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M 80
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M 80

February 19, 2005
Globular Cluster M80 (NGC 6093), class II, in Scorpius

Right Ascension: 16 : 17.0 (h:m)
Declination: -22 : 59 (deg:m)
Distance: 32.6 (kly)
Visual Brightness: 7.3 (mag)
Apparent Dimension: 10.0 (arc min)

Discovered 1781 by Charles Messier.

M80 is a fine 8th mag globular. Its 10' angular diameter corresponds to roughly 95 light years linear dimension at its distance of 27,400 light years. Its appearance resembles very much that of a comet.

This dense stellar swarm contains several 100,000s of stars, held together by their mutual gravitational attraction. It is one of the densest globulars in our Milky Way Galaxy. As was found by astronomers from observations with the Hubble Space Telescope in 1999 in the visible and UV part of the electromagnetic spectrum, M80 contains a large number of so-called "Blue Stragglers" in its core, about twice as much as any other globular investigated with the HST. These stars are blue and bright stars which appear near the main-sequence of the Hertzsprung-Russell diagramm, and thus appear more massive and younger than the globular clusters age. The reason is very probably that these stars lost their cooler envelopes in close encounters with other stars. Their large number in M80 indicates an exceptionally high stellar collision rate in the core of this globular cluster.

Globular cluster M80 was one of the original discoveries of Charles Messier, who found it on January 4, 1780, and cataloged it as a "Nebula without a star, .. resembling the nucleus of a comet." William Herschel was the first to resolve it (before 1785), and found it was "one of the richest and most compressed clusters of small stars I remember to have seen."

On May 21, 1860, a nove occurred in M80, completely changing the appearance of this globular cluster for some days. This nova, also designated T Scorpii was discovered by Auwers at Berlin, had mag 7.0 on May 21 and 22, and faded to mag 10.5 on June 16. It was independently seen by Pogson. It was reported that Pogson had seen a rebrightening in early 1864, but this appears improbable, as nobody else could confirm this notion. The maximum brighteness of this nova corresponds to an absolute magnitude of about -8.5, if it was a cluster member. At its maximum, the nova was considerably brighter than the whole cluster !

A second nova occured in globular cluster M14 in 1938 but was only photographically observed, and found years later. A further nova was V 1148 Sagittarii which appeared near NGC 6553, but in this case a physical correlation is uncertain. Other cataclysmic variable observations in globulars are occasionally reported: Early observations of dwarf novae were recorded for M5, M30 and NGC 6712, according to Cecilia Payne-Gaposhkin's book, Stars and Clusters.

In M80, however, investigations with the Hubble Space Telescope have lead to the detection of only two nova-like close binary stars, far fewer than expected theoretically, based on the stellar collision rate.

M80, though not very conspicuous, can be located quite easily as it is situated almost exactly half-way between Antares (Alpha Scorpii) and Graffias (Beta Scorpii), just below the declination parallel of Dschubba (Delta Scorpii). It is seen as a bright but small, round ball with brighter nucleus; its surface brightness decreases to the outer regions. Messier determined a diameter of 2 arc min, but better moderate-sized amateur telescopes will show it as a mottled, nebulous object of size 3-5 arc mins, at best very poorly resolved. A better resolution into stars requires larger aperture telescopes.

In the same low-power field of view, there are two faint variable stars, R and S Scorpii, both discovered in 1854 by J. Chacornac: # R Scorpii; 10.4 to 15.0 mag, period 223 days. # S Scorpii; 10.5 to 14.6 mag, period 117 days. The field of M80, especially to the East and South, exhibits a large number of dark and some bright diffuse nebulae, clouds of interstellar matter.