Massive stars, typically around eight times more massive than our Sun, undergo spectacular explosions known as supernovae at the end of their lives. These explosions, which are some of the most energetic events in the universe, often outshine their entire host galaxies for months, leaving behind either a neutron star or a black hole as a remnant. However, astronomers have recently observed an anomaly—a massive star that seemingly skipped the supernova explosion altogether and directly collapsed into a black hole. This discovery challenges long-standing theories about stellar death and opens up new avenues for research in stellar evolution.
Stars maintain a delicate balance between two opposing forces: the outward pressure from nuclear fusion at their cores, and the inward pull of gravity. When a massive star exhausts its hydrogen fuel, fusion weakens, and the outward pressure can no longer counteract gravity. As a result, the star collapses, triggering a supernova explosion. This explosion expels the outer layers of the star and leaves behind a remnant, which could either be a neutron star or, in cases where the mass is great enough, a black hole.
In some instances, however, massive stars seem to fail in the traditional supernova process, collapsing directly into black holes without the characteristic explosion. This is the case with M31-2014-DS1, a hydrogen-depleted supergiant star located in the Andromeda Galaxy (M31). Researchers observed this star over several years, and their findings, published in the preprint study titled “The disappearance of a massive star marking the birth of a black hole in M31,” offer compelling evidence that this star underwent a “failed” supernova event.
The study, led by Kishalay De, a postdoctoral scholar at the MIT Kavli Institute for Astrophysics and Space Research, sheds new light on how some massive stars may fail to explode. M31-2014-DS1 was initially spotted brightening in mid-infrared (MIR) light in 2014. Over the following years, its luminosity remained remarkably constant for about 1,000 days, before dramatically fading between 2016 and 2019. By 2023, the star was no longer detectable in deep optical and near-infrared (NIR) observations. This unique behavior raised questions, as variability in massive stars usually results from pulsations or other processes that don’t typically produce such a sudden and sustained fading.
The research team concluded that M31-2014-DS1 was a massive star, born with an initial mass of around 20 times that of the Sun, but having exhausted its hydrogen and ended up with roughly 6.7 solar masses at its core. Observations revealed that the star was surrounded by a shell of recently ejected dust, which is consistent with what would be expected from a supernova explosion. However, there was no optical outburst to confirm a typical supernova event. The authors described the star’s fading as an indication that nuclear burning ceased, but the subsequent shock wave failed to overcome the infalling material, preventing an explosion. In essence, the star’s core collapsed directly into a black hole rather than erupting in a supernova.
So, what could cause a star to fail to explode as a supernova? The answer lies in the complex physics of the core-collapse process. During this phase, the density inside the collapsing core becomes so high that electrons and protons combine to form neutrons and neutrinos in a process known as neutronization. This generates a burst of neutrinos, which carries away about 10% of the star’s rest mass energy. In a typical supernova, this neutrino burst triggers a shock wave that expels the outer layers of the star. However, this shock can stall before it fully expels the outer layers. If it is revived, it leads to an explosion. If not, the material falls inward, and the star collapses directly into a black hole.
In the case of M31-2014-DS1, the neutrino shock was never revived, meaning that instead of an explosive outburst, the core collapsed in on itself, forming a black hole. The researchers estimate that approximately 98% of the star’s mass (about 5 solar masses) collapsed into the core, resulting in the formation of a black hole with a mass of around 6.5 solar masses.
While M31-2014-DS1 provides compelling evidence of a “failed” supernova, it is not the first such candidate. In 2009, astronomers observed N6946-BH1, a supergiant star in the “Fireworks Galaxy” (NGC 6946), which disappeared without a typical supernova explosion. This star had a mass of about 25 solar masses, and its luminosity briefly increased to a million times that of the Sun before fading away. Like M31-2014-DS1, N6946-BH1 left behind only a faint infrared glow, suggesting a collapse into a black hole rather than an explosion.
A recent survey using the Large Binocular Telescope, which monitored 27 nearby galaxies, suggests that between 20% and 30% of massive stars could end their lives as failed supernovae. However, M31-2014-DS1 and N6946-BH1 are the only two confirmed cases so far, and much more research is needed to fully understand the frequency and mechanisms of these enigmatic events.
The discovery of stars that fail to explode as supernovae offers exciting new insights into stellar evolution and the end stages of massive stars. If a significant portion of massive stars indeed collapse directly into black holes, this could have profound implications for our understanding of black hole formation and the evolution of galaxies. Future observations and studies will be crucial in confirming whether this phenomenon is more common than previously thought, and how it might influence the larger cosmic environment.
Source: Universe Today