Galactic chandeliers
- Thomas Grohmann
- 3. Juni
- 11 Min. Lesezeit
This travel report does not take us to remote regions on our blue planet, but into the infinite expanses of outer space. It is also a journey through time, as we look at relics from the beginnings of the universe: the globular clusters.
They are truly wondrous companions, these 151 ancient spheres known to us, orbiting our galaxy, the Milky Way, in its halo and even crossing the galactic plane along their paths. At 8 to over 13 billion years old, they are almost as old as the universe itself. Their typically hundreds of thousands or even millions of stars contain very little metal, and they belong to the densest gravitationally bound systems we know. Globular clusters have also been detected in other galaxies—for example, the elliptical galaxy M87 hosts over 10,000 of them. However, the formation and cosmological classification of globular clusters remained entirely unclear until recently. Only a few years ago, researchers proposed an explanatory model based on simulations, suggesting that the oldest globular clusters could have formed within a few million years in colliding and merging dwarf galaxies during the early days of the universe.
Current simulations now allow their origins to be described in a completely natural way by applying the known physics of star formation to the conditions of the early cosmos. According to the authors of these studies, globular clusters are the inevitable result of intense star formation in the young universe. Shortly after the Big Bang, the gas clouds in galaxies were much denser than they are in modern galaxies. From these dense clouds, star clusters with millions of stars could form very efficiently. The models suggest that the first globular clusters either formed alongside their galaxies or were captured from the surrounding environment. Over the following billions of years, more globular clusters were added, ultimately concentrating around a large galaxy and evolving into the globular clusters we observe today. In 2024, astronomers using NASA's James Webb Space Telescope were able to detect five globular clusters in a 13.3-billion-year-old dwarf galaxy.
Globular clusters appear to us as relatively bright objects—some refer to them as galactic chandeliers—whose observation and photography are quite feasible even during the brighter summer nights, under city lights, or in moonlight. Their brightness is likely also the reason why the famous Messier catalog of 1771, which includes 110 objects, contains as many as 29 globular clusters.
We will now take a closer look at a selection of these beautiful objects. The following images were taken in Schmitten with partially multi-hour exposure times in August 2024 as well as in May/June 2023, and the image from the Southern Hemisphere was captured in the summer of 2017 in the Kalahari border region in Namibia. The imaging optics, except for Namibia, were each a Newtonian telescope with a focal length of 1200mm and an f/4 aperture ratio.
Messier 15 (NGC7078)
Among all the globular clusters in the Milky Way system, M15 has the highest central stellar density and has already undergone a so-called core collapse (contraction). This gravitational effect leads to a densification and very small distances between the cluster stars in the core. Even the Hubble Space Telescope was barely able to resolve the dense core. Through binoculars or a small telescope, the cluster appears as a fuzzy star.
Messier 15 is located about 33,600 light-years from Earth and has a diameter of about 175 light-years. It has an absolute magnitude of -9.2, corresponding to a luminosity of 360,000 Suns. Estimates of the number of stars in M15 vary significantly, but it is believed to contain several hundred thousand stars. M15 is estimated to be 13.2 billion years old, making it one of the oldest known globular clusters. The mentioned core collapse could indicate the presence of a black hole. M15 also emits X-rays; two bright X-ray sources were resolved by the Chandra X-ray Observatory. These also provide evidence for a black hole at the center.
M15 also offers another remarkable feature: it is the first globular cluster in which a planetary nebula, Pease 1, was identified in 1928. A so-called planetary nebula is a shell of gas and plasma ejected by a star at the end of its life. To this day, Pease 1 is one of only four known planetary nebulae in a globular cluster. The following images show a magnified section of M15’s core with photographic evidence of Pease 1 (red arrow), which—making things more challenging—is located in the immediate vicinity of the core region.
Messier 15 also contains over one hundred variable stars, as well as at least 8 pulsars, including a double neutron star system.
The nearly 6-hour recording of a larger section of the sky above shows M15 embedded in the so-called Galactic Cirrus (known in English as IFN, Integrated Flux Nebula), which appears distinctly L-shaped from our perspective. IFNs are a relatively new astronomical phenomenon and are still not fully understood. Unlike the typical and well-known gas nebulae within the plane of the Milky Way, IFNs are located outside the main body of the galaxy. These nebula clouds, an important component of the interstellar medium, consist of dust particles, hydrogen, carbon monoxide, and other elements. They are incredibly faint and require hours of exposure time to capture at all. The following orientation map from Sky Safari shows the galactic position of M15, an area where IFN occurrences are frequent (primarily toward the northern and southern celestial poles):
Messier 2
Messier 2, or NGC 7089, is, similar to M15, a large and bright globular cluster, strongly concentrated with a dense core. In a moderately populated star field in the western part of the constellation Aquarius, M2 is an unmistakable object even with binoculars. It is one of the richer and more compact globular clusters and has a strikingly elliptical shape. The core is symmetrical, with a slight north-south elongation. In the outer corona, there is an abundance of stars, and perhaps 100 to 200 stars can be resolved against the hazy background of the cluster. Star chains that wind outward from the core extend the halo to a diameter of 16'. Several dark lanes can be seen, the most prominent of which is in the northeastern part. M2 is about 37,500 light-years away and lies far beyond the center of the Milky Way. It has a diameter of about 175 light-years and contains about 150,000 stars. The dense central core has a diameter of only 3.7 light-years. The age of M2 is estimated to be around 13 billion years. It moves on a highly eccentric (e=0.76) orbit in the halo of the Milky Way, which carries it to an enormous distance of 171,000 light-years from the galactic center and 165,000 light-years above and below the galactic plane.
