International effort to peek into stars’ ‘natal clouds’ reveals that massive star clusters emerge earlier than smaller ones
AMHERST, Mass. — Using data from both the James Webb and Hubble space telescopes, an international team of astronomers, including the University of Massachusetts Amherst’s Distinguished Professor Daniela Calzetti , have been able to, for the first time, glimpse one of the most mysterious parts of a star’s life: the moments just after birth, when the star is still wrapped in its “natal cloud.” This glimpse does more than shine a bright light on stars’ lifecycle; it also answers one of the thorniest questions from the early universe: how did the early universe manage to re-ionize itself after it cooled down from the Big Bang?
“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,” says Calzetti. “And then something happened: a massive burst of energy forced the protons and electrons between galaxies apart, in an event that astronomers call ‘Reionization.’ The questions that astronomers have been asking is, what was the source of this energy?”
Some astronomers have suspected that quasars were the source; others, the process of star formation; still others believed it to be a combination of the two.
The answer lay hidden inside what astronomers call the “natal cloud” of gas that surrounds newly born star clusters. As more massive stars are born in a collapsing cloud, strong stellar winds, harsh ultraviolet radiation and the supernova explosions at the end of the life of the massive stars eventually disperse the cloud, ending star formation before all the gas is depleted. Once the cloud of gas surrounding a nascent star cluster is gone, the star’s light can bear down on other regions of the galaxy where stars are forming. This process, called stellar feedback, means that most of the gas in a galaxy never gets used for star formation.
But that natal cloud has been difficult to see through, until now.
Studies of the closest star-forming regions, in the Milky Way galaxy and the dwarf galaxies that orbit it, allow astronomers to dissect star clusters in the smallest details, but Earth’s position in the disc of our galaxy means only a few such regions are visible to us. By observing nearby galaxies, astronomers can survey thousands of star-forming regions and characterize entire populations of star clusters at many stages of evolution—a feat made possible with the launch of space telescopes, most prominently the NASA/ESA/CSA James Webb Space Telescope and its ability to penetrate gaseous curtains with its infrared instruments.
The state of the art has been even further developed by pairing Webb’s data with Hubble’s, which can detect the ultraviolet and optical light from those regions. Together, the two space satellites were able to provide a broad-spectrum view of thousands of young star clusters. The international team, led by Angela Adamo with her student Alex Pedrini, both of Stockholm University and the Oskar Klein Center, has pored over images of four nearby galaxies — Messier 51 , Messier 83 , NGC 628 , and NGC 4449 — from the FEAST observing program (# 1783 ), trying to solve this mystery. Their results, published recently in Nature Astronomy , show that it is the most massive star clusters that clear away their gaseous shroud the fastest, and begin lighting their galaxy the earliest. Calzetti, along with her current UMass Amherst graduate student Drew Lapeer, former graduate student Benjamin Gregg and former postdoc Sean Linden, are all members of the team and co-authors of the Nature Astronomy paper.
The team identified nearly 9,000 star clusters in the four galaxies in different evolutionary stages: young clusters deeply embedded in clouds of gas, clusters that had partially dispersed the gas (both from Webb images), and fully unobstructed clusters visible in optical light (found in Hubble images).
With Webb’s ability to peer inside the gas clouds, they were able to then estimate the mass and age of each cluster from its light spectrum. The most massive clusters had fully emerged and dispersed the clouds of gas after around 5 million years, while less massive clusters were between seven and eight million years old when they emerged from their nurseries.
Answering this open question of which star clusters clear away their birth clouds the fastest advances our understanding of galaxy formation. “Simulations of star formation and stellar feedback have struggled to reproduce how star clusters form and emerge from their natal clouds. These results give us important new constraints on that process,” explained Adamo, a lead author on the study and principal investigator of FEAST.
It also helps explain the mystery of how the universe came to be reionized. “It had to be the formation of massive star clusters that helped drive the reionization of the universe,” says Calzetti. “The fact that the most massive clusters can emerge from their natal clouds in just 5 million years means that they had enough time for producing the photons that reionized the universe.”
Massive star clusters with their abundances of hot stars naturally emit most of the ultraviolet light in galaxies, but this work confirms that they also get a head start on producing stellar feedback over lighter clusters. Knowing where and when this stellar feedback is strongest throughout the lifetime of a galaxy allows astronomers to better predict how star-forming fuel is pushed around the galaxy and how stars, and star clusters, are likely to form.
Current theories of how planets form are also affected by this research. The faster gas is cleared away within a star cluster, the earlier protoplanetary discs around stars are exposed to harsh ultraviolet radiation from other stars, and the less opportunity they have to attract further gas from the nebula. This reduces the opportunities they have to grow dust and create planets.
“This work brings together researchers simulating star formation and those working with observations, as well as groups researching planet formation,” said 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.”
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