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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.
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NEBULA TICKETS
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Formation
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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 before galaxies were known, they were classified as spiral nebulae.
- H II regions, which encompass diffuse nebulae, bright nebulae, and reflection nebulae.
- Planetary nebulae
- Supernova remnant
- Dark nebula
This classification did not encompass all known cloud-like structures. An example is a
Herbig–Haro object.
Diffuse nebulae
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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.
[1] 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.
[2] 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.
[3]
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.
[4] 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
- Ant Nebula
- Barnard's Loop
- Boomerang Nebula
- Cat's Eye Nebula
- Crab Nebula
- Eagle Nebula
- Eskimo Nebula
- Eta Carinae Nebula
- Helix Nebula
- Hourglass Nebula
- Horsehead Nebula
- Orion Nebula
- Pelican Nebula
- Red Square Nebula
- Ring Nebula
- Rosette Nebula
- Tarantula Nebula
;Nebula catalogs
- Sharpless catalog
- Gum catalog
- RCW catalog
See also
- H I region
- H II region
- List of diffuse nebulae
- Magellanic Clouds
- Messier object
- Orion Molecular Cloud Complex
- Timeline of the interstellar medium and intergalactic medium
References
- The Messier Catalog: Diffuse Nebulae
- The Physical Universe
- Astronomy: a beginner's guide to the universe
- A Starfish Preplanetary Nebula: IRAS 19024+0044