Black hole stars challenge our idea of the universe
A potentially new cosmic object raises the question of which came first: black holes, or stars and galaxies?
Anjana Ahuja
© Andy CarterThe mysterious celestial objects became known as the “little red dots”.
The scarlet blobs, scattered over images of deep space collected by the orbiting James Webb Space Telescope, defied explanation.
The dots looked like bright mature galaxies but, puzzlingly, seemed anchored in a universe too young to host them.
It was like flicking through an album of cosmic baby photos and finding portraits of a teenager.
Now astronomers have come up with a potential explanation: the dots belong to a new class of cosmic object called black hole stars.
These are not stars in the usual sense of being powered by nuclear fusion; instead, they comprise a supermassive black hole wrapped in dense hydrogen gas.
As the central black hole devours surrounding matter and emits radiation, the illuminated gas cloud shines like the outer layer of a star.
The analysis, published this month in the journal Astronomy and Astrophysics, may well challenge ideas about how the first celestial objects formed, and in what order.
Did stars and galaxies give rise to black holes, or was it the other way around?
It also illustrates how studying extremes can push boundaries by forcing scientists to come up with new ideas.
It was an outlier among the little red dots that energised the idea of black hole stars — a concept that, though still speculative, has been well-received by cosmologists.
The James Webb Space Telescope, launched by Nasa in 2021, is a window on to the early universe, particularly a period called the cosmic dawn, the first few hundred million years after the Big Bang.
The instrument is designed to gather up the faint light emitted from the earliest cosmic objects; because the universe is expanding, this antique light stretches, or “redshifts”, as it travels billions of light-years across the cosmos.
When Webb surveys of the early universe started coming through in 2022, they were speckled with objects that astronomers struggled to explain.
Young galaxies are meant to appear blue; these objects glowed red.
Each compact dot shone with the dazzle of millions or billions of suns, a seemingly impossible stellar density.
A team led by Anna De Graaff at the Max Planck Institute for Astronomy in Heidelberg, including members from Penn State University and Princeton University, took a closer look at the nature of the light coming from one of the most prominent little red dots lying around 12bn light years away; some aspects of its dramatic spectra looked characteristic of a single star.
In addition, the pattern of wavelengths did not match the distinctive signatures of known celestial objects, such as dusty galaxies or quasars.
The best fit for the mystery object, they concluded, was a supermassive black hole surrounded by turbulent hydrogen gas.
As this cosmic monster feeds on its surroundings, it emits radiation that looks very much like starlight.
It is a plausible scenario, according to Martin Rees, the cosmologist and former astronomer royal, because of the clumpy way in which matter spread across the early universe as it expanded.
Most astronomers, he says, think the clumps fragmented into individual stars, making small galaxies — but that might not be the case.
Rees explains: “What seems to be happening — a surprise — is that sometimes the supermassive cloud doesn’t fragment but mainly collapses into one piece, making a black hole a million times more massive than an ordinary star.
The remaining gas swirls into it, heating up and creating a ‘superstar’.”
Black hole stars would presumably consume their gas cloud, which might explain why such objects are not seen today in our 13.8bn-year-old universe.
Any material in its neighbourhood that does not get sucked in, could go on to form stars and galaxies.
Rees notes that every galaxy, including our own Milky Way, is thought to have a black hole at its centre.
Solving the conundrum of which came first may depend on further observations, not only by the Webb telescope but also by the Square Kilometre Array, a radio telescope under construction in South Africa and Australia, and the Extremely Large Telescope being built in Chile.
Both should be operational in the next few years: astronomers and cosmologists hoping to catch a glimpse of the cosmic dawn will have more windows to look through.
The writer is a science commentator
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