An Etymological Dictionary of Astronomy and Astrophysics
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The best known → ionized nebula and one of the nearest regions to the Sun in which stars are presently being formed. It is visible to the naked eye in the constellation → Orion south of → Orion's Belt as a fuzzy patch. It lies about 1,500 → light-years away and measures about 30 light-years across. The Orion Nebula is ionized and made visible by a small group of → O-type and → B-type stars known as the → Trapezium cluster. Other designations: M 42, NGC 1976. See also: → Orion molecular cloud; → Huygens Region ; → Orion association; → Orion Bar; → Orion Bright Bar.

→ Orion; → nebula.

A → planetary nebula in the constellation → Ursa Major, one of the four planetary nebulae in → Messier catalog. It is one of the more complex planetary nebulae known. Its appearance has been interpreted as that of a cylindrical torus shell viewed obliquely, so that the projected matter-poor ends of the cylinder correspond with the Owl's eyes. Also known as M97 or NGC 3587.

Discovered by Pierre Méchain in 1781, the name goes back to Lord Rosse, who first used it in 1848. → owl; → nebula.

An → H II region, also known as IC 5067 and IC 5070, about 2,000 → light-years away in the constellation → Cygnus. It is part of a much larger, complex star-forming region also containing the larger and bright → North America Nebula.

So named because of its resemblance to a pelican on long exposure images. M.E. pellican; O.E. pellicane, from L.L. pelecanus, from Gk. pelekan "pelican:" → nebula.

A small part of the → Vela supernova remnant with a narrow appearance. The Pencil Nebula measures about 0.75 → light-years across, is about 5 light-years long, and lies about 800 light-years from Earth. It is moving through the → interstellar medium at about 650 000 kilometres per hour.

→ pencil; → nebula.

An extended complex of → molecular clouds in the constellation → Ophiuchus, apparently shaped like a smoker's pipe, about 5° in size and located about 5° away from the → Galactic center. It has a mass of 10⁴ solar masses, lies at a distance of about 130 pc, and is among the closest molecular clouds to Earth. The Pipe Nebula includes a number of → dark nebulae, including Barnard 59, 65, 66, 67, 77, 244, and 256. It is of particular interest because of the almost complete lack of → star formation within it. This cloud is an extremely rare example of a relatively massive molecular cloud that may be in a state of evolution prior to the onset of significant star-forming activity.

→ pipe; → nebula.

A hot envelope of gas ejected from a central evolved star before becoming a → white dwarf. At the end of the → asymptotic giant phase the pulsating → red giant star is surrounded by an extended shell formed by the material ejected from it. As the evolved star contracts, its → effective temperature rises considerably. When it reaches about 30,000 K, the radiated photons become energetic enough to ionize the atoms in the nebula. The nebula becomes then visible in the optical. It shines essentially in a few → emission lines, produced by cascades during recombination or by collisional excitation with electrons. The central stars of planetary nebulae, → CSPNe, are typically 0.6 to 0.8 solar masses. They have → main sequence masses in the range 1 to 8 solar masses, with an average mass of 2.2 solar masses for a standard → initial mass function. Thus a total of about 1.6 solar masses is in average lost during the → AGB and planetary nebula phases. The life-time of planetary nebulae is relatively short. A typical planetary nebula lasts only a few 10,000 years.

→ planetary; → nebula. The name comes from the fact that these objects appear as planetary disks in a low-resolution telescope. The first planetary nebula, designated NGC 7009 or the → Saturn Nebula, was discovered in 1782 by the German-born English astronomer William Herschel (1738-1822), who described it as "planetary nebula."

An evolved star whose → planetary nebula has dissipated.

→ post-; → planetary; → nebula; → star.

A short-lived transition object between the → asymptotic giant branch (AGB) and → planetary nebula phases. See also → post-asymptotic giant branch star (post-AGB).

→ pre-; → planetary; → nebula. The more commonly used term, → protoplanetary nebula, is a misnomer and must be avoided. Indeed → protoplanetary is widely used to refer to disks around → pre-main sequence stars. Since the term → protoplanet is used to denote planets undergoing formation, the use of the term "protoplanetary nebula" to mean a completely different kind of object is an unfortunate choice (Sahai et al. 2005, ApJ 620, 948).

