A nebula (from Latin: "cloud"; pl. nebulae or nebulæ, with ligature or nebulas) is an interstellar cloud of dust, hydrogen gas, helium gas and other ionized gases. 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 massive enough to form stars. The remaining materials are then believed to form planets, and other planetary system objects.

History

Around A.D. 150, Claudius Ptolemaeus (Ptolemy) recorded, in books VII-VIII of his Almagest, 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. The first true nebula, as distinct from a star cluster, was mentioned by the Persian astronomer, Abd al-Rahman al-Sufi, in his Book of Fixed Stars (964). He noted "a little cloud" where the Andromeda Galaxy is located. He also cataloged the Omicron Velorum star cluster as a "nebulous star" and other nebulous objects, such as Brocchi's Cluster. The supernova that created the Crab Nebula, the SN 1054, was observed by Arabic and Chinese astronomers in 1054.

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 also 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 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 was 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 rest 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 many 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 are transformed in some manner.

Formation

Many nebulae or stars 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, 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 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 labelled 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 enough material, its temperature increases and the ultraviolet radiation it emits can ionize the surrounding nebula that it has thrown off. The nebula is 97% Hydrogen and 3% Helium with trace materials.

Types of nebulae

Classical types

Nebulae are classified in four major groups(stars). Galaxies and globular clusters were previously 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.

Planetary nebulae

Supernova remnant

Dark nebula

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 classification 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 differently at optical wavelengths, they are all bright sources of emission at infrared wavelengths. This emission comes chiefly 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 an HII region because most 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 the nebulae resembled the disks of planets, although they are not related to planets. Our Sun is believed to become one of these 12 billion years after the Sun's formation.

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 following planetary nebula (PN) phase. During the AGB phase, the star undergoes mass loss, emitting a circumstellar shell of hydrogen gas. When this phase comes to an end, the star enters the PPN phase.

The PPN is energized by the central star, causing it to emit strong infrared radiation and become a reflection nebula. Collaminated stellar winds from the central star shape and shock the shell into an axially symmetric form, while producing a fast moving molecular wind. The exact point when a PPN becomes a planetary nebula (PN) is defined by the temperature of the central star. The PPN phase continues until the central star reaches a temperature of 30,000 K, after which is it hot enough to ionize the surrounding gas.

Supernova remnants

A supernova occurs when a high-mass star reaches the end of its life. When nuclear fusion in the core of the star stops, the star collapses. 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 diffuse nebula. Although much of the optical and X-ray emission from supernova remnants originates from ionized gas, a great 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

Lagoon Nebula

Orion Nebula

Pelican Nebula

Red Square Nebula

Ring Nebula

Rosette Nebula

Tarantula Nebula

Kronberger 61

;Nebula catalogs

Gum catalog

RCW catalog

Sharpless 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

External links

Fusedweb.pppl.gov

Information on star formation, geocities.com

Clickable table of Messier objects, space-and-telescope.com

NASAN.gov, nebula worldbook

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