Ancient Globular Clusters Cradle Supermassive Stars
By Judith E Braffman-Miller
Our majestic barred-spiral Milky Way Galaxy is surrounded by more than 150 ancient globular clusters, each hosting hundreds of thousands of sparkling stars that are packed tightly together and held in place by gravity. Globular clusters are spherical collections of stars that orbit around the core of their host galaxies as satellites, and the stars that inhabit these dense stellar clusters are very old. Indeed, they are almost as old as the Universe itself. In June 2018, a team of international astrophysicists announced that they may have solved an intriguing mystery that has bewitched and bewildered scientists for more than half a century: why are the elderly stars that perform their mysterious ballet within globular clusters made of material that is different from other stars found in our Galaxy? Even though globular clusters host some of the most ancient stars born in any galaxy, both their origins and the role that they play in galactic evolution are not well understood. However, it does appear that they formed as part of the star formation occurring in their host galaxies, rather than as separate galaxies. It is not known if the stars that inhabit these clusters are born as a single generation, or if they are born as members of many stellar generations over several hundred million years. However, because in many clusters most of the stars are approximately at the same stage of stellar evolution, it seems to indicate that they were all born at about the same time.
Since the 1960s, astrophysicists have understood that most of the stellar inhabitants of these beautiful clusters do not contain the same chemical elements as the other stars in our Milky Way–and these elements could not have been manufactured within the stars themselves. This is because the required temperatures to produce them are approximately 10 times higher than the temperatures of these stars.
Astrophysicists from the University of Surrey in the UK now propose that supermassive stars–that have masses tens of thousands of times greater than the mass of our Sun–were born at the same time as their host globular clusters. At that time, the clusters were brimming with dense gas from which new baby stars were forming. As the newborn protostars gathered more and more gas, they got so close to one another that they physically collided and merged together to create a supermassive star as the result of a runaway collision process. The supermassive star is born hot enough to manufacture all the observed atomic elements–and to then go on to “pollute” its sibling stars with the same peculiar atomic elements observed by astronomers today.
Ancient Stellar Behemoths
Supermassive stars are stars with masses that are more than 50 times that of our Sun, and these giant stars follow very different paths from smaller stars when they depart from the main-sequence of the Hertzsprung-Russell Diagram of Stellar Evolution (H-R Diagram). A main-sequence star that exceeds 8 solar masses has the ability to fuse atomic elements heavier than carbon in its searing-hot core, which results in the massive star having a very different fate than that of its less massive stellar kin.
In the terminology that astronomers use, a metal refers to all of the atomic elements heavier than helium. Only hydrogen, helium, and traces of lithium were produced in the Big Bang birth of the Universe 13.8 billion years ago. All of the heavier metals were manufactured by the stars.
Like all stars, very massive stars depart from the hydrogen-burning main-sequence when they have managed to burn their necessary supply of hydrogen fuel in their cores. In addition, supermassive stars experience a few initial events that are similar to those of their lower-mass kin. This means that, at first, a massive star sports a shell of hydrogen that is then followed by a core that fuses helium into carbon. After that, the helium-fusing core is encircled by helium-and hydrogen-burning shells.
The path that a star of 8 solar-masses takes across the H-R Diagram is essentially a straight line–it stays at just about the same luminosity as it cools off. However, eventually an 8 solar-mass star perishes in a violent and brilliant supernova blast. The extremely heavy star, after it has been blown to pieces, leaves behind a black hole of stellar mass. The batch of heavy metals that the massive progenitor star has managed to produce, by way of the process of stellar nucleosynthesis, are hurled out into interstellar space where they may again meet up with another giant molecular cloud–just like the one that served as a cradle to the now-dead massive progenitor star. At this point, the process of star-birth begins anew, with the metals manufactured by the dead progenitor star being incorporated into new, bright, and fiery stellar babies.
In general, there are three stellar generations. Population I stars, like our Sun, are the most youthful and contain the greatest metal content, because they have been richly endowed by earlier generations of stars that produced them in their nuclear-fusing furnaces. Population III stars are the most ancient, and are believed to have been born differently from the two younger stellar generations. This is because Population III stars formed from only the lightest pristine atomic elements born in the Big Bang birth of the Universe about 14 billion years ago. Population III stars are supermassive, as well as depleted of metals because no earlier stellar generation had existed before them to contribute their newly forged supply of heavy metals. The oxygen you breathe, the iron in your blood, the ground beneath your feet, the water that you drink, and the calcium in your bones were all formed in the nuclear-fusing hearts of the Universe’s myriad stars.
