Nebula: Definition & Theory

Instructor: Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

A nebula is a large cloud of gas and dust particles. Nebulae are known as nurseries for stars, while at the same time most are created by the death of stars. This lesson will define and explain nebulae.


A nebula is an interstellar cloud of dust, hydrogen, helium, and other gases. Nebulae (more than one nebula) are often star-forming regions, where gas, dust, and other materials 'clump' together to form larger masses, which attract further matter, and eventually will become massive enough to form stars. The remaining materials are then believed to form planets, and other planetary system objects.

Nebulae can be described as stellar nurseries, meaning stars form out of the gas clouds that make up nebulae. As gravity attracts the hydrogen and helium particles towards one another, they begin to heat as pressure builds. Eventually the size and density of the gas cloud grows so large that the pressure deep down causes the hydrogen atoms to fuse together, creating helium, and resulting in the formation of a star.

The Eagle Nebula contains many new stars.
crab nebula

In 1054 AD, a bright explosion was seen in the sky by Chinese astronomers. Almost 1,000 years later, in 1921, astronomers put the pieces together that the Crab Nebula was the remnants of the supernova that was seen in 1054 AD. Take a look at the picture of the Crab Nebula as seen by the Hubble Telescope.

The Crab Nebula is the remnant of a supernova.
Crab Nebula

Types of Nebulae

Diffuse Nebulae

Nebulae can be described as diffuse when they extend and spread out until there is no clear boundary of where they end. These gas clouds are typically detected in relation to the light of stars. There are two types of diffuse nebulae: reflection nebulae and emission nebulae.

Nebula diffusing out to a thin cloud

Reflection Nebulae

A reflection nebula doesn't emit a large amount of light, but becomes visible when nearby stars reflect their light off of it. These nebulae are usually blue.

Emission Nebulae

If the nebula is dense enough, the gas clouds will emit heat that can be detected by telescopes here on Earth. This occurs when the gas becomes ionized and begins to give off light. These are called emission nebulae and they often appear red in color.

Carina Nebula

Dark Nebulae

Dark nebulae are clouds of dust which are simply blocking the light from whatever is behind them. Physically, they resemble reflection nebulae. Depending on where light from nearby stars is coming from, the nebula may either be considered dark or reflected. Dark nebulae are also seen often in conjunction with reflection and emission nebulae.

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