In the early universe, old stars hung in groups.
Spherical clusters consist of hundreds of thousands of ancient stars, tightly packed together in the shape of a sphere and orbiting the core of the galaxy.
While studying globular clusters in the nearby Andromeda Galaxy in October 2019, a group of astronomers came across a cluster that just didn’t look quite right.
This strange asterism of the stars was shockingly low for the heavy metals, the very things that feed star formation.
The discovery is described in detail in a study published Thursday in the journal Science.
Soren Larsen, an associate professor at Radbud University and lead author of the new study, has been studying the composition of globular clusters for years, looking for clues to how galaxies form and evolve over time.
In October last year, Larsen and his team were waiting for data from a different purpose of the study. In order to pass the time, they decided to randomly choose a different cluster to observe. Their boredom soon turned to disbelief.
“It was quite surprising,”
The globular cluster Larsen and his chosen colleagues is known as RBC EXT8 and is located in the Andromeda Galaxy. Andromeda is the closest galaxy to our own Milky Way, located just 2.5 million light-years away. This may seem far away, but Andromeda is a very close space neighbor – so close that one day the galaxies will collide.
Scientists have found that the cluster in question has about 800 times less heavy metals than the Sun. Together, it is three times lower in the metal than the previous globular cluster with the lowest metallicity. The discovery calls into question how many of these heavy metals are needed to create stars.
In context, heavy metals are chemical elements that have a relatively high density and are considered heavier than hydrogen and helium. These heavy metals were thought to be essential for star formation, especially at the high levels required to form a globular cluster. But these results show otherwise.
“Our finding shows that massive globular clusters can form in the early universe from gas that has received only a small” spray “of elements other than hydrogen and helium,” Larsen said. “We have to try to figure out how we can form these massive star clusters with very small amounts of heavy elements.”
In essence, the data disputes what astronomers believe they know about massive groups of stars – indicating that scientists are missing something in their theories about how stars formed. The team is now on the hunt for other clusters that may show similar properties.
“We know there is at least one,” Larsen said. “Now we’d like to find out more and we can be a little more systematic about that.”
Scientists are working to form stars to gain a better understanding of how galaxies form. One theory suggests that galaxies begin as small clouds of stars and dust that merge with other clouds over time, eventually creating a galaxy like the Milky Way.
“I think there are still a lot of things we don’t know about how galaxies are built or when they first formed their first stars,” Larsen said. “There are many more details to fill out.”
Summary: Spherical clusters (GC) are dense, gravitationally connected systems of thousands to millions of stars. They are associated primarily with the oldest components of galaxies, and therefore measurements of their composition may give an idea of the accumulation of chemical elements in galaxies in the early universe. We report a massive GC in the Andromeda Galaxy (M31), which is extremely depleted in heavy elements. Its abundance of iron is about 800 times lower than that of the Sun and about three times lower than in the most iron GCs known so far. It is also highly depleted in magnesium. These measurements challenge the idea of a metal bottom for GC and the theoretical expectation that massive GCs could not form at such low metals.