The Triangulum Emission Garren Nebula
NGC 604 video by
NASA]]
A
nebula (from
Latin: "cloud"
; pl.
nebulae or
nebulæ, with
ligature or
nebulas) is an
interstellar cloud of
dust,
hydrogen gas,
helium gas and
plasma. 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
planetary system objects.
History
Evidence exist to support that the
Maya may have known about nebulae before the invention of a
telescope. The information which supports this theory comes from a folk tale that deals with the Orion constellation's area of the sky. It mentions that there is a smudge around the glowing fire.
At about 150 A.D.,
Claudius Ptolemaeus (Ptolemy) recorded, in books vii-viii of his
Almagest, that five stars that appeared nebulous. He also noted a region of nebulosity between the
constellations
Ursa Major and
Leo that was not associated with any star. In his 964 AD work the
Book of Fixed Stars,
Abd al-Rahman al-Sufi noted the presence of "a little cloud" where the
Andromeda Galaxy is located.
For reasons unknown, Al-Sufi failed to note the
Orion Nebula, which is at least as prominent as the Andromeda galaxy in the night sky. On November 26, 1610,
Nicolas-Claude Fabri de Peiresc discovered the Orion Nebula using a telescope. This nebula was also observed by
Johann Baptist Cysat in 1618. However, the first detailed study of the Orion Nebula wouldn't be performed until 1659 by
Christian Huygens, who likewise believed himself to be the first person to discover this nebulosity.
In 1715,
Edmund Halley published a list of six nebulae. This number steadily increased during the century, with
Jean-Philippe de Cheseaux compiling a list of 20 (including eight not previously known) in 1746. From 1751–53,
Nicolas Louis de Lacaille cataloged a total of 42 nebulae from the
Cape of Good Hope, with most of them being previously unknown.
Charles Messier then compiled a catalog of 103 nebulae by 1781, although his primary goal in doing so was to avoid the false detection of
comets.
The number of nebulae were then greatly expanded by the efforts of
William Herschel and his sister
Caroline Herschel. Their
Catalogue of One Thousand New Nebulae and Clusters of Stars was published in 1786. A second catalog of a thousand was published in 1789 and the third and final catalog of 510 appeared in 1802. During much of their work, William Herschel believed that these nebulae were merely unresolved clusters of stars. In 1790, however, he discovered a star surrounded by nebulosity and concluded that this was a true nebulosity, rather than a more distant cluster.
Beginning in 1864,
William Huggins examined the spectra of about 70 nebulae. He found that roughly a third of them had the absorption spectra of a gas. The remainder showed a continuous spectrum and thus were thought to consist of a mass of stars.
A third category was added in 1912 when
Vesto Slipher showed that the spectrum of the nebula that surrounded the star
Merope matched the spectra of the
Pleiades open cluster. Thus the nebula radiates by reflected star light.
Slipher and
Edwin Hubble continued to collect the spectra from a number of diffuse nebulae, finding 29 that showed emission spectra and 33 had the continuous spectra of star light.
In 1922, Hubble announced that nearly all nebulae are associated with stars, and their illumination comes from star light. He also discovered that the emission spectrum nebulae are nearly always associated with stars having spectral classifications of B1 or hotter (including all
O-type main sequence stars), while nebulae with continuous spectra appear with cooler stars. Both Hubble and
Henry Norris Russell concluded that the nebulae surrounding the hotter stars is transformed in some manner.
Formation
Many nebulae form from the
gravitational collapse of gas in the
interstellar medium or ISM. As the material collapses under its own weight, massive stars may form in the center, and their
ultraviolet radiation ionises the surrounding gas, which creates plasma, making it visible at optical
wavelengths. Examples of these types of nebulae are the
Rosette Nebula and the
Pelican Nebula. The size of these nebulae, known as HII regions, varies depending on the size of the original cloud of gas. These are sites of where star formation occurs. 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 materials thrown off from the
supernova explosion are ionized by the energy and the compact object that it can produce. One of the best examples of this is the
Crab Nebula, in
Taurus. The supernova event was recorded in the year 1054 and is designated as
SN 1054. The compact object that was created after the explosion lies in the center of the Crab Nebula and is a
neutron star.
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. The nebula is 97%
Hydrogen and 3%
Helium with trace materials. The main goal in this stage is to achieve equilibrium.
Types of nebulae
Classical types
Nebulae are classified in four major groups. In the past galaxies and
globular clusters were thought to be other types of nebulae. Spiral nebula were used to explain the spiral structures of
galaxies.
- H II regions, which encompass diffuse nebulae, bright nebulae, and reflection nebulae.
This classification does not encompass all known cloud-like structures. An example is a
Herbig–Haro object.
Diffuse nebulae
The diffuse nebulae near the stars are examples of
reflection nebula.
Most 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.
Planetary nebulae
Planetary nebulae are nebulae that form from the gaseous shells that are ejected from low-mass asymptotic giant branch stars 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 ionized. 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 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 forms 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
electrons oscillating within
magnetic fields.
Notable named nebulae
Nebula catalogs
See also