→ preplanetary nebula.

→ proto-; → planetary; → nebula.

Same as → plerion.

→ pulsar; → wind; → nebula.

A type of nebula that is visible from its reflection of starlight. Bright stars near reflection nebulae emit light into the region that is reflected by the large amount of dust there. The size of the dust grains causes blue light to be reflected more efficiently than red light, so these reflection nebulae frequently appear blue in color.

→ reflection; → nebula.

A bright → planetary nebula in the constellation → Lyra, also called M57 or NGC 6720. In small telescopes it has the appearance of a slightly elliptical luminous ring around a central hot star (15th magnitude). The radius is one-third of a → light-year, and the nebula is about 2,000 light-years away.

→ ring; → nebula.

A giant H II region of about 1° in diameter, lying about 5000 light-years away in the Milky Way, the constellation → Monoceros. It is ionized by the cluster NGC 2244, a group of hot young stars at the center of the nebula. Also called M16, the brighter portions of the nebula have been assigned different NGC numbers: 2237, 2238, 2239, and 2246.

Rosette "a rose-shaped ornament," from Fr. rosette, from O.Fr. rosette, diminutive of rose "rose;" L. rosa, probably from Gk. wrodon (Aeolic), then rhodon, a loan from Iranian, as below; → nebula.

Miq, → nebula; golsân "resembling rose, flower," from gol "flower, rose," variants vard (sohre-vard "red rose"), Semnâni dialect vela "rose;" Mid.Pers. *vard, gul, loaned in Arm. vard and Ar. ward; Av. varəδa- "rose;" loaned in Gk. wrodon (Aeolic), then rhodon; + -sân "manner, semblance" (variant sun, Mid.Pers. sân "manner, kind," Sogdian šôné "career").

A planetary nebula in the Aquarius constellation discovered by William Herschel in 1782. It has a size of about 0.3 x 0.2 light-years and lies about 1400 light-years away. Also known as NGC 7009.

→ Saturn, such named by Lord Rosse in the 1840s, because the object has a vague resemblance to the planet Saturn in low-resolution telescopes; → nebula.

The cloud of interstellar gas and dust from which the Sun and the rest of the solar system initially formed.

→ solar; → nebula.

An obsolete term used to describe nebular objects with spiral shape before it was understood that they are independent galaxies lying outside our → Milky Way galaxy. Now called → spiral galaxy.

→ spiral; → nebula.

Same as → Omega Nebula.

→ swan; → nebula.

The largest and brightest → H II region in the → Large Magellanic Cloud. This → giant H II region has a diameter of over 800 → light-years, and contains half a million → solar masses of ionized gas. The ionization is produced by several clusters of → O-type and → B-type stars, including the very powerful and compact cluster → R136 near its centre. The nebula's name comes from its spider-like shape. Also known as → 30 Doradus and NGC 2070.

Tarantula "any of several large, hairy spiders of the family Theraphosidae," from M.L. tarantula, from It. tarantola, from Taranto "seaport city in southern Italy in the region where the spiders are frequently found," from L. Tarentum, from Gk. Taras; → nebula.

Miq, → nebula; roteyl "large, hairy spider, tarantula."

A large luminous → H II region in the constellation → Sagittarius. Also known as M20, NGC 6514. Conspicuous → dust lanes radiating from the center appear to divide the nebula in three → lobes. It is a combined → emission nebula and → reflection nebula, extending for nearly 30' on the sky. Its estimated distance is 4100±200 → light-years (Kuhn et al., 2018, arXiv:1807.02115).

Trifid, from L. trifidus "split in three," from → tri- + fidus "divided," from findere "to split;" → nebula.

Miq, → nebula; sepâré "split in three," from sé, → three, + pâré "piece, part, portion, fragment;" Mid.Pers. pârag "piece, part, portion; gift, offering, bribe;" Av. pāra- "debt," from par- "to remunerate, equalize; to condemn;" PIE *per- "to sell, hand over, distribute; to assigne;" cf. L. pars "part, piece, side, share," portio "share, portion;" Gk. peprotai "it has been granted;" Skt. purti- "reward;" Hitt. pars-, parsiya- "to break, crumble."

A part of the → Cygnus loop, an old → supernova remnant.

→ veil; → nebula.