The theory is that a star could collapse directly into a black hole without first exploding as a supernova. In fact, this is relatively common. However, despite this, astronomers have found little observational evidence to support this.
But it could also have happened in our neighbor, the Andromeda galaxy, and astronomers almost missed it.
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The discovery was published in a study titled “The disappearance of massive stars in the Andromeda galaxy due to black hole formation.” The paper is published in the journal Science, and the lead author is Kisharai De, a professor of astronomy at Columbia University.
The researchers looked at serial images of M31 to look for sources of variation. Images were taken every six months from 2009 to 2022. “Using periodic six-month observations from 2009 to 2022, we looked for bright mid-infrared transients associated with failed dusty stellar eruptions such as SNe,” they explain. They discovered M31-2014-DS1, and over a two-year period beginning in 2014, this source increased mid-infrared flux by 50%.
After increasing brightness for 2 years, it decreased to below its initial luminous flux in 1 year. The dimming continued until 2022.
“This was probably the most surprising discovery of my life,” lead author De said in a press release. “The evidence for star extinction lay dormant in public archive data, but no one noticed it for years until we found it.”
This region is often observed by other ground-based and space telescopes, and the researchers used those observations to obtain the object’s optical light curve. Between 2016 and 2019, its optical light dimmed by a factor of about 100. The object is no longer detectable by ground-based optical observations in 2023.
Hubble happened to photograph it in 2022, but found nothing optically, only a faint source in the near-infrared (NIR). Follow-up NIR observations and spectroscopic analysis by Keck in 2023 confirmed a faint NIR source.
“The star’s dramatic and sustained dimming is highly unusual and suggests that the star’s core did not explode into a supernova and instead collapsed directly into a black hole,” De said.
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The authors say that whether a star collapses directly into a black hole without exploding as a supernova depends on neutrinos. When a massive star reaches the end of its life, its external radiation can no longer support its own mass. The core of the star collapses, releasing neutrinos, which send shock waves into the star’s outer layer, or the star’s envelope.
If the shock is strong enough, the envelope is ejected and the star explodes as a supernova. “If the shock fails to eject the envelope, it is predicted to fall onto the collapsing core, creating a stellar-mass black hole (BH) and killing the star,” the researchers wrote.
The star started out at about 13 solar masses. When it died, it had only about 5 solar masses. Most of its mass has been lost to powerful stellar winds.
“It has long been thought that stars of this mass always explode as supernovae,” De said. “The fact that there is no indication that stars of the same mass can either succeed or fail to explode is probably due to how gravity, gas pressure, and powerful shock waves interact chaotically inside a dying star.”
Astronomers are aware of another directly collapsing black hole candidate. It was observed in 2010 in the Grand Design spiral galaxy NGC 6946, about 25 million light-years away. However, it is about 10 times further away than M31-2014-DS1. The candidate, named N6946-BH1, was also a supergiant ancestor. It glowed and then slowly faded away, just like Andromeda’s celestial body.
Unfortunately, N6946-BH1 is much farther away, so it is much fainter, and the observational data is not as high quality as M31-2014-DS1. However, this new discovery makes N6946-BH1 relevant again.
“We know that black holes must come from stars,” said Morgan MacLeod, a lecturer in astronomy at Harvard University and a co-author of the paper. “With these two new events, we’re witnessing it happen, and in the process we’re learning an enormous amount about how the process works.”
It took a lot of effort to find M31-2014-DS1. This study is the largest study ever conducted on a tunable infrared source. They looked at star populations in the Milky Way and other nearby galaxies looking for such objects, but found only one. While supernovae are hard to miss and make their presence known by months of extreme brightness, the opposite is true for directly collapsing black holes.
“It’s easy to spot supernovae because they outshine an entire galaxy for several weeks, but it’s much harder to find individual stars that die without exploding,” De said.
Astronomers were buried under a mountain of astronomical data and almost missed this object. The question is, how many more are there? How common is it?
“It’s shocking to learn that a huge star basically disappeared (and died) without exploding and no one noticed it for more than five years,” De said. “This has major implications for our understanding of the mass catalog of stellar deaths in the universe. It shows that these events are occurring quietly outside Earth and may easily go unnoticed.”
As with many problems in astronomy and astrophysics, only more samples and better observations can advance our understanding of these directly collapsing black holes. Vera Rubin Observatory has the potential to discover many more of them in its 10-year legacy survey of space and time.
A version of this article originally appeared on Universe Today.
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