Webb and Hubble Reveal How Giant Star Clusters Reionized the Universe

The early universe was dark and hot, with unbound particles racing in every direction. Eventually, it began to cool.

Electrons joined with protons, making it difficult for light to travel freely as space became neutral. That period of calm was short-lived.

A source then unleashed sufficient energy throughout the cosmos to split many of those particles apart once more. Astronomers refer to this episode as reionization, and its cause has been debated for years.

A fresh study strongly supports one explanation: enormous newly formed star clusters that burst through dense gas clouds much sooner than anticipated.

Using the James Webb Space Telescope together with the Hubble Space Telescope, researchers examined thousands of young star clusters in nearby galaxies.

The study has offered astronomers one of their most detailed views so far of the fleeting period when stars have formed but remain concealed within thick gas and dust clouds. This obscured stage may have played a part in transforming the early universe.

Violent beginnings for young stars

Stars are born as vast gas clouds collapse because of gravity. Once increasing numbers of stars ignite within these clouds, conditions quickly become turbulent.

Huge stars release powerful stellar winds and ultraviolet radiation before ultimately exploding as supernovae. These effects force away surrounding gas and prevent additional stars from forming in the nearby area.

Astronomers describe this phenomenon as stellar feedback.

For many decades, researchers have found it difficult to establish precisely how rapidly young star clusters disperse their birth clouds. Resolving this could clarify both galaxy evolution and the reionization of the early universe.

A clue to cosmic reionization

The research was carried out by an international group headed by scientists at Stockholm University and the Oskar Klein Center. Daniela Calzetti, Distinguished Professor at the University of Massachusetts Amherst, was among the co-authors.

“As the universe started to cool after the Big Bang, all the electrons and protons that had been scattered everywhere started to find each other and bind together until the universe assumed a neutral charge,” she said.

Later, an immense release of energy separated protons from electrons between galaxies during the event called reionization.

Astronomers have spent a long time seeking to identify the source of energy responsible for this change.

Webb and Hubble finally crack the cloud

Scientists have extensively investigated nearby star-forming regions in the Milky Way, although Earth’s location within the galaxy obscures much of them. Observing other galaxies instead allows them to study thousands of star clusters at varied points in their lives.

Webb’s infrared equipment made this possible because it can penetrate dense veils of gas and dust. Hubble supplied the complementary view through ultraviolet and visible-light observations.

Combined, the two telescopes enabled the team to assess nearly 9,000 star clusters in four nearby galaxies: Messier 51, Messier 83, NGC 628, and NGC 4449.

Certain clusters remained heavily embedded in gas, while others had begun to clear their local environment. Some had become completely visible.

Researchers identified a clear trend: large star clusters left their natal clouds considerably more quickly than smaller clusters.

The biggest clusters emerged after approximately 5 million years. By comparison, less massive clusters remained shrouded in gas for about 7 to 8 million years before appearing.

Although that difference may seem slight, it is enormously significant in astronomical terms.

Why giant star clusters matter

Massive star cluster formation probably contributed to the universe’s reionization.

The team determined that the largest clusters can leave their natal clouds in only 5 million years, allowing enough time to generate the photons required to reionize the universe.

Massive stars inherently emit vast quantities of ultraviolet radiation. When the gas around them is removed swiftly, that energy can reach space far earlier.

This may account for the universe’s shift from a faint, neutral environment to the luminous, structured cosmos observed today.

The results may also help refine computer models of galaxy evolution.

“Simulations of star formation and stellar feedback have struggled to reproduce how star clusters form and emerge from their natal clouds,” said Angela Adamo, a lead author on the study and principal investigator of FEAST. “These results give us important new constraints on that process.”

Planet formation under pressure

These findings have implications beyond ideas about galaxies. They could also alter how researchers understand planet formation.

Young stars are commonly encircled by rotating gas-and-dust structures known as protoplanetary discs. Planets eventually form from these discs.

However, planet formation could be halted at an early stage if nearby massive stars rapidly remove gas and bathe the region in ultraviolet radiation. Consequently, some star systems may be deprived of the material needed to create large planets.

Webb peers into stellar nurseries

“This work brings together researchers simulating star formation and those working with observations, as well as groups researching planet formation,” said Alex Pedrini.

“Using Webb, we can look into the cradles of star clusters and connect planet formation to the cycle of star formation and stellar feedback.”

The James Webb Space Telescope was built to probe far back into cosmic history, and research of this kind illustrates why its launch was so eagerly awaited by scientists.

Rather than simply detecting faraway galaxies, Webb reveals the concealed processes that moulded the universe well before Earth came into existence.

The complete study appeared in the journal Nature Astronomy.