Messier 20, NGC 6514 - Trifid Nebula

Messier 20 (NGC 6514) is also known as the Trifid Nebula. Named for its three-lobed appearance, it is one of the most famous objects in the sky. This object is an unusual combination of an open cluster of stars, an emission nebula, a reflection nebula, and a dark nebula that divides the emission nebula into three parts.

Discovery and History

M 20 was discovered by Le Gentil before 1750. Charles Messier added it as the 20th entry in his catalog in June, 1764, and described it as "a cluster of stars of 8th to 9th magnitude, enveloped in nebulosity". Messier's remark on nebulosity also includes the nearly open cluster M 21.

The famous three-lobed appearance of M 20 may have caused William Herschel to assign four different numbers to parts of this nebula: H IV.41, H V.10, H V.11, H V.12. Herschel, who normally avoided numbering Messier's objects in his own catalog, may have done so because Messier merely described it as "Cluster of Stars." The name "Trifid" was first used by John Herschel, who assigned only one catalog number to the whole object (h 1991, h 3718, GC 4355), which became NGC 6514 in J.L.E. Dreyer's New General Catalog.

The dark nebulae which give the Trifid its appearance were cataloged by E. E. Barnard as B 85.

Amateur Observation

M 20 is easily spotted in binoculars as an ellipse of haze. Viewed through a small telescope, the Trifid Nebula is a perennial favorite of amateur astronomers. M 20 is situated roughly 2 degrees northwest of the larger Lagoon Nebula (M 8), and even closer to the open cluster M 21; these objects form a nice target for wide field photographs.

The Trifid is nearly the size of the full moon, and contains both reddish emission and bluish reflection nebulosity. The red emission nebula, and young star cluster near its center, is surrounded by a blue reflection nebula which is particularly conspicuous at the northern end. The emission nebula spans a diameter of 15' around the central star. The fainter reflection nebula to the north, surrounding a yellowish magnitude 7.5 star, appears about half this size. Both sections of the nebula are enveloped by a faint outer haze that reaches a diameter of 30', and is more extensive to the east.

The relatively high surface brightness of M 20 provides a good contrast to its three dark, radial dust lanes. The three sections into which the dark lanes divide the nebula are unequal in area; the northern is largest, and the SW the smallest. The dark lanes themselves are unequal in length and width: the NE lane is longest and most distinct; the west lane is broad and short; and the south lane is thin and short. The lanes do not converge directly, but lead to a circular, mottled central area. High power reveals a short and very thin lane headed straight north from the inner half of the west lane.

The emission nebula's central star, HD 196692 or ADS 10991, is not located in the mottled area, but at the tip of the nebula's eastern segment. It is a multiple system of integrated magnitude 7. Its two brightest components (AC, 10.6" apart) each have two faint companions, making a sextuple system of individual magnitudes 7.6 (A), 10.7 (B), 8.7 (C), 10.7 (D), 12.6 (E), 14.0 (F), and 13.4 (G). The presence of these bright stars makes magnitude estimates for the nebula difficult; they vary widely, from 6.8 to 9.0.

Situated on M 20's northern edge is HD 164514. This supergiant, of visual magnitude 7.4 and spectral type A5 Ia, illuminates M 20's blue reflection nebulosity.

Properties and Evolution

M 20 is estimated to lie about 5,200 light years away, on the far side of the same complex of nebulosity that includes the Lagoon Nebula, M 8. The Trifid's exact distance is rather uncertain, with estimates ranging from 2,200 to 9,000 light years. At the value of 5,200 light years adopted here, the Trifid spans a diameter about 10 light years across.

M 20 is only about 300,000 years old, making it among the youngest emission nebulae known. All of its bright central stars are extremely hot, of spectral type O5 to O7. They illuminate a dense pillar of gas and dust, producing a bright rim on the side facing them. Star formation is no longer occurring in the immediate vicinity of the central star cluster, because its intense radiation has blown away the gas and dust from which new stars are made.

In 1997, the Hubble Space Telescope zoomed in on the stellar nursery in the Trifid, being torn apart by radiation from its central stars. It observed evaporating gaseous globules (EGGs) similar to those seen in the Eagle Nebula's "Pillars of Creation" region, as well as stellar jets protruding from young stellar objects deep within the cloud. In 2005, the Spitzer Space Telescope, observing at near infrared wavelengths, discovered 30 embryonic stars and 120 newborn stars in the Trifid Nebula which are invisible in the optical part of the spectrum.

Images of the Trifid Nebula appear on the set of the star ship Enterprise in many episodes of the Star Trek television series.