Messier 68

This article is about the astronomical object. For other uses, see M68 (disambiguation).

Messier 68
M68 from Hubble Space Telescope in 2008; 3.32′ view
Observation data (J2000 epoch)
ClassX[1]
ConstellationHydra
Right ascension12h 39m 27.98s[2]
Declination–26° 44′ 38.6″[2]
Distance33.6 kly (10.3 kpc)[3]
Apparent magnitude (V)7.8[4]
Apparent dimensions (V)11.0′
Physical characteristics
Mass2.23×105[3] M☉
Radius53.5 ly[5]
Metallicity = –2.23[3] dex
Estimated age11.2 Gyr[6]
Notable featuresRelatively metal poor.[3]
Other designationsM68, NGC 4590, GCl 20[7]
See also: Globular cluster, List of globular clusters

Messier 68 (also known as M68 or NGC 4590) is a globular cluster found in the east south-east of Hydra, away from its precisely equatorial part. It was discovered by Charles Messier in 1780. William Herschel described it as "a beautiful cluster of stars, extremely rich, and so compressed that most of the stars are blended together". His son John noted that it was "all clearly resolved into stars of 12th magnitude, very loose and ragged at the borders".[8]

M68 is centred about 33,600 light-years away from Earth. It is orbiting our galaxy's galactic bulge with a great eccentricity of 0.5. This takes it to 100,000 light years from the center.[8] It is one of the most metal-poor globular clusters, which means it has a paucity of elements other than hydrogen and helium. The cluster may be undergoing core-collapse, and it displays signs of being in rotation. The cluster may have been acquired in its gravitational tie to the Milky Way through accretion from a satellite galaxy.[9]

As of 2015, 50 variable stars have been identified in this cluster; the first 28 being identified as early as 1919–20 by American astronomer Harlow Shapley. Most of the variables are of type RR Lyrae, or periodic variables. Six of the variables are of the SX Phoenicis variety, which display short pulsating behavior.[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 Cite error: The named reference apj742_1_51 was invoked but never defined (see the help page).
  4. ^ "Messier 68". SEDS Messier Catalog. Retrieved 30 April 2022.
  5. ^ distance × sin( diameter_angle / 2 ) = 53.5 ly. radius
  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 stoyan2008 was invoked but never defined (see the help page).
  9. ^ a b Cite error: The named reference aa578_A128_23 was invoked but never defined (see the help page).