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.
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.
An emission nebula is a 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.
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 Nebula and the Coalsack Nebula.
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 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.
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.