# Messier 28

Messier 28
Messier 28 by Hubble Space Telescope; 2.5′ view
Credit: NASA/STScI/WikiSky
Observation data (J2000 epoch)
Class IV[1]
Constellation Sagittarius
Right ascension 18h 24m 32.89s[2]
Declination –24° 52′ 11.4″[2]
Distance 17.9 kly (5.5 kpc)[3]
Apparent magnitude (V) +7.66[2]
Apparent dimensions (V) 11′.2[4]
Physical characteristics
Mass 5.51×105[3] M
VHB 15.55 ± 0.10[6]
Metallicity ${\displaystyle {\begin{smallmatrix}\left[{\ce {Fe}}/{\ce {H}}\right]\end{smallmatrix}}}$ = –1.32[3] dex
Estimated age 12.0 Gyr[7]
Notable features Contains first pulsar discovered in a globular[8]
Other designations M 28, NGC 6626, GCl 94[2]
See also: Globular cluster, List of globular clusters

Messier 28 (also known as M28 or NGC 6626) is a globular cluster in the constellation Sagittarius. It was discovered by French astronomer Charles Messier on July 27, 1764.[9] He briefly described it as a "nebula containing no star... round, seen with difficulty in 3½-foot telescope; Diam 2′." The 2′ at the end indicates an angle of two arcminutes.[10]

In the sky it is less than a degree to the northwest of the 3rd magnitude star Kaus Borealis. This cluster is faintly visible as a hazy patch with a pair of binoculars[9] and can be readily found in a small telescope with a 8 cm (3.1 in) aperture, showing as a nebulous feature spanning 11.2 arcminutes. At 15 cm (5.9 in), the core becomes visible and a few individual stars can be resolved along the periphery. Larger telescopes will provide greater resolution,[4] with a 25 cm (9.8 in) telescope revealing a 2′ core.[9]

M28 is at a distance of about 17,900 light-years away from Earth.[3] It has a combined 551,000[3] times the mass of the Sun and is 12 billion years old.[7] 18 RR Lyrae type variable stars have been observed in this cluster. In 1986, M28 became the first globular cluster where a millisecond pulsar, PSR B1821–24, was discovered with the Lovell Telescope at Jodrell Bank Observatory.[8] A total of 11 additional millisecond pulsars have since been detected in the cluster with the Green Bank Telescope. As of 2011, this is the third largest known population of pulsars in a cluster following Terzan 5 and 47 Tucanae.[11]

Messier 28 on 2MASS; wide angle
Map showing location of M28 (Roberto Mura)

## References

1. ^ Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin (849): 11–14, Bibcode:1927BHarO.849...11S.
2. ^ a b c d "SIMBAD Astronomical Database". Results for NGC 6626. Retrieved 2006-11-16.
3. Boyles, J.; et al. (November 2011), "Young Radio Pulsars in Galactic Globular Clusters", The Astrophysical Journal, 742 (1): 51, arXiv:1108.4402, Bibcode:2011ApJ...742...51B, doi:10.1088/0004-637X/742/1/51.
4. ^ a b Inglis, Mike (2004), Astronomy of the Milky Way: Observer's guide to the northern sky, Patrick Moore's Practical Astronomy Series, 1, Springer, p. 21, ISBN 1852337095.
5. ^ distance × sin( diameter_angle / 2 ) = 30 ly radius
6. ^ Testa, Vincenzo; et al. (February 2001), "Horizontal-Branch Morphology and Dense Environments: Hubble Space Telescope Observations of Globular Clusters NGC 2298, 5897, 6535, and 6626", The Astronomical Journal, 121 (2): 916–934, Bibcode:2001AJ....121..916T, doi:10.1086/318752
7. ^ a b Koleva, M.; et al. (April 2008), "Spectroscopic ages and metallicities of stellar populations: validation of full spectrum fitting", Monthly Notices of the Royal Astronomical Society, 385 (4): 1998–2010, arXiv:0801.0871, Bibcode:2008MNRAS.385.1998K, doi:10.1111/j.1365-2966.2008.12908.x
8. ^ a b "JBO - Stars". Jodrell Bank Observatory. Retrieved 2007-06-01.
9. ^ a b c Thompson, Robert Bruce; Thompson, Barbara Fritchman (2007), Illustrated Guide to Astronomical Wonders, Diy Science, O'Reilly Media, Inc., p. 402, ISBN 0596526857.
10. ^ Burnham, Robert (1979), Burnham's celestial handbook: an observer's guide to the universe beyond the solar system, Dover books explaining science, 3 (2nd ed.), Dover Publications, p. 1609, ISBN 0486236730
11. ^ Bogdanov, Slavko; et al. (April 2011), "Chandra X-ray Observations of 12 Millisecond Pulsars in the Globular Cluster M28", The Astrophysical Journal, 730 (2): 81, arXiv:1101.4944, Bibcode:2011ApJ...730...81B, doi:10.1088/0004-637X/730/2/81