Deep Sky Objects (DSO)

Emission nebulae are vast clouds of gas glowing because radiation from nearby hot stars energises them. Hydrogen produces much of the characteristic red glow seen in astronomical photographs. Many are stellar nurseries, where fresh stars are forming inside clouds that can stretch across many light-years of space.

Supernova remnants are the remains of stars that ended their lives in enormous explosions. The expanding material crashes into surrounding gas, creating glowing filaments, shells and delicate structures. The Veil Nebula is a famous example and forms part of the much larger Cygnus Loop in the constellation Cygnus.

Despite the name, planetary nebulae have nothing to do with planets. They form when stars rather like our Sun approach the end of their lives and shed their outer layers into space. The Dumbbell Nebula is one of the best-known examples, with a small hot stellar core at its centre.

Reflection nebulae do not produce their own visible light. Instead, fine dust reflects and scatters light from nearby stars. They often appear blue in photographs, because shorter blue wavelengths scatter more strongly. The beautiful blue nebulosity around the Pleiades is a familiar and striking example.

Some deep sky objects are visible because they block light. Dense clouds of dust appear as dark shapes against stars or glowing gas behind them. The famous Horsehead Nebula is a dark cloud, catalogued as Barnard 33, silhouetted against the red glow of IC 434.

Galaxies are enormous collections of stars, gas and dust held together by gravity. Our Solar System lies inside the Milky Way. The light from the Andromeda Galaxy has travelled about 2.5 million years to reach us, hinting at the extraordinary scale involved when we photograph another galaxy.

Star clusters are groups of stars associated with one another. Open clusters are usually loose, young gatherings, while globular clusters are ancient and densely packed. The Hercules Globular Cluster, M13, contains hundreds of thousands of stars gathered into a roughly spherical cluster orbiting the Milky Way.

Markarian's Chain, in the Virgo Cluster, shows this beautifully. Many of the apparently small objects in such an image are entire galaxies, each containing millions or billions of stars and lying many millions of light-years away.