Galaxies

A galaxy is a giant family of stars held together by gravity, along with vast clouds of gas, dust and probably an invisible substance called dark matter. A small galaxy might contain a few hundred million stars; a giant one can hold over a trillion. Almost every galaxy has a supermassive black hole at its centre. We live in a spiral galaxy called the Milky Way, and the universe holds about 2 trillion others.

• Galaxies in the universeapprox. 2 trillionBest estimate from telescopes like Hubble
• Our galaxyMilky WayA barred spiral, 100,000 light years across
• Closest big galaxyAndromeda2.5 million light years from us
• Main galaxy types4Spiral, elliptical, irregular, lenticular
• Stars in the Milky Wayapprox. 100 to 400 billionOur home galaxy alone
• Largest known galaxyIC 1101Over 50 times the size of the Milky Way

Types of galaxy

Astronomers sort galaxies by shape. Spiral galaxies like our Milky Way have long curving arms of stars rotating around a bright central bulge. Elliptical galaxies are smooth and egg-shaped, often filled with older red stars. Irregular galaxies have no clear shape and are often the result of two galaxies colliding. Lenticular galaxies are a halfway type, with a bulge but no spiral arms.

How big and how far?

Galaxies are so far apart that astronomers measure the distances in light years. A light year is the distance light travels in one year: about 9.5 trillion kilometres. The Milky Way is about 100,000 light years across, and Andromeda, our nearest big neighbour, is 2.5 million light years away. The light you see from Andromeda tonight set out before humans had even invented writing.

How do galaxies grow?

Galaxies are not born at their full size. The first galaxies were small and irregular. Over billions of years they have grown by pulling smaller galaxies in and merging with them. The Milky Way is still doing this today: it is in the slow process of pulling in the small Sagittarius Dwarf Galaxy, and in approximately 4.5 billion years it will merge with its much bigger neighbour, the Andromeda Galaxy.

Most galaxies sit in groups or huge clusters held together by gravity. The Milky Way is part of a small group of about 50 galaxies called the Local Group, which is itself a tiny corner of a much bigger structure called the Virgo Supercluster.

Fact The Milky Way and the Andromeda Galaxy are pulling each other in. In approx. 4.5 billion years they will collide and merge into a single, larger galaxy. Don't panic: galaxies are mostly empty space, so almost no individual stars will hit each other. The night sky just gets a spectacular new shape.

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