The globular cluster Messier 13, or the Hercules Cluster, as seen by the Hubble Space Telescope. Somewhere at the heart of this dense stellar crowd may lie cosmic monsters known as superstars. (Image credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA); Acknowledgment: C. Bailyn (Yale University), W. Lewin (Massachusetts Institute of Technology), A. Sarajedini (University of Florida), and W. van Altena (Yale University))
Astronomers have discovered signs of supermassive stars at the center of globular clusters born not long after the Big Bang.
Cosmic monsters dwell in dense star clusters born just a few hundred million years after the universe's birth, new observations by the James Webb Space Telescope (JWST) suggest.
These monsters are supermassive stars, which JWST spotted signs of in globular clusters born about 13.4 billion years ago.
Globular clusters are found in almost every galaxy; our own, the Milky Way, hosts at least 180 of them. Not only are globular clusters the most massive and most ancient of star groupings, often containing up to a million stars born together as early as 440 million years after the Big Bang, but these stars can show anomalies not found in any other stellar collections.
For example, globular cluster stars tend to show high levels of compositional variation, despite the fact that they were born together at the same time from the same collapsing cloud of cool gas and dust. The proportion of oxygen, nitrogen, sodium and aluminum varies from one star to another in globular clusters. Explaining these so-called "abundance anomalies" has become a significant challenge for astronomers.
One potential explanation for this mystery, suggested back in 2018, is that supermassive stars "pollute" the original gas cloud as globular clusters form. This leads to the infant stars being unevenly enriched with chemical elements as they are forming.
Now, a team of researchers has announced that JWST has spotted chemical traces suggesting that monstrous stars are indeed lurking in stellar clusters, thus providing the first observational evidence for this enrichment theory.
"Today, thanks to the data collected by the JWST, we believe we have found a first clue of the presence of these extraordinary stars," study lead author Corinne Charbonnel, an astronomy professor at the University of Geneva in Switzerland, said in a statement(opens in new tab).
These supermassive stars are between 5,000 and 10,000 times as massive as the sun and are as hot as 135 million degrees Fahrenheit (75 million degrees Celsius) at their cores, compared to 27 million degrees F (15 million degrees C) or so at the heart of the sun. But, despite their intimidating size and fearsome temperatures, these stellar beasts are not always easy to locate. This is because they burn through their fuel for nuclear fusion quickly and thus have short lifespans.
"Globular clusters are between 10 and 13 billion years old, whereas the maximum lifespan of superstars is two million years," team member Mark Gieles, of the University of Barcelona, said in the same statement. "They therefore disappeared very early from the clusters that are currently observable. Only indirect traces remain."
To spot signs of these supermassive stars, the study team turned to the infrared vision of JWST to try and catch globular clusters earlier in their existence. The powerful space telescope saw light emitted by one of the most distant and earliest galaxies found to date, GN-z11. The galaxy is located around 13.3 billion light-years away, and JWST sees it as it was when it was just a few tens of millions of years old, making it a good choice as a hunting ground for young globular clusters.
Because chemical elements absorb and emit light at certain frequencies, the spectrum of light from cosmic sources contains "fingerprints" that point to the composition of celestial objects. The astronomers took light from GN-z11 seen by JWST and broke it down, finding two valuable pieces of information in the process.
"It has been established that it [GN-z11] contains very high proportions of nitrogen and a very high density of stars," said study team member Daniel Schaerer, an astronomy professor at the University of Geneva.
These facts suggest that several globular clusters are being born in GN-z11 as we see it, but also that those clusters still host active supermassive stars. This is because the strong presence of nitrogen can only be explained by the combustion of hydrogen at extremely high temperatures — temperatures that can be reached only in the cores of supermassive stars, Charbonnel said.
The results strengthen the model of supermassive star enrichment suggested by the team to explain the strange compositions of globular clusters. The next step in this investigation will be to look at more globular clusters in distant galaxies as seen by JWST to see if the same patterns hold.
The team's research was published this month in the journal Astronomy and Astrophysics(opens in new tab).
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