Messier 55

Messier 55
Globular cluster Messier 55 in Sagittarius
Observation data (J2000 epoch)
ClassXI[1]
ConstellationSagittarius
Right ascension19h 39m 59.71s[2]
Declination−30° 57′ 53.1″[2]
Distance17.6 kly (5.4 kpc)[3]
Apparent magnitude (V)6.3[4]
Apparent dimensions (V)19.0′
Physical characteristics
Mass2.69×105[3] M☉
Radius48 ly[5]
Metallicity = –1.94[3] dex
Estimated age12.3 Gyr[6]
Other designationsM55, NGC 6809, GCl 113, C 1936-310[7]
See also: Globular cluster, List of globular clusters

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]

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]

  1. ^ Cite error: The named reference hcob849_11 was invoked but never defined (see the help page).
  2. ^ a b Cite error: The named reference aj140_6_1830 was invoked but never defined (see the help page).
  3. ^ a b c d e Cite error: The named reference apj742_1_51 was invoked but never defined (see the help page).
  4. ^ "Messier 55". SEDS Messier Catalog. Retrieved April 29, 2022.
  5. ^ From trigonometry: radius = distance × sin( diameter_angle / 2 ) = 17,600 × sin(19′/2) = 48.6 ly.
  6. ^ Cite error: The named reference aaa481_3_701 was invoked but never defined (see the help page).
  7. ^ Cite error: The named reference simbad was invoked but never defined (see the help page).
  8. ^ a b Cite error: The named reference thompson2007 was invoked but never defined (see the help page).
  9. ^ Cite error: The named reference aa60_3_245 was invoked but never defined (see the help page).


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