Clusters Messier 7, NGC 6444, NGC 6453

Messier 7
Messier 7: Section of the STScI Digitized Sky Survey POSS2/UKSTU Red [160]

History

Messier 7 is a large and brilliant open star cluster that can easily be seen with the naked eye. He was already mentioned in Ptolemy's catalog and in the Latin translation from the 16th century of Amalgest he appears together with Messier 6 as Girus Zonen nebulosus. The Arabic name Tali al Shaulah is the equivalent of the Latin translation of Ulug Beg's title: «Stella nebulosa quae sequitur aceleum Scorpionis» (which follow the sting of the scorpion). Hevelius includes M 7 in a list published in 1690 and the cluster also appears in W. Derham's short catalog of «nebulous stars» in 1730. M 7 is the southernmost object recorded by Messier.

Physical Properties

From our line of sight, the star cluster M 7 appears projected in front of numerous distant Milky Way stars. The distance is about 600 light years. The central area covers about 30 arcminutes. The cluster contains 80 stars brighter than 10th magnitude in a field of 1.2° diameter. The group is reminiscent of Praesepe (M44) in the constellation Cancer, but a little smaller. Spectral studies showed that the group members brighter than 7.5 magnitudes have already developed out of the main sequence. The age of the cluster is estimated to be around 260 million years - older than the Pleiades, but much younger than globular clusters. The cluster contains eight spectroscopic and at least three visual, but very close binary stars with a component spacing of 0.1 to 0.5 arcseconds. [4]

«Revised New General Catalogue and Index Catalogue» Dr. Wolfgang Steinicke, 2021 [277]
DesignationNGC 6475
TypeOCL (II2r)
Right Ascension17h 53m 50.0s
Declination-34° 47' 36"
Diameter75.00 arcmin
Photographic (blue) magnitude3.5 mag
Visual magnitude3.3 mag
Dreyer DescriptionCl, vB, pRi, lC, st 7…12
IdentificationM 7, OCL 1028, ESO 394-SC9, Ptolemy's cluster

Nearby Star Clusters

In the shadow of M 7 stands the rather inconspicuous open star cluster NGC 6444. Dreyer described the open star cluster somewhat euphorically as very large and very rich in stars of 12th and 13th magnitude. [142] It is really big and has many stars, so many that it hardly stands out from the Milky Way in the DSS section.

NGC 6453, a faint globular cluster, lies to the west in the field of view of M 7. It was first noticed in June 1837 by John Herschel. The core only appears to be about a arcminute in diameter. The globular cluster is moving towards us at 91.16 km/s and is at a distance of 11.6 kpc (37800 light years). [4, 251]

NGC 6444
NGC 6444: Open Cluster. Section of STScI Digitized Sky Survey POSS2/UKSTU Red [160]
NGC 6453
NGC 6453: Globular Cluster. Section of STScI Digitized Sky Survey POSS2/UKSTU Red [160]
«Revised New General Catalogue and Index Catalogue», «Historically Corrected New General Catalogue», Dr. Wolfgang Steinicke, 2021 [277]
NameRADecTypeBmagVmagDimDreyer DescriptionIdentification
NGC 6444 17 49 35.1-34 49 11OCL (III2m)12.00Cl, vL, vRi, st 12…13OCL 1023, ESO 393-SC30
NGC 6453 17 50 51.8-34 35 53GCL (IV)10.27.60cL, iR, pmbM, rGCL 79, ESO 393-SC36

Finder Chart

The open star cluster Messier 7 is fairly easy to track down, especially since it is visible to the naked eye on good nights. It is on the line connecting Al Nasl (γ Sagittarii) and G Scorpii, on the third closer to G Scorpii. To identify the individual small PN, use the close-up on the location card.

Chart Messier 7
Chart created using SkySafari 6 Pro and STScI Digitized Sky Survey. [149, 160]

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
142NGC 2000.0, The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters by J.L.E. Dreyer; edited by Roger W. Sinnott; Sky Publishing Corporation and Cambridge University Press (1988); cdsarc.unistra.fr/viz-bin/cat/VII/118 (2021-02-18)
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
251«Young Radio Pulsars in Galactic Globular Clusters» Jason Boyles, Duncan R. Lorimer, Phil J. Turk, Robert Mnatsakanov, Ryan S. Lynch, Scott M. Ransom, Paulo C. Freire, Khris Belczynski; The Astrophysical Journal, Volume 742, Number 1; DOI:10.1088/0004-637X/742/1/51
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)