Messier 28

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.[9][lower-alpha 1] He briefly described it as a "nebula containing no star... round, seen with difficulty in 3½-foot telescope; Diam 2′."[10]

Messier 28
Globular cluster Messier 28 in Sagittarius
Credit: NASA/ESA/Hubble/J.E. Grindlay et al
Observation data (J2000 epoch)
ClassIV[1]
ConstellationSagittarius
Right ascension18h 24m 32.89s[2]
Declination–24° 52′ 11.4″[2]
Distance17.9 kly (5.5 kpc)[3]
Apparent magnitude (V)+7.66[2]
Apparent dimensions (V)11′.2[4]
Physical characteristics
Mass5.51×105[3] M☉
Radius30 ly[5]
VHB15.55 ± 0.10[6]
Metallicity = –1.32[3] dex
Estimated age12.0 Gyr[7]
Notable featuresContains first pulsar discovered in a globular[8]
Other designationsGCl 94, M 28, NGC 6626[2]

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[9] 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,[4] one of 25 cm (9.8 in) revealing a dense 2′ core, with more density within.[9]

It is about 17,900 light-years away from Earth.[3] It is about 551000 M☉[3] 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.[7] 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.[8] 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.[11]

  • Messier 28 taken by Hubble.[12]
  • 2.5' view of M28 taken by Hubble Space Telescope.
  • Messier 28 on 2MASS; wide angle
  • Map showing location of M28 (Roberto Mura)

References and footnotes

References
  1. Shapley, Harlow; Sawyer, Helen B. (August 1927), "A Classification of Globular Clusters", Harvard College Observatory Bulletin, 849 (849): 11–14, Bibcode:1927BHarO.849...11S.
  2. "M 28". SIMBAD. Centre de données astronomiques de Strasbourg. 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. 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, Bibcode:2003amwn.book.....I, ISBN 978-1852337094.
  5. From trigonometry: radius = distance × sin( diameter_angle / 2 ) = 30 ly
  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. 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. "JBO - Stars". Jodrell Bank Observatory. Retrieved 2007-06-01.
  9. Thompson, Robert Bruce; Thompson, Barbara Fritchman (2007), Illustrated Guide to Astronomical Wonders, Diy Science, O'Reilly Media, Inc., p. 402, ISBN 978-0596526856.
  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 978-0486236735
  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
  12. "Nebulous, but no nebula". www.spacetelescope.org. Retrieved 11 March 2019.

Footnotes
  1. On July 27

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