Types of Nebula and Nebulae
A Nebula (or Nebulae plural) is a mixture of interstellar dust, hydrogen gas, helium gas and plasma. The name comes from the latin meaning cloud.
The term nebula was originally used to describe any extended object, including galaxies, outside the Milky Way until galaxies were discovered by Edwin Hubble in 1923.
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.
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 too distant to be resolved as stars were also classified as nebulosity.
Planetary nebula M57 (the Ring Nebula) in Lyra.
An emission nebula is a cloud of ionized gas or plasma which is emitting light at various wavelengths. The most common source for ionization 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 ionizing the gas.
The color depends on its chemical composition and degree of ionization. Due to the prevalence of hydrogen in interstellar gas, and its relatively low energy of ionization, many emission nebulae appear red due to the strong emissions of the Balmer series. If more energy is available, other elements will be ionized 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.
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 right) where the diffuse nebulae near the stars are examples of reflection nebula. These irregular clouds consist of interstellar matter, gas and dust within which stars can be born.
Dark nebulae 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 emission. They are usually very dense and can obscure the light from stars behind them. A good example is the Horsehead and the Coalsack.
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)
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.
The Crab Nebula is an example of a 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 ionized gas. Much of the optical and X-ray emissions originate from the ionized 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.