Huge clouds of dust and gas, nebulae are often the birthplace of stars. From the Latin word for cloud, the word “nebula” originally referred to distant galaxies, but today, the term is generally reserved for the dust and gas clouds that often harbor forming stars. The gas in nebulae is predominantly hydrogen, and the size of nebulae can range from one light-year across to several hundred. There are two primary types of nebulae: diffuse and planetary, both of which are also called gaseous nebulae.

Diffuse Nebulae

Diffuse nebulae are the largest kind, and some have enough material to form 100,000 stars the size of the Sun. Within this category are several kinds of nebulae, including:

Emission nebulae: An emission nebula glows, and forms near a hot, bright star. Ultraviolet light from the star excites hydrogen atoms, allowing the mass to glow. Emission nebulae are frequently home to forming stars--as gravity causes some of the nebula’s dust and gas to pull together, it contracts into a smaller, denser body, and after millions of years, the mass reaches a hot enough temperature to shine.

Reflection nebulae: A reflection nebula forms near a cool star, whose ultraviolet light is too weak to interact with the nebula’s hydrogen atoms. However, the dust particles in the nebula reflect the star’s light, generally forming a blue light. Reflection nebulae are usually the sites of star formation, and are often seen alongside emission nebulae.

Dark nebulae: Dark nebulae are dust clouds that block out any stars or other bodies behind them. A dark nebula has a similar physical structure as a reflection nebula, and are often a few hundred light-years across. Dark nebulae form in areas with no stars nearby, and are not easily visible because they don’t reflect or emit much light.

Planetary Nebulae

The second category, planetary nebulae, are not related to planets, but earned the name because they resemble planets when viewed through a small telescope. They occur when the star they surround begins throwing off its outer atmosphere. Planetary nebulae form from gas ejected by hot stars, such as red giants or supergiants, near the end of their lives.