Globular Cluster Messier 70

Messier 70
Messier 70: Image taken with the Hubble Space Telescope [245]

History

Charles Messier discovered the globular cluster M 70 on August 31, 1780, the same night he checked the discovery of Lacaille (M 69). He published the following note: «Nebula without a star, right next to the previous one [M 69] and on the same parallels: right next to it is a 9th magnitude star and under the nebula four small stars, almost on the same straight line, very close to each other seen through an inverting telescope; the nebula with the same star ε in Sagittarius is determined.» [281]

Physical Properties

The core region of M 70 experiences a phenomenon known as core collapse: More and more stars are pushing their way into the center, so that the star density there is higher than in average globular clusters. The stars in a globular cluster all orbit a common center of mass. Some have almost circular orbits, others in turn strongly elliptical that bring them far outwards. As the stars interact with each other over a longer period of time, the lighter stars gain speed and move to the edge of the cluster. The heavier stars slow down and concentrate towards the center. This effect causes the typical dense center of a globular cluster that has undergone a core collapse. About a fifth of the 150 known globular clusters in our Milky Way had experienced such a core collapse. M 70 is around 70'000 light years away. [215]

«Revised New General Catalogue and Index Catalogue» Dr. Wolfgang Steinicke, 2021 [277]
DesignationNGC 6681
TypeGCL (V)
Right Ascension18h 43m 12.7s
Declination-32° 17' 29"
Diameter8.00 arcmin
Visual magnitude7.8 mag
Dreyer Descriptionglobular, B, pL, R, gbM, st 14…17
IdentificationM 70, GCL 101, ESO 458-SC3

Finder Chart

M 70 is located in the constellation Sagittarius on the lower edge of the «Teapot» in the middle between the two stars Ascella (ζ Sagittarii) and Kaus Australis (ε Sagittarii). The globular cluster M 69 lies on the same declination only 2.5° to the west. In the months of June to August this constellation climbs highest over the southern horizon at night.

Chart M 70
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

References

149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
215Explore - The Night Sky | Hubble’s Messier Catalog; nasa.gov/content/goddard/hubble-s-messier-catalog (2020-12-31)
245Bucking the trend; esahubble.org/images/potw1921a (2021-01-22)
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)
281«Catalogue Nébuleuses et des Amas D'Étoiles» Observées à Paris, par M. Messier, à l'Observatoire de la Marine, hôtel de Clugni, rue des Mathurins. «Connoissance des temps ou connoissance des mouvements célestes, pour l'année bissextile 1784 » Page 227; gallica.bnf.fr/ark:/12148/bpt6k6514280n/f235 (2021-02-21)