Globular Clusters Messier 4, NGC 6144

Messier 4

Messier 4
Messier 4: Image taken with the Hubble Space Telescope [249]

Messier 4 is a beautiful globular cluster, one of the largest and closest to us. It was first observed by P. L. de Cheseaux in 1746. In May 1764 Messier described it as a cluster of very small stars, which with smaller telescopes appears more like a nebula.

Messier 4
Messier 4: Globular cluster in Scorpius; Vixen FL 102s; 40 min auf Kodak Ektar 1000; Bernese Highlands; © 20. 6. 1993 Bernhard Blank, Dragan Mihajlovic

The globular cluster M 4 is one of the closest to the solar system. Distances vary from 5700 to 7500 light years and the cluster is moving away from us at a speed of about 65 km/s. More recent estimates tend to be in the upper range. The population is estimated to be a few hundreds of thousands of stars visible from earth-based telescopes. In addition, around 40'000 white dwarfs are suspected, the weakest of which have about a fortieth of the luminosity of the brightest in the cluster. Despite its relatively large angular diameter, M 4 is one of the smaller globular clusters in our Milky Way.

The Hertzsprung–Russell diagram of this cluster shows that the starlight is reddened by about 0.8 magnitudes due to dark clouds in this region. The brightness of the brightest stars is 10.8 mag and that of the horizontal branch in the HR diagram is 13.4 mag. [4]

Messier 4
Messier 4: HR-Diagram [175]

The globular cluster M 4 was also the target of a study using the Hubble Space Telescope. A large number of «stellar corpses», so-called white dwarfs, were discovered. Using the measurement results, predictions about the cooling rate of white dwarfs can be refined - an important tool to be able to make reliable statements about the age of the Milky Way and the universe. White dwarfs are burned-out cores of collapsed stars that slowly cool down like a piece of embers and then go out. The universe is not yet old enough to accommodate completely burned-out and cold white dwarfs - so-called black dwarfs. Age estimates of the universe vary e.g. From eight to 20 billion (109) years old - paradoxically, the age of M 4 is estimated to be around 14 billion (109) years. M 4 is so old that all stars with at least 80% solar mass have already developed into red giants, followed by a collapse into a white dwarf. [175]

«Revised New General Catalogue and Index Catalogue» Dr. Wolfgang Steinicke, 2021 [277]
DesignationNGC 6121
TypeGCL (IX)
Right Ascension16h 23m 35.5s
Declination-26° 31' 29"
Diameter36.00 arcmin
Visual magnitude5.4 mag
Dreyer DescriptionCl, 8 or 10 B st in line, with 5 st, rrr
IdentificationM 4, GCL 41, ESO 517-SC1

NGC 6144

NGC 6144
NGC 6144: Crop of STScI Digitized Sky Survey [160]

In the shadow of Messier 4 and the red giant Antares, behind the nebula of Sharpless 2-9, one can find NGC 6144, another globular cluster. It was found by Wilhelm Herschel on May 22, 1783 and cataloged under the designation VI 10. Herschel's class «VI» stood for very compressed and rich star clusters. He described it as follows: «A very compact and concentrated, large cluster of the smallest imaginable stars, all of a somber red color. The next step to an easy-to-dissolve nebula.» [463] Dreyer described it as being of «considerable size, highly concentrated, brighter towards the middle and easily resolvable into individual stars». NGC 6144 has a very low central compression with a concentration index of 11 (1 = highest density). [4]

«Revised New General Catalogue and Index Catalogue» Dr. Wolfgang Steinicke, 2021 [277]
DesignationNGC 6144
TypeGCL (XI)
Right Ascension16h 27m 14.1s
Declination-26° 01' 27"
Diameter7.40 arcmin
Visual magnitude9.0 mag
Dreyer DescriptionCl, cL, mC, gbM, rrr
IdentificationIC 4606, GCL 42, ESO 517-SC6
Ophiuchus Molecular Cloud
Ophiuchus Molecular Cloud: Open cluster Collinder 302 with Ophiuchus molecular cloud (Antares Nebula vdB 107, nebula IC 4603, ρ Ophiuchi nebula IC 4604, nebula IC 4605, Sharpless 2-9, many dark clouds), plus globular clusters M 4 und NGC 6144. North is left; Canon EOS 20Da, Zoom-Objektiv 131 mm f:5.6 ; 145 min @ ISO 1600; Astrofarm Tivoli, Namibia, 1345 m elevation; © 5. 6. 2011 Eduard von Bergen, Hansjörg Wälchli

Finder Chart

The two globular clusters Messier 4 and NGC 6144 are very easy to find. The beautiful red supergiant Antares (α Scorpii) is set in the telescope with a large field eyepiece and the telescope is then swiveled slightly towards the west. Voilà, there are the two little rascals. It really doesn't get any easier than that. The best time to observe is May to July, when the constellation Scorpius is highest over the southern horizon at night.

Chart Messier 4
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
149SkySafari 6 Pro, Simulation Curriculum; skysafariastronomy.com
160The STScI Digitized Sky Survey; archive.stsci.edu/cgi-bin/dss_form
175Hubble Space Telescope Finds Stellar Graveyard; Press Release No. STScI-PR95-32; oposite.stsci.edu/pubinfo/pr/95/32.html
249Ancient orbs; esahubble.org/images/potw1236a (2021-01-24)
277«Historische Deep-Sky Kataloge» von Dr. Wolfgang Steinicke; klima-luft.de/steinicke (2021-02-17)
463«Catalogue of one thousand new nebulae and clusters of stars» William Herschel, Philosophical Transactions of the Royal Society of London, 1 January 1786; DOI:10.1098/rstl.1786.0027