Globular Clusters Reveal The Primeval Universe's Secrets

By Judith E Braffman-Miller

Our large spiral Milky Way Galaxy blazes with the fires of 200 billion stars, and when it is observed high above us in the sky on a clear, dark night, it is a magnificent sight--extending like a celestial smile from horizon to horizon. Before our Galaxy existed, the Universe was filled with mostly hydrogen gas, as well as smaller amounts of helium, and this gas was destined to evolve into myriad stars, their entourage of orbiting planets, moons, as well as conscious life-forms on Earth--and probably elsewhere. Globular clusters are spherical collections of stars that orbit a galaxy's core as satellites, and are bound to their host galaxies by gravity. The globular clusters that perform a dazzling dance around our own Milky Way are almost as old as the Universe itself. In April 2018, a team of astronomers announced that their supercomputer simulations indicate that these very ancient clusters were born as a result of the same mechanisms that created our Galaxy's first stars, which makes them natural relics of primordial star formation in the ancient Universe.

The supercomputer simulations were created by a team of astronomers led by Dr. Joel Pfeffer of Liverpool John Moores University (United Kingdom) and Dr. Diederik Kruijssen of Heidelberg University (Germany). According to the scientists, "this approach elegantly solves one of the greatest mysteries in astronomy."

Globular clusters are very tightly bound by gravity, which provides them with a spherical shape, as well as relatively high stellar densities toward their centers. The name of this type of star cluster is derived from the Latin globulus--meaning a small sphere. A globular cluster is sometimes referred to more casually as simply a globular.

Globulars are usually found in a galaxy's halo, and they host considerably more stars and are much older than less dense open stellar clusters, which inhabit the disk of a galaxy. Globulars are very common, and there are approximately 150 to 158 currently known to circle our Milky Way. However, it is thought that there may be as many as 10 to 20 more that have not yet been discovered. Our Galaxy's globulars circle it at radii of about 130,000 light years--or more. The larger the galaxy, the more globulars it possesses. For example, the Andromeda galaxy--a large spiral like our Milky Way--may have as many as 500 of these star clusters. In dramatic contrast. some giant elliptical galaxies may have as many as 13,000 globulars.

Indeed, every galaxy sporting sufficient mass inhabiting the Local Group of galaxies--of which our Milky Way is a member--has an associated retinue of globulars. In fact, almost every large galaxy so far studied by astronomers is orbited by a system of these spherical objects. The Sagittarius Dwarf galaxy and the controversial Canis Major dwarf galaxy, which are both satellites of our Milky Way, seem to be in the process of contributing their constituent globular clusters to our Galaxy. This provides an important clue about how many globular clusters may have been snatched up from other galaxies by our Milky Way in the past.

Even though globular clusters are believed to be populated by the first generation of stars to be born in the Universe, their origins, as well as their role in galactic evolution, are not well understood. However, it does appear that globulars are different from dwarf elliptical galaxies, and that they formed as part of the star formation process going on within their parent galaxy, rather than as separate galaxies in their own right.

Galaxies And Globulars

Our Milky Way Galaxy is only one of billions of other galaxies in the visible Universe. As our home galaxy, it has been studied extensively by astronomers, and currently scientists have a firm understanding of its nature. Long before our Star, the Sun, and its family of planets, moons, and smaller objects came to be, our Milky Way Galaxy existed. The ancient Universe was brimming with gas, and this gas (mainly hydrogen and helium) eventually evolved into stars and their surrounding solar systems. But before solar systems, such as our own, could form, galaxies had to exist.

Many astronomers think that our Milky Way was born within a giant, fairly spherical cloud of cold gas, rotating slowly and majestically in space. At some point, the frigid, giant cloud of gas started to collapse in on itself (condense), in much the same way that the clouds that gave birth to individual stars also collapsed. At first, some stars may have been born as the cloud of gas started to fragment around its edges--with each individual fragment collapsing further to form a baby star or group of stars. Because the cloud was spherical at that time, astronomers have observed some very elderly stars scattered in a spherical halo around the outside of our Galaxy today. The first stars born in the primordial Universe were made up of only the hydrogen and helium gas that composed their natal cloud.

