Hubble Discovers Thousands Of Starlit Snowballs

Globular Clusters look like big starlit snowballs as they orbit the hearts of their host galaxies as satellites. These beautiful celestial objects are really spherical collections of stars that are very tightly "glued" together by gravity, which is the reason why they formed their characteristic spherical shapes and relatively high stellar densities inward towards their centers. Every galaxy of sufficient mass in our own Local Group of galaxies has an accompanying retinue of globulars, and almost every large galaxy surveyed displays its own orbiting system of these sparkling, starry snowballs. In November 2018, a team of astronomers announced that by gazing 300 million light-years into an enormous "city" of galaxies, they had developed a comprehensive census of some of its tiniest constituents. Using the Hubble Space Telescope (HST), the astronomers discovered a twinkling treasure trove filled with globular clusters--amounting to a whopping 22,426 starlit snowballs!
The survey results, published in the November 9, 2018 issue of The Astrophysical Journal, will enable astronomers to use the globular cluster field to map the distribution of "ordinary" atomic (baryonic) matter and the mysterious, exotic, non-atomic dark matter, contained within the Coma Galaxy Cluster. The Coma Cluster hosts more than 1,000 galaxies that are packed tightly together.
Because globular clusters are both considerably more abundant and smaller than entire galaxies, they are a much better tracer of how the fabric of Space is warped by the Coma Cluster's powerful gravity. Indeed, the Coma Cluster was one of the first places in space where observed gravitational distortions were deemed to be signs of great amounts of unseen mass lurking in the Cosmos--this abundant, invisible, and ghostly mass was eventually named dark matter.
Globular Clusters are some of the most ancient structures in the Universe, and these snowball shaped objects host several hundred thousand very old stars. Indeed, Globulars play an important role in the birth and growth of a galaxy. Approximately 150 of these starlit spherical structures dance around our Milky Way Galaxy, and, since they host the most ancient known stars in the Universe, they were already present in the primordial formative years of our Galaxy.
Snowballs In The Sky
The first known Globular Cluster, dubbed M22, was discovered in 1665 by the German amateur astronomer Abraham Ihle (1627-1699). However, the primitive telescopes of that era could not resolve the individual stellar inhabitants of Globulars until the French astronomer Charles Messier (1730-1817) observed M4 in 1764. Messier listed the first eight Globulars to be discovered in his catalogue, and later the French astronomer Abbe Lacaille (1713-1762) listed several more in his own 1751-52 catalogue. The M before a number designates Messier's catalogue, while NGC is taken from the New General Catalogue by the Danish-Irish astronomer John Dreyer (1852-1926).
When the German-British astronomer William Herschel (1738-1822) began his own comprehensive survey of the sky, using the improved large telescopes available in 1782, there were only 34 known Globulars. Herschel discovered an additional 36 himself, and he was also the first astronomer to resolve literally all of their individual stars. Indeed, Herschel is credited with coining the term "Globular Cluster" in his own Catalogue of a Second Thousand New Nebulae and Clusters of Stars published in 1789.
The number of known Globulars continued to skyrocket, reaching 83 in 1915, 93 in 1930, and 97 by 1947. A total of 152 of these spherical clusters have currently been discovered in our Milky Way--out of a predicted total of approximately of 180, give or take 20. These additional, still undiscovered, clusters are thought to be well-hidden behind the obscuring veil of gas and dust of our Galaxy.
Beginning in 1914, the American astronomer Harlow Shapley (1885-1972) began a series of studies of Globular Clusters, published in about 40 scientific papers. Shapley also studied the RR Lyrae Variable stars inhabiting the clusters (which he mistook for Cepheid Variables) and used their period-luminosity relationship to derive distance estimates. Later, it was found that RR Lyrae Variables are fainter than Cepheid Variables. This caused Shapley to overestimate the distances of the clusters.
Most of our Galaxy's Globulars are located in a halo surrounding the Galactic core, and the majority are situated in the celestial sky centered on the Milky Way's core. In 1918, this extremely asymmetrical distribution was used by Shapley to make a determination of the overall dimensions of our Galaxy. Even though Shapley's distance estimate was in significant error, it nonetheless correctly demonstrated that our Milky Way's dimensions were considerably larger than had been proposed previously. Shapley's measurements also revealed that our Sun is relatively far from the center of our Galaxy--also at odds with what had previously been inferred from the apparently nearly even distribution of ordinary stars. Most ordinary stars are located within our Galaxy's disk while those stars that reside in the direction of its center and beyond are enshrouded by gas and dust--whereas Globulars are situated beyond the disk and can be observed at considerably greater distances.
