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Universe

Galaxies

Nearly 10,000 galaxies are visible in this image Nearly 10,000 galaxies are visible in this image Stars do not exist in isolation but gather together in vast "cities" in space, called galaxies. The galaxy in which the Sun is situated, the Milky Way Galaxy, is a colossal spiral of about 200 billion stars that measures approximately 100,000 light years across. There are reckoned to be roughly 170 billion other galaxies in the Universe, each of them containing billions of stars. Most galaxies are elliptical (oval) in shape. Many are spirals like the Milky Way, or barred spirals, which have a bar-shaped centre rather than a bulging nucleus. There are also other galaxies that have irregular shapes.


The Andromeda GalaxyThe Andromeda Galaxy

Composition

A galaxy is composed of stars and the remnants of old, dying stars. In between them is what is known as the interstellar medium: a region of gas, dust and cosmic rays (showers of high-energy subatomic particles). Dark matter, matter that gives out neither light nor any other form of electromagnetic radiation, makes up about 90% of the mass of most galaxies. Supermassive black holes may exist at the centre of many—perhaps all—galaxies.







The Local Group

The Andromeda Galaxy looks like a small smudge in the night sky when viewed with the naked eye, but it appears more than six times as wide as the Full Moon when viewed through a powerful telescope.

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