The secrets of galaxy formation revealed by mysterious globular clusters
This roundness is what distinguishes globular clusters from other types of star clusters (and gives them their name, from the Latin “small sphere”). They are large enough and contain enough stars to have enough gravity to draw them into spherical shapes.
Globular clusters are incredibly old. Of the approximately 150 within the Milky Way, the youngest are no less than 8 billion years old, while the oldest are nearly 12 billion years old. They have had no new star formation cycles in billions of years, so what remains inside them are remnants of other stars (white dwarfs, black holes, etc.) or small, dim red stars.
Whatever. had them form it happened a long, long time ago, and they simply changed little in all those aeons. In fact, Omega Centauri is one of the oldest things that can be seen with the naked eye. When the solar system was formed, that globular cluster was already very ancient.
Of course, "young" is a relative term here. These younger ones tend to be 8 to 10 billion years old. They also tend to be closer to the Milky Way's central bulge and have far more metals than other globular clusters. In astronomical jargon, "metals" means any element other than hydrogen and helium. These heavier elements are forged inside stars through nuclear fusion, and in a normal galaxy, the continuous cycles of star formation and star destruction continually enrich the galaxies.
Credits: Hubble NASA / ESA
The other older globular clusters date back to a period between 10 and 12 billion years. These are much more common; about two thirds of the globular clusters of the Milky Way come from this period of formation. They tend to be farther from the galactic center, have all kinds of random orbits, and are nearly metal-free. Astronomers suspect young globular clusters formed with the same 8 to 10 billion Milky Way Galaxy. years ago, while the older ones formed before our galaxy even began to form. Those globular clusters likely formed with small dwarf galaxies that were demolished by the Milky Way. The dwarf galaxies were torn apart, but the small and dense globular clusters managed to survive to the present day (however forced to orbit the same galaxy that destroyed their parents). We therefore have two types of globular clusters. But how did they form?
The biggest clue to the origins of globular clusters is that they have no dark matter. The measurements of their mass with different techniques (adding all the light sources, calculating the gravity necessary to keep them round and so on) do not require a hidden and invisible component. This means that globular clusters are completely different from galaxies. It is a bit difficult to find a definition of a "galaxy", but since almost all galaxies are made of at least 80% dark matter, it is necessary to include this factor in their definition.
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To date, astronomers are not exactly sure which scenario is more likely. In both cases, globular clusters are intriguing because they are so obviously linked to the formation of galaxies and can therefore also provide answers on how galaxies form and evolve, as astronomers are not exactly sure how this happened. By studying these gigantic time capsules, they hope to peer into our ancient past and unravel the final mysteries of how our galaxy was born.
