One idea, two cosmic mysteries—linking Little Red Dots and globular clusters

A new study led by astronomers at The University of Texas at Austin proposes a theory that could solve two astronomical riddles at once: the nature of Little Red Dots and the origin of globular clusters. Rather than representing distinct objects, the study suggests that one may instead be the ancestor of the other: Little Red Dots are, in fact, an early form of globular clusters. The findings are published in The Astrophysical Journal Letters.

First detected by the James Webb Space Telescope (JWST) in 2022, Little Red Dots are mysterious objects that appear 600 million years after the Big Bang, only to seemingly disappear 1.5 billion years later. They are compact, luminous and shine with a distinctive combination of red and ultraviolet light.

One theory is that Little Red Dots represent supermassive black holes, enshrouded in dense clouds of gas, that pull young stars into dramatic deaths. This scenario explains many of the objects’ signature properties. However, other scenarios could also fit.

For example, an early globular cluster with a supermassive star at its heart would also look a lot like a Little Red Dot.

“These may not be just a strange new JWST population with no connection to the universe around us today,” said John Chisholm, an astronomer at UT Austin and lead author on the study. “Instead, Little Red Dots may persist past the early universe, evolving into something relatively familiar.”

An old mystery in orbit

Globular clusters are dense collections of ancient stars found orbiting galaxies. The Milky Way alone contains around 150 of them. They can host hundreds of thousands or even millions of stars. Although astronomers have studied these objects for well over a century, their origins remain unknown.

“We usually see them after billions of years of evolution, at a time when their massive stars are gone, their gas has been cleared out, and dynamical processes have changed their masses and structures,” explained Danielle Berg, an astronomer at UT Austin and co-author on the study. “That makes it very hard to reconstruct the original conditions they formed in.”

The stars in globular clusters are all roughly the same age, developing during a burst of stellar activity in the early universe. While astronomers would expect stars from this era to have relatively straightforward chemistry, some clusters show unexpected patterns that are hard to explain. They contain an abundance of helium, nitrogen, sodium and aluminum, while being low in carbon, oxygen and magnesium.

“This specific pattern indicates nuclear fusion at very high temperatures, much higher than in the cores of even massive normal stars,” said UT Austin’s Mike Boylan-Kolchin, a co-author on the study. “A supermassive star is precisely the kind of environment that could produce this combination.”

A chemical trail to follow

Such a gigantic star—up to hundreds of thousands of times more massive than our sun—could have formed from a series of stellar collisions early in a globular cluster’s life. As stars merged with one another, over and over again, a central supermassive star would eventually form. Though short-lived, this star would be an incredibly powerful chemical furnace, forging material in its core in unusual ways.

“When they die, they would blow that material back out,” explained Berg, “seeding the next generation of stars with the chemical fingerprints we still see in globular clusters today.”

“In our model,” added Chisholm, “the supermassive star that helps make the object look like a Little Red Dot would live for only a short time. Once that star dies, the object may no longer look like a Little Red Dot, even if the cluster itself survives billions of years.”

Clues that line up

While chemistry provides a compelling connection between the two objects, additional clues could also link them. For one, the distribution of Little Red Dots in the early universe corresponds with the distribution of globular clusters in the present day. Models of Little Red Dot evolution also show that their mass could readily transform into that of today’s globular clusters. What’s more, Little Red Dots appear in the universe at roughly the same time the oldest globular clusters are expected to have formed.

“There’s no single smoking gun at this point that says Little Red Dots are globular clusters, but it would explain a lot of diverse and surprising observations,” said Boylan-Kolchin.

“Little Red Dots could be galaxies, they could involve black holes, or they could be something even more unexpected,” continued Chisholm. “Our work shows that forming globular clusters with supermassive stars should be part of that conversation.”

Who’s behind this story?

Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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