Messier 55

Messier 55
250px
Observation data (J2000 epoch)
Class	XI
Constellation	Sagittarius
Right ascension	19h 39m 59.71s
Declination	−30° 57′ 53.1″
Distance	17.6 kly (5.4 kpc)
Apparent magnitude (V)	+7.42
Apparent dimensions (V)	19′.0
Physical characteristics
Mass	2.69×105 M☉
Radius	48 ly
Metallicity	–1.94 dex
Estimated age	12.3 Gyr
Other designations	M55, NGC 6809, GCl 113, C 1936-310

File:M55HunterWilson.jpg

Messier 55 with amateur telescope

File:M55map.png

Map showing the location of M55

Messier 55 (also known as M55 or NGC 6809 or "The Summer Rose Star") is a globular cluster in the constellation Sagittarius. It was discovered by Nicolas Louis de Lacaille in June 16, 1752 while observing from South Africa. Starting in 1754, Charles Messier made several attempts to find this object from Paris, France, but the low declination meant it never rose sufficiently far above the horizon to allow for easy observation. He finally observed and catalogued it in 1778. The cluster can be seen with a pair of 50 mm binoculars, although resolving the individual stars requires a medium-sized telescope.

M55 is at a distance of about 17,600 light-years away from Earth. It has a mass of about 269,000 times that of the Sun. As with other Milky Way globular clusters, it has a low abundance of elements other than hydrogen and helium compared to the Sun—what astronomers term the metallicity of the cluster. This quantity is normally listed as the base–10 logarithm of the proportion relative to the Sun; for NGC 6809 the metallicity is given by: [Fe/H] = –1.94 dex. Taking this exponent to the powers of 10 yields an abundance equal to 1.1% of the proportion of such elements in the Sun.

Only about 55 variable stars have been discovered in the central part of M55.

External links

• Messier 55, SEDS Messier pages
• Messier 55, Galactic Globular Clusters Database page
• Messier 55 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images