One of the brightest stars in the Andromeda galaxy quietly collapsed into a black hole without any of the fanfare of a spectacular supernova.
What makes this startling discovery even more remarkable is
that the first signs of the transformation were recorded back in 2014 – data
that is crucial for understanding the different ways black holes can form after
the death of a giant star.
"This has probably been the most surprising discovery
of my life," says astronomer Kishalay De of Columbia University in the US,
who led the research. "The evidence of the disappearance of the star was
lying in public archival data, and nobody noticed for years until we picked it
out."
When a massive star many times heavier than the Sun dies,
it's not expected to go quietly. Once nuclear fusion in the core can no longer
generate sufficient outward pressure against the inward pull of gravity, the
core collapses.
This can send a giant shock tearing outward through the
star, triggering a supernova explosion that sends the star's outer material
flying, while the core transforms into either a neutron star or a black hole.
However, this is not the only way this transformation can
take place. In some scenarios, the outward shock stalls. Instead of ripping the
star apart, the explosion fizzles out, and the material ends up falling back
onto the newly formed black hole. Because this is a much less dramatic process
than a supernova, clear evidence of it is relatively rare.
"Unlike finding supernovae, which is easy because the
supernova outshines its entire galaxy for a few weeks, finding individual stars
that disappear without producing an explosion is remarkably difficult," De
explains.
Only one such event had been documented previously, a star
recorded vanishing around 2010 in a galaxy 22 million light-years away. Now, by
carefully looking over archival observations of the Andromeda galaxy, De and
his colleagues have found another, and it's even clearer than the previous
example.
M31-2014-DS1 was a supergiant star that started out about 13
times the mass of the Sun and shone brightly, even across the 2.5 million
light-year distance between the Milky Way and Andromeda.
Then, in 2014, NASA's NEOWISE telescope recorded it suddenly
shining more intensely in infrared, increasing its brightness by about 50
percent over about two years.
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An illustration of a failed supernova, surrounded by hot
dust. (Keith Miller, Caltech/IPAC - SELab) |
Then, between 2016 and 2022, it dimmed dramatically to the
point where, by 2023, it completely vanished from view in optical wavelengths.
But it wasn't just visible light that faded: The star's
total brightness across the entire spectrum dimmed by at least a factor of 10.
Today, it's detectable only in mid-infrared light, shining at roughly a tenth
of its former infrared brightness.
"This star used to be one of the most luminous stars in
the Andromeda Galaxy, and now it was nowhere to be seen," De says.
"Imagine if the star Betelgeuse suddenly disappeared. Everybody would lose
their minds! The same kind of thing [was] happening with this star in the
Andromeda Galaxy."
The team's detailed analysis shows the sequence of events is
consistent with a failed supernova. First, the infrared brightening is
consistent with dust ejected from the dying star settling into a surrounding
cocoon instead of being blasted out into space.
Then, the dramatic dimming across all wavelengths shows that
light levels didn't drop because of dust blocking its light, as we saw with
Betelgeuse back in 2019. If the culprit was just dust, infrared brightness
would not have dropped, since infrared light can penetrate dust clouds.
The entire-spectrum dimming indicates that the entire energy
output of the star dropped, consistent with a cessation of fusion.
"The dramatic and sustained fading of this star is very
unusual, and suggests a supernova failed to occur, leading to the collapse of
the star's core directly into a black hole," De says.
"Stars with this mass have long been assumed to always
explode as supernovae. The fact that it didn't suggests that stars with the
same mass may or may not successfully explode, possibly due to how gravity, gas
pressure, and powerful shock waves interact in chaotic ways with each other
inside the dying star."
The resulting object, the researchers calculated, is likely
to be a black hole with about five times the mass of the Sun – an event horizon
about 30 kilometers (18 miles) across.
Because these events happen so quietly, the fact that
astronomers have now found two of them just a few years apart suggests two
really exciting things. The first is that our ability to see what the Universe
is getting up to, even when it's subtle, is improving. The second is that these
failed supernovae may actually be a more common pathway than we thought.
"It comes as a shock to know that a massive star
basically disappeared (and died) without an explosion and nobody noticed it for
more than five years," De says. "It really impacts our understanding
of the inventory of massive stellar deaths in the Universe. It says that these
things may be quietly happening out there and easily going unnoticed."
The research has been published in Science.