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.
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.