What Nebula and Nebulae, What are the Types of Nebula?
A look at what nebula and nebulae are, how they are created and the types of nebula we can observe in the universe with examples and photos.

By Tim Trott | Celestial Objects | May 8, 2009

852 words, estimated reading time 3 minutes.

| (0) | (2)

Astronomical Objects

This article is part of a series of articles. Please use the links below to navigate between the articles.

The term nebula was originally used to describe any extended object, including globular clusters, open clusters, and other objects outside the Milky Way, such as galaxies until they were discovered as something else by Edwin Hubble in 1923.

The first nebula was mentioned by the Persian astronomer Abd al-Rahman al-Sufi, in his Book of Fixed Stars (964). He noted "a little cloud" where the Andromeda Galaxy is located. The supernova that created the Crab Nebula, SN 1054, was observed by Arabic and Chinese astronomers in 1054.

Nebula is a singular term, and "nebulae" is the plural of nebula. A Nebula is a mixture of interstellar dust, hydrogen gas, helium gas and plasma. The name comes from the Latin meaning cloud.

The "Pillars of Creation" from the Eagle Nebula imaged by the Hubble Space Telescope

Nebulae are often star-forming regions such as the Eagle Nebula also known as the "Pillars of Creation". In these regions, the formations of gas, dust and other materials 'clump' together to form larger masses, which attract further matter, and eventually will become big enough to give birth to new stars. The remaining materials are believed to go on to form planets and other planetary system objects. These areas are sometimes called stellar nurseries or molecular clouds and are mainly composed of the elements hydrogen and helium.

Let's take a look at the 5 main types of nebula.

Types of Nebula

There are many different types of nebulae and they are classified as being emission, diffuse, reflective or dark after the way they interact with energy from nearby stars. Before their nature was understood, galaxies and star clusters were too distant to be resolved as stars were also classified as types of nebula.

Emission Nebula Type

Planetary nebula M57 (the Ring Nebula) in Lyra.

An emission nebula is a gas cloud of ionised gas or plasma which is emitting light at various wavelengths. The most common source for ionisation comes from high-energy photons emitted by a nearby star. Among the different types of emission nebula are HII regions in which star formation is taking place and young massive stars are the source of the ionising photons. Planetary nebulae consist of a dying star which has thrown off its outer layers, with the exposed hot core then ionising the gas.

The colour depends on its chemical composition and degree of ionisation. Due to the prevalence of hydrogen in interstellar gas, and its relatively low energy of ionisation, many emission nebulae appear red due to the strong emissions of the Balmer series. If more energy is available, other elements will be ionised and green and blue nebulae become possible. By examining the spectra astronomers deduce their chemical content. Most are about 90% hydrogen, with the remainder helium, oxygen, nitrogen, and other elements.

Diffuse Nebulae Type

A deep-sky exposure of the Pleiades star cluster by amateur astronomer Robert Gendler.

Most nebulae can be described as diffuse, which means that they are extended and contain no well-defined boundaries. A good example is the Pleiades (shown left) where the diffuse nebulae near the stars are examples of a reflection nebula. These irregular clouds consist of interstellar matter, gas and dust within which stars can be born.

Dark gas clouds are similar to the diffuse kind, but they are not seen by their emitted or reflected light. Instead, they are seen as dark clouds in front of more distant stars or in front of emissions. They are usually very dense and can obscure the light from the stars behind them. A good example of this type of nebula is the Horsehead Nebula and the Coalsack Nebula.

Reflection Nebulae Type

Reflection nebula IC2118, the Witch Head Nebula, in the Orion constellation.

Reflection nebulae are clouds of dust which reflect the light of a nearby star. The energy from the nearby star is insufficient to ionize the gas to create an emission nebula but is enough to give sufficient scattering to make the dust visible, thus the frequency spectrum shown by reflection is similar to that of the illuminating stars.

Reflection nebulae are usually blue because the scattering is more efficient for blue light than red. This is the same scattering process that gives us blue skies and red sunsets.

Planetary Nebulae Type

The Cat's Eye Nebula (NGC6543) is an example of a planetary nebula.

Planetary nebulae form from the gaseous shells that are ejected from low-mass giant stars when they reach the end of their lives and transform into white dwarfs. These are emission nebulae with spectral emission that is similar to that found in star formation regions.

Supernova Remnant

The last of our types of the nebula is the Supernova Remnant. A supernova occurs when a high-mass star reaches the end of its life. When this happens, the star sheds its outer layers as a superheated cloud of ionised gas. Much of the optical and X-ray emissions originate from the ionised gas, but a substantial amount of the radio emission is a form of non-thermal emission called synchrotron emission which originates within the magnetic field.