The star-birthing cloud continued to collapse more and more, and more and more stars were born as it did so. Because the cloud was rotating, the spherical shape eventually flattened out into a disk--and the baby stars that had been born at this ancient time began to heavily populate this disk region. As the birth of new baby stars continued, some of the older stars had enough time to approach the end of that long stellar road and end their hydrogen-burning lives on the Hertzsprung-Russell Diagram of Stellar Evolution. When stars have finished burning their necessary supply of hydrogen fuel, they are doomed to "die". The process of nuclear-fusion keeps a star active and youthful, as it fuses its lighter atomic elements into heavier things--such as carbon, oxygen, neon, nickel and iron. Once a massive star contains a core of iron--that's it! The unfortunate massive star explodes in the fiery fury of a supernova blast. In the process, these stellar senior citizens enrich their surroundings with these freshly forged heavier atomic elements. This is because they blast these elements out into the space between stars where they can be incorporated into younger stellar populations. The youngest generation of stars, of which our own Sun is a member, are classified as Population I stars. The oldest generation of stars, born in the ancient Universe, are designated Population III stars. Population II stars are very ancient, but not as ancient as Population III stars--and not nearly as young as our bouncy Population I Sun, and other stars of its youthful generation.

According to galaxy classification, spiral galaxies, like our Milky Way, are made up of a flat, rotating disk heavily populated by stars, gas, and dust. Spirals also contain a central collection of stellar inhabitants termed a bulge. The entire spiral galaxy is surrounded by a faint halo that also houses stars, many of which are denizens of globular clusters.

The first known globular cluster, now named M22, was discovered in 1665 by Johann Abraham Ihle (1627-1699?), an amateur astronomer from Germany. However, individual stars inhabiting globulars could not be resolved until the French astronomer Charles Messier (1730-1817) observed another globular, Named M4, in 1764. When the German-English astronomer William Herschel (1738-1822) began his comprehensive survey of the sky using large telescopes in 1782, there were 34 known globulars, and he discovered another 36. Herschel was also the first to resolve literally all of them into their component stars. He is credited with coining the term globular clusters in his Catalogue of a Second Thousand New Nebulae and Clusters of Stars published in 1789.

The number of known globulars inhabiting our Galaxy has continued to increase with the passage of time, reaching 83 in 1915, 93 in 1930, and 97 by 1947. A total of 152 globular clusters have now been discovered in our Milky Way, out of an estimated total of 180 give or take 20. These additional, still undiscovered globulars are believed to be veiled behind the obscuring gas and dust of our Galaxy.

Globulars Tell Their Ancient Secrets

Our Galaxy today is very different from the cloud of cold gas that it was born from billions of years ago. It is no longer a spherical mass composed mostly of hydrogen. While astronomers can't "stand back" and observe our Milky Way as a whole, they can peer out into intergalactic space and observe other galaxies which are thought to be similar to our own.

Even though it has long been understood that galaxies like ours are surrounded by hundreds of globulars, the question of how these spherical clusters formed in the first place remains one of the most intriguing mysteries in astrophysics. The astronomers from Liverpool and Heidelberg have now presented their new supercomputer simulations to solve this difficult question. To achieve this, the scientists used current models of globular cluster formation, and were aided in their efforts by using the state-of-the-art EAGLE simulation of galaxy birth. The project, entitled Modeling Star cluster population Assembly in Cosmological Simulations within EAGLE--or E-MOSAICS for short--shows how the changing conditions occurring within galaxies over the passage of 13 billion years influence the formation and evolution of their globulars.

For their new study, the astronomers began with the idea that globulars formed in the same way as new-born stellar clusters emerging from heavily gas-laden regions of nearby galaxies today. E-MOSAICS helped them test this theory on an observed population of globulars surrounding our Milky Way. "The simulations show that the first star clusters form just a few hundred million years after the Big Bang. In the billions of years that follow, they are joined by other globular clusters and ultimately concentrate around a large galaxy, arranged much like those around the Milky Way," explained Dr. Kruijssen in an April 6, 2018 University of Heidelberg Press Release. Dr. Kruijssen is research group leader at the Institute of Astronomical Computing at the Heidelberg University Center for Astronomy (ZAH).

Before this study, some unusual assumptions were needed to explain the mysterious origin of globular clusters. However, the new supercomputer simulations reveal an explanation that is completely natural by using familiar stellar formation physics, and then applying it to the conditions that existed in the primordial Universe. According to Dr. Pfeffer, the study's lead author, globulars are the inevitable outcome of intensive star formation when our Cosmos was young. Soon after the Big Bang birth of the Universe about 13.8 billion years ago, the clouds of gas floating around within galaxies were much denser than those in today's galaxies.

Dr. Kruijssen explained in the April 6, 2018 University of Heidelberg Press Release that "These dense clouds very efficiently fueled the formation of star clusters of up to a million stars. Some of them survived to become the globular clusters we observe today."

The astronomers are looking forward to using E-MOSAICS to reconstruct the formation history of galaxies based on the formation of primordial star clusters. In this way, they hope to gain a new understanding of the formation of our Milky Way Galaxy. The new results were published in the Monthly Notices of the Royal Astronomical Society.

Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various newspapers, magazines, and journals. 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.