The Triangulum Emission Nebula NGC 604A nebula (from
Latin: "mist"; pl. nebulae or nebulæ, with
ligature) is an
interstellar cloud of
dust,
hydrogen gas and
plasma. It is the first stage of a
star's cycle. Originally nebula was a general name for any extended
astronomical object, including
galaxies beyond the
Milky Way (some examples of the older usage survive; for example, the
Andromeda Galaxy was referred to as the
Andromeda Nebula before galaxies were discovered by
Edwin Hubble). Nebulae often form star-forming regions, such as in the
Eagle Nebula. This nebula is depicted in one of
NASA's most famous images, 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 form stars. The remaining materials are then believed to form
planets, and other
solar system objects.
Many nebulae form from the
gravitational collapse of diffuse gas in the
interstellar medium or ISM. As the material collapses under its own weight, massive stars may form in the centre, and their
ultraviolet radiation ionises the surrounding gas, making it visible at optical
wavelengths. An example of this type of nebula is the
Rosette Nebula or the
Pelican Nebula. The size of these nebulae, known as HII regions, varies depending on the size of the original cloud of gas, and the number of stars formed can vary too. As the sites of star formation, the formed stars are sometimes known as a young, loose cluster.
Some nebulae are formed as the result of
supernova explosions, the death throes of massive, short-lived stars. The material thrown off from the
supernova explosion is ionised by the supernova remnant. One of the best examples of this is the
Crab Nebula, in
Taurus. It is the result of a recorded supernova in the year 1054 and at the centre of the nebula is a
neutron star, created during the explosion.
Other nebulae may form as
planetary nebulae. This is the final stage of a low-mass star's life, like
Earth's
Sun.
Stars with a
mass up to 8-10 solar masses evolve into
red giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost a sufficient amount of material, its temperature increases and the ultraviolet radiation it emits is capable of
ionizing the surrounding nebula that it has thrown off.
Diffuse nebulaeMost nebulae can be described as
diffuse nebulae, which means that they are extended and contain no well-defined boundaries. In visible light these nebulae may be divided into
emission nebulae and
reflection nebulae, a categorization that depends on how the light we see is created. Emission nebulae contain
ionized gas (mostly ionized
hydrogen) that produces
spectral line emission. These emission nebulae are often called
HII regions; the term "HII" is used in professional astronomy to refer to ionized hydrogen. In contrast to emission nebulae, reflection nebulae do not produce significant amounts of visible light by themselves but instead reflect light from nearby stars.
Dark nebulae are similar to diffuse nebulae, 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 nebulae.
Although these nebulae appear different at optical wavelengths, they all appear to be bright sources of emission at
infrared wavelengths. This emission comes primarily from the
dust within the nebulae.
Specific types of nebulae
While diffuse nebulae have poorly-defined boundaries, a few nebulae may actually be described as discrete objects with identifiable boundaries.
Planetary nebulae
Planetary nebulae are nebulae that form from the gaseous shells that are ejected from low-mass
red giants when they transform into
white dwarfs. These nebulae are emission nebulae with spectral emission that is similar to the emission nebulae found in
star formation regions. Technically, they are a type of HII region because the majority of
hydrogen will be ionised. However, planetary nebulae are denser and more compact than the emission nebulae in star formation regions. Planetary nebulae are so called because the first
astronomers who observed these objects thought that the nebulae resembled the disks of planets, although they are not at all related to planets.
Protoplanetary nebula
A
protoplanetary nebula (PPN) is an astronomical object which is at the short-lived episode during a star's rapid stellar evolution between the late
asymptotic giant branch (LAGB) phase and the subsequent planetary nebula (PN) phase. A PPN emits strong in infrared radiation, and is a kind of reflection nebula. The exact point when a PPN becomes a planetary nebula (PN) is defined by the temperature of the central star.
Supernova remnants
A
supernova occurs when a high-mass star reaches the end of its life. When
nuclear fusion ceases in the core of the star, the star collapses inward on itself. The gas falling inward either rebounds or gets so strongly heated that it expands outwards from the core, thus causing the star to explode. The expanding shell of gas form a
supernova remnant, a special type of
diffuse nebula. Although much of the optical and
X-ray emission from supernova remnants originates from ionized gas, a substantial amount of the
radio emission is a form of non-thermal emission called
synchrotron emission. This emission originates from high-velocity and
electrons oscillating within
magnetic fields.
