Messier 55

Messier 55 (also known as M55, NGC 6809, or Specter Cluster) is a globular cluster in the south of the constellation Sagittarius. It was discovered by Nicolas Louis de Lacaille in 1752^[a] while observing from what today is South Africa.^[8] Starting in 1754, Charles Messier made several attempts to find this object from Paris but its low declination meant from there it rises daily very little above the horizon, hampering observation.^[b] He observed and catalogued it in 1778. The cluster can be seen with 50 mm binoculars; resolving individual stars needs a medium-sized telescope.^[8]

Messier 55
Globular cluster Messier 55 in Sagittarius
Observation data (J2000 epoch)
Class	XI[1]
Constellation	Sagittarius
Right ascension	19h 39m 59.71s[2]
Declination	−30° 57′ 53.1″[2]
Distance	17.6 kly (5.4 kpc)[3]
Apparent magnitude (V)	6.3[4]
Apparent dimensions (V)	19.0′
Physical characteristics
Mass	2.69×105[3] M☉
Radius	48 ly[5]
Metallicity	${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = –1.94[3] dex
Estimated age	12.3 Gyr[6]
Other designations	M55, NGC 6809, GCl 113, C 1936-310[7]
See also: Globular cluster, List of globular clusters

It is about 17,600 light-years away from Earth. It contains about 269,000 solar masses (M_☉).^[3] As with other Milky Way globular clusters, it has few elements other than hydrogen and helium compared to the Sun. Messier 55 therefore has "low metallicity". This quantity is normally listed as the base 10 logarithm of the proportion of the Sun; for NGC 6809 the metallicity is given by: [Fe/H] = −1.94 dex, whereby −2 would be 100 times less iron than the Sun.^[3] This means the cluster has 1.1% of the proportion of the Sun's iron compared to hydrogen and helium.

Only about 55 variable stars have been found in the central part of M55.^[9]

Gallery

• 
  Messier 55, wide field view
• 
  Map showing the location of M55

See also

• List of Messier objects