Messier 28

Messier 28
Globular cluster Messier 28 in Sagittarius
Observation data (J2000 epoch)
ClassIV[1]
ConstellationSagittarius
Right ascension18h 24m 32.89s[2]
Declination–24° 52′ 11.4″[2]
Distance18.26 ± 0.98 kly (5.6 ± 0.3 kpc)[3]
Apparent magnitude (V)6.8[4]
Apparent dimensions (V)11′.2[5]
Physical characteristics
Mass5.51×105[6] M☉
Radius30 ly[7]
VHB15.55 ± 0.10[8]
Metallicity = –1.32[6] dex
Estimated age12.0 Gyr[9]
Notable featuresContains first pulsar discovered in a globular[10]
Other designationsGCl 94, M 28, NGC 6626[2]
See also: Globular cluster, List of globular clusters

Messier 28 or M28, also known as NGC 6626, is a globular cluster of stars in the center-west of Sagittarius. It was discovered by French astronomer Charles Messier in 1764.[11][a] He briefly described it as a "nebula containing no star... round, seen with difficulty in 31⁄2-foot telescope; Diam 2′."[12]

In the sky it is less than a degree to the northwest of the 3rd magnitude star Kaus Borealis (Lambda Sgr). This cluster is faintly visible as a hazy patch with a pair of binoculars[11] and can be readily found in a small telescope with an 8 cm (3.1 in) aperture, showing as a nebulous feature spanning 11.2 arcminutes. Using an aperture of 15 cm (5.9 in), the core becomes visible and a few distinct stars can be resolved, along the periphery. Larger telescopes will provide greater resolution,[5] one of 25 cm (9.8 in) revealing a dense 2′ core, with more density within.[11]

It is about 18,300 light-years away from Earth.[3] It is about 551000 M☉[6] and its metallicity (averaging −1.32 which means more than 10 times less than our own star), coherency and preponderence of older stellar evolution objects, support its dating to very roughly 12 billion years old.[9] 18 RR Lyrae type variable stars have been found within.

It bore the first discovery of a millisecond pulsar in a globular cluster – PSR B1821–24. This was using the Lovell Telescope at Jodrell Bank Observatory, England.[10] A total of 11 further of these have since been detected in it with the telescope at Green Bank Observatory, West Virginia. As of 2011, these number the third-most in a cluster tied to the Milky Way, following Terzan 5 and 47 Tucanae.[13]

  1. ^ Cite error: The named reference hcob849_11 was invoked but never defined (see the help page).
  2. ^ a b c "M 28". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2006-11-16.
  3. ^ a b Cite error: The named reference Oliveira was invoked but never defined (see the help page).
  4. ^ "Messier 28". SEDS Messier Catalog. Retrieved 21 July 2024.
  5. ^ a b Cite error: The named reference inglis2004 was invoked but never defined (see the help page).
  6. ^ a b c Cite error: The named reference apj742_1_51 was invoked but never defined (see the help page).
  7. ^ From trigonometry: radius = distance × sin( diameter_angle / 2 ) = 30 ly
  8. ^ Cite error: The named reference aj121_2_916 was invoked but never defined (see the help page).
  9. ^ a b Cite error: The named reference mnras385_4_1998 was invoked but never defined (see the help page).
  10. ^ a b "JBO - Stars". Jodrell Bank Observatory. Retrieved 2007-06-01.
  11. ^ a b c Cite error: The named reference thompson2007 was invoked but never defined (see the help page).
  12. ^ Cite error: The named reference burnham1979 was invoked but never defined (see the help page).
  13. ^ Cite error: The named reference apj730_2_81 was invoked but never defined (see the help page).


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