A vast system of billions of stars, gas and dust held together by gravity. The Andromeda Galaxy (M31), 2.5 million light-years away, is visible to the naked eye as an elongated smudge. In a telescope most look like faint grey glows: their light set off millions of years ago.
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Related terms
- Milky Way
Our galaxy, a disc about 100,000 light-years across holding more than 100 billion stars; the Sun lies about halfway out. Seen from inside, the disc forms the milky band across dark night skies. It is brightest in summer towards Sagittarius, where the galactic centre lies.
- Surface brightness
The brightness of an extended object spread over its area, in magnitudes per square arcsecond. A large magnitude 9 galaxy can be harder to see than a small magnitude 11 nebula. It is what matters when judging how visible a deep-sky object will be.
- Messier catalogue
The list of 110 deep-sky objects, M1 to M110, that the comet hunter Charles Messier published from 1774 so as not to mistake them for comets (the last entries were added later). It gathers the finest clusters, nebulae and galaxies visible from mid-northern latitudes. It is the ideal observing programme for beginners.
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