Messier 71
For a long time, M71 was thought to be an open star cluster. However, the age of its stars suggests that it is actually a globular cluster. And the longer it is exposed in photographs, the more clearly its spherical shape emerges. M71 is the least concentrated globular cluster in the Messier catalog and is rather small and poor in stars. It has likely lost many stars due to encounters with other objects in the Milky Way. Its estimated total mass is only 40,000 solar masses. Its orbit around the center of the Milky Way takes 160 million years, and it always remains within the galactic disk, as shown by the orientation map from Sky Safari:
Messier 3
Messier 3 is one of the most beautiful globular clusters. With an apparent brightness of 6.2 mag, it is visible to the naked eye under dark conditions - and an excellent object even with the least optical aids.
At a distance of 33,900 light-years, M3 is farther from us than the center of the Milky Way. Its distance from the galactic center is 40,000 light-years. It moves along an inclined orbit that takes it 49,000 light-years above and below the galactic plane; at present, it is located about 33,000 light-years above this plane. It takes 300 million years to complete one orbit. M3 contains about 500,000 stars, making it very rich in stars but only moderately concentrated. It is famous for the large number of variable stars discovered within it, the first of which were found by E. C. Pickering in 1889. The cluster is about 8 billion years old and consists mainly of old, red stars. However, it also contains a relatively large number of so-called “blue stragglers,” which appear much younger than the rest of the globular cluster’s stellar population. These stars, which once puzzled astronomers, are now understood to have formed through stellar interactions; their cooler outer layers are stripped away during close encounters, which occur when passing through the dense central regions of the cluster.
Messier 5
M5 is a impressive globular cluster with 800,000 solar masses, slightly elliptical in shape and moderately concentrated. Its age is estimated at 8.9 to 10 billion years – which makes it a real youngster among globular clusters. One orbit around the galactic center takes 1 billion years. In addition to some "blue stragglers", the cluster contains over 140 variable stars, including a dwarf nova.
Messier 92
Not quite as spectacular as its bigger brother M13 in the constellation Hercules (see the picture at the beginning of the blog post and the next entry), M92 is nevertheless one of the brightest and oldest globular clusters in the northern sky. Its core is even denser than that of M13 and tends to become overexposed in photographs. North of the globular cluster (left side of the image) there are larger galaxy clusters; some galaxies are also visible with discernible structures. Due to the precession of Earth’s axis, M92 will pass the celestial north pole at a distance of only 1° in 14,000 years. We should keep that in mind as a future navigational aid ;-)
Messier 53 and NGC 5053 in the constellation Coma Berenices – cosmic neighbors with a shared past?
About 60,000 light-years away from us and also from the galactic center, two ancient globular clusters travel together through the galactic halo: M53 and NGC 5053. They take about 1 billion years to complete one orbit around the Milky Way’s center, and they are separated by only about 6,500 light-years in space. Both formed approximately 12.5 to 13 billion years ago, making them among the oldest known stellar systems in our galaxy. Their stars are extremely poor in heavy elements – a clear indication that they originated in an era when the universe was still young.
Precise measurements from the Gaia mission, as well as spectroscopic investigations of its stars, present a complex picture. Both star clusters have similar spatial motions, comparable distances, and indications of extended stellar populations beyond their nominal boundaries. Other studies show that they may even interact and could possibly be connected by a tidal star bridge and a shell-like structure. This suggests that they are not merely close to each other by chance but may share a common origin—perhaps as remnants of a dwarf galaxy long ago absorbed by the Milky Way.
The differences between them, however, could hardly be greater: M53 is a dense, massive sphere of stars with 500,000–750,000 members, while NGC 5053 appears much lighter, looser, and almost ghostly diffuse (estimated at 3,500–15,000 stars). This loose structure makes NGC 5053 particularly sensitive to the tidal forces of both M53 and the Milky Way, which slowly strip stars from the cluster.
