Way, Andromeda, the Triangulum galaxy and dozens of smaller galaxies, spread over about 10 million light-years. It is itself part of a larger structure, the Virgo Supercluster.