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.
Related terms
- Nebula
A cloud of gas and dust in space. Depending on how it shines, or does not, it is called an emission, reflection, dark or planetary nebula. The word once also covered galaxies, whose nature was unknown.
- 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.
← The whole glossary