A of stars. There are two types of

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.

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Definition

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
carina

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.

Planetary Nebulae

Then there are planetary nebulae. Despite their name, planetary nebula are not planets. These nebulae occur near the end of a star’s life. A dying star ejects its outer layers in waves of glowing gas over a few thousand years.

It is possible that this is the fate of our sun in about 5 billion years.

Lesson Summary

Nebulae are large interstellar gas clouds that are the breeding grounds for star formation. Nebulae can form from supernovas, i.e. the explosion of large stars. These explosions, like one seen in 1054 AD, send gas and dust across the universe for the process to start over again.

Terms to Know

  • Nebula: interstellar cloud of dust and gases
  • Diffuse Nebula: a nebula with no clear boundaries
  • Reflection Nebula: type of diffuse nebula that becomes visible when stars reflect their light on it
  • Emission Nebula: type of diffuse nebula that gives off heat detectable by telescopes
  • Dark Nebula: a nebula that blocks the light of whatever is behind it
  • Planetary Nebula: a nebula that occurs at the end of a star’s life

Learning Outcomes

After completing this lesson, you should be ready to:

  • Define nebula and understand how nebulae are formed
  • Identify and describe several different types of nebulae
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