Nebulae are clouds of interstellar gas where stars are born as the gaseous material condenses into a tightly packed mass. Emission nebulae have one or more bright stars nearby that cause the gases to emit energy primarily in the reddish hydrogen-alpha part of the spectrum. Reflection nebulae have one or more bright stars nearby but not close enough to cause emission, and only bluish light from the stars is reflected.

These deep sky objects are tiny and not like the large emission and reflection nebulae where stars are born. A planetary nebula is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from a star late in its life, and only exists for a few tens of thousands of years. They are also not related to planets, but were given this name when first discovered in the 1700s because they resembled the size and shape of planets.

Supernovae are powerful explosions of stars when their internal nuclear fuel supplies burn out and the stars quickly collapse, resulting in massive shock waves causing stars to explode.

The Crab Nebula, while quite small and really difficult to photograph without a telescope, is the remnant of the great supernova of 1054 A.D. This supernova was so bright in 1054 that it was visible in the daylight for 23 days, and some medieval rulers added its representation to their coinage as a divine signal for their rule.

Open star clusters are loosely bound by mutual gravitational attraction. Note the blue nebulosity around the stars of the Pleiades. Globular clusters are spheroidal conglomerations of stars, and often contain more than a million stars in a single cluster.