Massive stars, about eight times as massive as the Sun, explode as supernovae at the end of their lives. The explosions that leave a black hole or neutron star are so energetic that they can outshine their parent galaxies for months. However, astronomers appear to have discovered a massive star that skipped the explosion and turned straight into a black hole.
Stars are balancing acts between the external force of fusion and the internal force of their own gravity. As a massive star enters its final stages of evolution, it begins to run out of hydrogen and its fusion weakens. The external force of its merger can no longer counteract the star's strong gravity, and the star collapses in on itself. The result is a supernova explosion, a catastrophic event that destroys the star and leaves behind a black hole or neutron star.
However, it appears that sometimes these stars don't explode as supernovae and instead turn directly into black holes.
New research shows that a massive, hydrogen-poor supergiant star in the Andromeda Galaxy (M31) failed to explode as a supernova. The research is “The disappearance of a massive star that marks the birth of a black hole in M31.” The lead author is Kishalay De, a postdoctoral researcher at the Kavli Institute for Astrophysics and Space Research at MIT.
These types of supernovae are called core collapse supernovae, also called Type II. They are relatively rare and appear in the Milky Way about every hundred years. Scientists are interested in supernovae because they are responsible for the creation of many heavy elements and their shock waves can trigger star formation. They also produce cosmic rays that can reach Earth.
This new research shows that we may not understand supernovae as well as we thought.
Artist's impression of a Type II supernova explosion. These supernovae explode when a massive star reaches the end of its life, leaving behind either a black hole or a neutron star. But sometimes the supernova doesn't explode and collapses directly into a black hole. Image source: ESO
The star in question is named M31-2014-DS1. In 2014, astronomers noticed a brightening in the mid-infrared (MIR). Its luminosity remained constant for a thousand days. Then, for another thousand days between 2016 and 2019, it subsided dramatically. It is a variable star, but that cannot explain these fluctuations. In 2023, it went undetected in deep optical and near-infrared (NIR) imaging observations.
The researchers say the star was born with an initial mass of about 20 stellar masses and reached its final phase of nuclear combustion at about 6.7 stellar masses. Their observations suggest that the star is surrounded by a recently ejected dust envelope, suggesting a supernova explosion, but there is no evidence of an optical burst.
“The dramatic and sustained fading of M31-2014-DS1 is exceptional in the variability landscape of massive, evolved stars,” the authors write. “The sudden drop in luminosity in M31-2014-DS1 suggests the end of nuclear combustion, along with a subsequent shock that the incoming material cannot overcome.” A supernova explosion is so powerful that it completely overwhelms incoming material .
“As there is no evidence of a luminous outburst in this vicinity, the observations of M31-2014-DS1 suggest signs of a 'failed' SN leading to the collapse of the star's core,” the authors explain.
What could cause a star to not explode as a supernova, even if it has the right mass to explode?
Supernovae are complex events. The density inside a collapsing nucleus is so extreme that electrons are forced to combine with protons, creating both neutrons and neutrinos. This process is called neutronization and produces a powerful burst of neutrinos that carry about 10% of the star's rest mass energy. The burst is called a neutrino shock.
Neutrinos get their name from the fact that they are electrically neutral and rarely interact with normal matter. Every second, about 400 billion neutrinos from our sun pass through every person on Earth. But in a dense star core, the density of neutrinos is so extreme that some of them release their energy into the surrounding stellar material. This heats the material, creating a shock wave.
The neutrino shock always stops, but sometimes it comes back to life. When revived, it triggers an explosion and ejects the outer layer of the supernova. If it is not revived, the shock wave will fail and the star will collapse, forming a black hole.
This image shows how the neutrino shock wave can stall and lead to a black hole without a supernova explosion. A shows the initial shock wave, with the cyan lines representing the emitted neutrinos and the red circle representing the outwardly propagating shock wave. b shows stalling of the neutrino shock, with white arrows representing incoming matter. The outer layers fall inward and the neutrino heating is not strong enough to revive the shock. C shows the resolution of the failed shock as a dotted red line and the thicker white arrows represent the acceleration of the collapse. The outer layers collapse quickly and the core becomes more compact. D shows the formation of the black hole, with the blue circle representing the event horizon and the remaining material forming an accretion disk. (Source: Original illustration created for this article.)
In M31-2014-DS1, the neutrino shock was not revived. The researchers managed to limit the amount of material ejected from the star, and it was well below what a supernova would eject. “These constraints imply that the majority of the stellar material (?5 solar masses) collapsed into the core, exceeding the maximum mass of a neutron star (NS) and forming a BH,” they conclude. About 98% of the star's mass collapsed, forming a black hole with about 6.5 solar masses.
M31-2014-DS1 is not the only failed supernova or possible failed supernova that astronomers have found. They are difficult to recognize because they are characterized by what doesn't happen rather than what does happen. A supernova is hard to miss because it is so bright and suddenly appears in the sky. Ancient astronomers recorded several of these.
In 2009, astronomers discovered the only other confirmed failed supernova. It was an oversized red star in NGC 6946, the “Fireworks Galaxy.” It is named N6946-BH1 and has about 25 solar masses. After it disappeared from view, it left only a faint infrared light behind. In 2009, its luminosity increased to one million solar luminosities, but by 2015 it had disappeared in optical light.
A study using the Large Binocular Telescope monitored 27 nearby galaxies in search of disappearing massive stars. The results suggest that between 20 and 30% of massive stars may end their lives as failed supernovae. However, M31-2014-DS1 and N6946-BH1 are the only confirmed observations.
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