For most of human history, the Milky Way galaxy contained almost everything we could perceive, yet obscuring layers still hide much of it from us. Naturally, astronomers keep trying to pull back the veils, most recently leading to the finding of five globular clusters that disrupt our expectations for these bundles of stars. Their nature could determine the debate on the origins of one of the galaxy's most important features: its central bulge.
Large galaxies are surrounded by globular clusters, spherical cities of stars sometimes compared to islands around a continent. “They were the first stellar systems formed in the early universe and may be considered true living fossils from which galaxies like the Milky Way were built,” Dr Denilso Camargo of the Colégio Militar de Porto Alegre, Brazil said to Astrowatch.net. This, he added, makes them “A powerful tool to study the formation and early evolution of galaxies, especially the Milky Way,"
The most familiar globular clusters lie our galaxy's halo. The Milky Way has at least 150, but as Camargo's discovery demonstrates, there are still more to find. The somewhat larger Andromeda Galaxy has around 500, and an astonishing 13,000 are thought to accompany the giant M87 galaxy.
Camargo's latest finds are intriguing because, as he reports in The Astrophysical Journal they may belong more to the Galactic bulge than the halo, lying less than 12,000 light-years from the center of the galaxy. However, they are as lacking in metals as those further out in the halo.
“The nature of the Galactic bulge remains not clearly understood,” Camargo told IFLScience. Models suggest two options; the merging of smaller galaxies and clusters or instabilities in the disk slowly coalescing.
This is where Camargo's clusters become important. They are very low in metal content and therefore very old. Their location, Camargo explained to IFLScience, “suggests the presence of the bulge already in the initial phase of the Milky Way's formation.”
Camargo told IFLScience the arrival of infrared detectors, paired with newly launched telescopes has allowed us to “penetrate the cocoon of dust that surrounds a star cluster in formation.” Many astronomers are using these technologies to study known clusters in detail; Camargo is one of the few using them to seek new ones. As a result, he has personally found an astonishing 1,106 previously unknown star clusters and stellar groups. Some of these are modest and undistinguished, but others have shown completely unexpected behavior that has altered our thinking on galaxy formation.
“A significant fraction of these findings are sparsely populated stellar groups,” Camargo said. “Which may suggest that many low mass stars like our sun are born in small stellar groups or in isolation rather than populous classical clusters.”