Types of Deep Sky Objects (DSOs)

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. Dark nebulae do not emit or reflect light, but can be seen if they are in front of bright nebulae.

Planetary nebulae 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.

Galaxies contain hundreds of billlions of stars, and are millions of light-years from Earth. They sometimes are gravitationally bound with other galaxies to form clusters.

Open star clusters are loosely bound by mutual gravitational attraction.

Globular clusters are spheroidal conglomerations of stars, and can contain more than a million stars in a single cluster.