If their connection is confirmed, M53 and NGC 5053 would represent a rare example of two ancient stellar systems that share a common origin yet have evolved entirely differently over billions of years.
Additionally, M53 contains numerous RR Lyrae variables, making it an important astrophysical laboratory for distance measurements and stellar evolution. Nearly 200 “blue stragglers” (collided cluster stars) and even a 33-millisecond pulsar have also been discovered there. In images of M53, the beautiful double star Struve 648 with its differently colored components also stands out.
Messier 13 - Great Hercules Cluster
Before we finally feature the star of all globular clusters, let us first take a look at the showpiece in the northern sky: Messier 13, the most spectacular globular cluster in our latitudes.
One of the reasons why M13 appears so large and bright is its relative proximity. Its distance is about 25,000 light-years, and it has an extent of about 145 light-years. With 775,000 solar masses, it is also one of the larger globular clusters in absolute terms. It contains several hundred thousand stars, with some sources even stating more than a million. Toward the center of M13, the concentration of stars is about 500 times higher than in the Sun’s surroundings. One orbit of M13 around the galactic center takes approximately 500 million years, during which its distance increases to about 80,000 light-years. The age of M13 has been estimated at 12 billion years.
Among the many blue stars, M13 contains a particularly young blue star for such an old cluster: Barnard No. 29, with spectral type B2. Its membership is confirmed by radial velocity measurements; apparently, it is a captured field star.
Visually and in photographs, dark structures can also be seen, but it is still uncertain whether these, if real, actually belong to the cluster. Dust should hardly be present in such very old globular clusters. They appear as small, irregular, dark spots and streaks in the core area of M13. Three of these streaks meet on the southeastern side of the central core and form a Y-shaped figure, first noticed by Lord Rosse. Also interesting is a large X made of two chains of stars in the central region of M13.
Omega Centauri (NGC 5139)
The star among all globular clusters must, of course, not be missing from our collection: Omega Centauri (NGC 5139) is the brightest globular cluster in the sky and the largest and brightest globular cluster orbiting the Milky Way. It is so massive that it is believed to be the core of a dwarf galaxy that merged with the Milky Way a long time ago. Located in the constellation Centaurus, this gem can only be seen from the Southern Hemisphere. The image below was taken in Namibia.
Omega Centauri has been known since antiquity. The Greek astronomer Ptolemy included it in his star catalog, which he compiled in the mid-2nd century AD. When Johannes Bayer assigned Greek letters to the brighter stars, he mistakenly considered this star cluster to be a star and named it Omega Centauri.
At a distance of 15,600 light-years, Omega Centauri is one of the globular clusters closest to the solar system. Its visual size of about 36' corresponds to an actual diameter of 175 light-years. Its photographic size is 65', which means a diameter of over 300 light-years. As with all globular clusters, the density of stars rapidly increases toward the center. The average distance between stars in its core is only about 0.1 light-years.
With an estimated 10 million stars and about 5 million solar masses, Omega Centauri is roughly ten times as massive as a typical large globular cluster and about as massive as the smallest of all galaxies. It is the brightest and most massive globular cluster orbiting the Milky Way, and among all globular clusters in the Local Group, only Mayall II (G1) in the Andromeda Galaxy (M31) is more massive and brighter.
Omega Centauri differs so greatly from the other globular clusters of the Milky Way that it is believed to have a different origin. It is about 12 billion years old, but as a 1999 study suggests, the stars of Omega Centauri did not all form at once but over a period of 2 billion years. The team that conducted this study suspects that Omega Centauri is the remnant core of a small galaxy that merged with the Milky Way long ago.
Used and suitable sources of this blog entry:
Stoyan, Messier Guide, 2020
Koch/Korth, Die Messier-Objekte, 2010
Max-Planck-Institut für Astrophysik, Entstehung von Kugelsternhaufen entschlüsselt, 2019 (Originalveröffentlichung: Nathalia Lahen, Thorsten Naab, Peter Johansson, Bruce Elmegreen, Chia-Yu Hu, and Stefanie Walch
The Formation of Low-metallicity Globular Clusters in Dwarf Galaxy Merger
The Astrophysical Journal Letters, Volume 879, Issue 2, article id. L18, 6 pp. 2019)
J. Pfeffer, J.M.D. Kruijssen, R.A. Crain and N. Bastian: The E-MOSAICS Project: simulating the formation and co-evolution of galaxies and their star cluster populations, Monthly Notices of the Royal Astronomical Society (2018)
Sang-Hyun Chun et al.: A wide-field photometric survey for extratidal tails around five metal-poor globular clusters in the Galactic halo
FAZ of 29.06.2024, Manfred Lindinger: KUGELSTERNHAUFEN ENTDECKT, Juwelen des jungen Universums
Sky Safari (App)
Various Wikipedia articles
DSC Deep⋆Sky Corner www.deepskycorner.ch
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