Between Populations I and III there is, of course, a Population II. Population II stars contain small amounts of metals produced by the earlier Population III stars. However, all stars–despite their generation–are primarily composed of hydrogen.
Globular clusters are usually found in the halo of a galaxy, and they commonly host hundreds of thousands of metal-poor, ancient stars. Globulars are free of gas and dust, and for this reason it is presumed that all of their supply of gas and dust was turned into stars very long ago.
Globular clusters sport spherical shapes because they are bound very tightly by the force of gravity. This is also the reason why these lovely clusters have relatively high stellar densities towards their centers.
Globular clusters got their name from the Latin globulus–a small sphere. While globulars are found in the galactic halo, their much less dense cousins, open stellar clusters, are generally found in the galactic disk. Globulars host more stars and are considerably older than open clusters. There are approximately 150 known globulars surrounding our Milky Way Galaxy, with perhaps as many as 10 to 20 more still undiscovered. Indeed, globular clusters are rather common occupants of the cosmic zoo. In general, larger galaxies host a greater number of these starry spherical objects than smaller galaxies. For example, our Milky Way’s large spiral neighbor, the Andromeda Galaxy, may host as many as 500 globulars inhabiting its halo. Some giant elliptical (football-shaped) galaxies–especially those that are located in the centers of galaxy clusters, such as M87–can possess as many as 13,000 globulars.
Every galaxy of sufficient mass in our Milky Way’s Local Group has an accompanying group of globular clusters. The Local Group of galaxies, that contains our own Galaxy and Andromeda, houses over 54 galaxies that are spread out over a diameter of almost 10 million light-years. Between 1 billion and 1 trillion years from now, they will all bump into one another and merge to create a single gigantic galaxy.
The Sagittarius Dwarf galaxy and the still-hypothetical Canis Major Dwarf galaxy seem to be in the process of contributing their attendant globulars (such as Palomar 12) to our much larger Milky Way. This provides an important hint of how many of our Galaxy’s globular clusters were acquired long ago.
Globulars belonging to the Large Magellanic Cloud (LMC), an amorphous satellite galaxy of our Milky Way, show a bimodal population. Long ago, when they were young, these LMC globulars possibly met up with giant molecular clouds that triggered an encore performance of star-birth. This star-birthing era was relatively short-lived, compared to the ancient ages of many globular clusters. Some astronomers have proposed that this multiplicity in stellar populations may have a dynamical origin. For example, in the Antennae galaxy, the Hubble Space Telescope (HST) has spotted clusters of clusters existing in regions of the galaxy that span hundreds of parsecs, where many of the clusters are destined to bump into one another and then merge. Many of them display a significant range in ages, and thus possible differing metallicities, and their future merger could possibly result in clusters showing a bimodal or even multiple distribution of populations.
Globulars arise primarily in regions of efficient star-birth, and where the interstellar medium is at a higher density than in more common star-birthing regions. Globular clusters have been observed to form in starburst regions and in interacting galaxies. Studies have also shown an intriguing correlation between the mass of central supermassive black holes and the extent of the globular cluster systems of elliptical and lenticular (“armless” spiral) galaxies. The mass of the resident supermassive black hole in ellipticals and lenticulars is frequently close to the combined mass of the host galaxy’s attendant globulars.
Supermassive black holes probably haunt the secretive hearts of every large galaxy in the Universe, and they sport masses that are millions to billions of times that of our Sun.
There are no known globular clusters that display active star-birth. This observation is consistent with the viewpoint that globulars are typically the most ancient objects belonging to a galaxy. This means that they were among the first collections of stars to be born. Vast regions of star formation termed super star clusters, such as Westerlund 1 in our own Galaxy, are thought to be the precursors of globular clusters.
The Strange Story Of Globulars And Supermassive Stars
Dr. Mark Gieles, Principal Investigator of the project from the University of Surrey, explained in a June 21, 2018 University of Surrey Press Release that “What is truly novel in our model is that the formation of the supermassive stars and the globular clusters are intimately linked, and this new mechanism is the first model that can form enough material to pollute the cluster, and with the correct abundances of different elements, which has been a long-standing challenge.”
The team proposes a number of different ways to test their new model of globular clusters and supermassive star formation with existing and upcoming telescopes, which will have the ability to peer deep into the mysterious, secretive regions where the globular clusters first formed long ago, when our Universe was young.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various magazines, journals, 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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