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Fact sheet: Galaxy

The Pinwheel Galaxy, also known as Messier 101, is a spiral galaxy. (Credit: NASA/ESA/K. Kuntz, JHU/F. Bresolin, U of Hawaii/J. Trager, JPL/J. Mould, NOAO, Y.-H. Chu, U of Illinois Urbana/STScI)


Galaxies are collections of stars, planets, gas, dust and dark matter. They are held together by the force of gravity. The majority of galaxies contain a supermassive black hole at their centre.


Our galaxy is called the Milky Way. The closest galaxy to it is Andromeda. Other well-known galaxies include Messier 87 and Messier 101.


The size of galaxies is measured in light-years, which is the distance light can travel in one year. Most galaxies are between 3,000 and 300,000 light-years in diameter.


The mass of galaxies is measured using a unit called solar mass, which is the mass of our Sun. Galaxies can weigh anywhere from a few million solar masses to several hundred trillion solar masses.


Galaxies are filled with different objects that vary in temperature. They are mostly empty space, though, so the average temperature of any given spot in a galaxy is likely to be quite cold.


Galaxies can look quite different depending on what type they are and what they contain. Spiral galaxies look like whirlpools of stars with a brighter spherical centre, and tend to be bluer. Elliptical galaxies look like spheres of stars, and tend to be redder. Galaxies can contain spiral arms, discs, bars, bulges, dust trails and rings.


Galaxies are found scattered throughout the universe. They are often found in clusters that can include thousands of galaxies.

The Andromeda Galaxy is the most distant object visible with the naked eye. Located about 2.5 million light-years from Earth, our huge galactic neighbour – which contains about a trillion stars – can be seen in the night sky at different times of the year. Use our stargazing tips to try spotting it tonight!

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