Globular Cluster Messier 28

Messier 28
Messier 28: Image taken with the Hubble Space Telescope in visible light, UV and infrared. © ESA/Hubble & NASA [240]

Object Description

The globular cluster is located about 0.8° northwest of the star Kaus Borealis (λ Sagittarii, 2.82 mag). It was first discovered by Charles Messier in July 1764. He described it as a «star-less nebula, round, difficult to see in a 3.5-foot telescope, diameter 2'». Presumably Wilhelm Herschel was the first to break down M 28 into its single stars. His son John wrote the following: «Very bright, round, strongly condensed, resolved into stars 14 ... 15 mag. A beautiful object.»

M 28 is somewhat in the shadow of its bigger brother M 22, which is only 3° away. The amount of dust in the region also contributes to this. It is estimated that this will result in a loss of 2.5 magnitudes. M 28 is one of the more compact and dense globular clusters. The integrated spectral type is F9 and the mean brightness of the brightest 25 stars is 14.73 mag. It is moving away from us at 11.11 km/s. The distance is estimated at 6 kpc (19'600 ly).[4, 145]

The globular cluster is particularly noteworthy as it is the first to have a millisecond pulsar found: PSR B1821-24. This dense neutron star rotates at an insane speed (once every three milliseconds) and emits radiation from its poles, the concentrated beam of which sweeps across the earth like that of a lighthouse. The pulsar was discovered in 1986 with the Lovell radio telescope in England. [240]

Revised+Historic NGC/IC, Version 22/9, © Dr. Wolfgang Steinicke [277]
DesignationNGC 6626
TypeGCL (IV)
Right Ascension18h 24m 32.9s
Declination-24° 52' 10"
Diameter13.8 arcmin
Visual magnitude6.9 mag
Metric Distance5.500 kpc
Dreyer Description!, globular, vB, L, R, geCM, rrr, st 14…16
Identification, RemarksM 28, GCL 94, ESO 522-SC23

Finder Chart

M 28 is really easy to find. It is located in the constellation Sagittarius, about 0.8° northwest of the star Kaus Borealis (λ Sagittarii, 2.82 mag). It can best be seen in the months of June to August. Then the constellation is highest above the southern horizon.

Chart Globular Cluster Messier 28
Globular Cluster Messier 28 in constellation Sagittarius. Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

More Objects Nearby (±15°)

References

4«Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System» by Robert Burnham; Dover Publications, Inc.; Voume I: ISBN 0-486-23567-X; Volume II: ISBN 0-486-23568-8; Volume III: ISBN 0-486-23673-0
145SIMBAD astronomical database; simbad.u-strasbg.fr/simbad
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
240Messier 28; nasa.gov/feature/goddard/2017/messier-28 (2021-01-22)
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)