The elderly stars that inhabit Globular Clusters are almost as old as the Universe itself, and they have puzzled astronomers for more than fifty years. That is because these stars are composed of material that is different from all of the other stars in our Milky Way.
Even though Globulars host some of the most ancient stars inhabiting any galaxy in the visible Universe, both their origins and the role that they play in galactic evolution are poorly understood. However, it is generally thought they were born as part of the star-birthing process occurring within their host galaxies, rather than as separate galaxies in their own right. It is unknown if the stars that inhabit these clusters were born as a single generation, or if they were born as members of various stellar generations over the course of several hundred million years. However, it has been observed that most of the stars inhabiting a particular cluster are at the same stage of stellar evolution. This strongly suggests that they were all born at approximately the same time.
Globular Clusters host considerably more stars and are much older than less dense open star clusters, which are usually found in the disk of a Galaxy. Our Sun and its myriad sister stars were born in an open stellar cluster.
Despite their beauty and mysterious origins, Globulars are fairly common in the Cosmos. Galaxies that are larger than our Milky Way can have many more Globular Clusters than our Galaxy. For example, the slightly larger nearby Andromeda Galaxy may have as many as 500 orbiting Globulars, and some of the giant elliptical galaxies (particularly the glaring behemoths that lurk in the centers of galaxy clusters), such as M87, may have as many as 13,000 orbiting Clusters.
Astronomers think that every galaxy that contains sufficient mass in our Milky Way's Local Group is circled by an associated collection of Globulars. For example, two of our own Galaxy's small satellites--the Sagittarius Dwarf Galaxy and the currently disputed Canis Major Dwarf Galaxy--appear to be in the midst of contributing their associated retinues of Globular Clusters (such as Palomar 12) to the gravitationally welcoming arms of our Milky Way. This reveals how many of our Galaxy's Globulars were snatched up in the past.
HST Finds Thousands Of Starry Snowballs
Some of our Milky Way's Globular Clusters appear as fuzzy-looking "stars" to the unaided human eye. However, at the distance of the Coma Cluster, its Globular denizens look like tiny specks of light even to HST's sharp vision. The new HST survey discovered Globular Clusters dispersed throughout the space between galaxies. These luckless clusters apparently were evicted from their host galaxy as a result of galaxy near-collisions within the crowded environment of their cluster. HST revealed that some Globular Clusters line up along bridge-like patterns. This is tattle-tale evidence for interactions between galaxies where they pull on one another like children tugging on taffy at a taffy-pull.
Dr. Juan Madrid, an astronomer of the Australian National Facility in Sydney, Australia first began to consider the distribution of Globulars in Coma when he was studying HST images. This is because the images show that the Globulars reach all the way to the very edge of any particular photograph of galaxies inhabiting the Coma Cluster.
Dr. Madrid was waiting for still more data to come in from one of the legacy surveys of HST. These surveys were designed to obtain data of the entire Coma Cluster, and they were named the Coma Cluster Treasury Survey. Alas, in 2006--only halfway through the program--HST's Advanced Camera for Surveys (ACS) experienced an electronic failure. The ACS was later repaired by astronauts during a 2009 HST servicing mission.
In order to fill in the survey gaps, Dr. Madrid and his colleagues carefully studied numerous images of the cluster obtained from different HST observing programs. These images are stored in the Space Telescope Science Institute's (STSI's) Mikulski Archive for Space Telescopes in Baltimore, Maryland. He then put together a mosaic of the central region of the cluster, working with students from the National Science Foundation's (NSF's) Research for Undergraduates program. "This program gives an opportunity to students enrolled in universities with little or no astronomy to gain experience in the field," Dr. Madrid commented in a November 29, 2018 Hubblesite Press Release.
The team of astronomers then developed algorithms in order to carefully sift through the mosaic images of the Coma Cluster that showed at least 100,000 potential sources. The program used the color of the Globulars--primarily dominated by the dying fires of ancient red stars--as well as spherical shape in order to eliminate unwanted objects. The unwanted objects were mainly background galaxies unassociated with the Coma Cluster. The main drawback of the HST--despite its great success--is that its otherwise sensitive detectors have tiny fields of view.
As Dr. Madrid continued to comment: "One of the cool aspects of our research is that it showcases the amazing science that will be possible with NASA's planned Wide Field Infrared Survey Telescope (WFIRST) that will have a much larger field of view than Hubble. We will be able to image entire galaxy clusters at once."
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics, she particularly loves writing about astronomy because it gives her the opportunity to communicate to others some of the many wonders of her field. Her first book, "Wisps, Ashes, and Smoke," will be published soon.
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