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.
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Related terms
- Galaxy
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.
- Light pollution
Artificial lighting that brightens the night sky and drowns faint stars, nebulae and the Milky Way. A dome of light spreads for tens of kilometres around towns. It also harms wildlife and health; some towns now switch off street lights in the middle of the night.
- Dark nebula
A dust cloud dense enough to hide the stars and glowing gas behind it. It shows up in silhouette, like the Horsehead in Orion or the Coalsack near the Southern Cross. The dark rifts in the summer Milky Way are made of them.
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