Black Hole Formation from a Disappearing Star in the Andromeda Galaxy
A new paper led by Kishalay De (accessible here), on which I was privileged to be a co-author, reports the discovery of a disappearing star which likely collapsed to a black hole. The star, labeled M31–2014-DS1, faded to darkness in the Andromeda galaxy — the companion of our Milky-Way galaxy at a distance of about 2.5 million light years.
Stars are nuclear fusion reactors, bound by gravity. They are fueled by hot gas but once the fuel is consumed, gravity overwhelms pressure and they collapse to a remnant. The Sun will end its life in 7.6 billion years as its core will shrink to a white dwarf, a cold metallic remnant the size of the Earth. This is not a theoretical speculation but a sober realization, as the graveyard of the Milky-Way galaxy contains billions of corpses of Sun-like stars that formed long before the Sun and died by now. We observe many of these white-dwarf remnants because the Sun formed relatively recently, in the last third of cosmic history.
Stars more massive than the Sun by a factor of more than 8, end their life quicker through a collapse to a neutron star — a city size (~12 kilometers) remnant with the density of an atomic nucleus, or a black hole — the ultimate prison from which even light cannot escape. Just as in Las Vegas, whatever happens inside a black hole, stays there. Whereas the birth of a neutron star is often accompanied by a bright supernova explosion, the collapse of a massive star to a black hole could be far less dramatic, akin to the silent death of old people during their sleep. Some black hole births result in gamma-ray bursts, observable all the way to the edge of our cosmic horizon.
The collapse of a star to a black hole causes it to abruptly disappear. Although we expect a large population of ten million black hole remnants in the Milky-Way galaxy alone, the sudden disappearance of massive stars is challenging to observe in real time because it requires monitoring many stars for a long time. This is understandable from my experience as a child. Growing-up on a farm, I used to collect eggs from 2,000 chickens every day, but I never saw a chicken lay an egg because the process is very short lived.
Remarkably, the star M31–2014-DS1 in the Andromeda galaxy was observed to exhibit
such a disappearance between 2014 and 2022, with properties consistent with the failed explosion of a yellow supergiant progenitor with an initial mass of about 12 solar masses. The collapse likely led to the formation of a black hole carrying about 5 solar masses with the remaining mass ejected in an outflow.
Our paper presents infrared observations of the stellar remnant from the Webb Telescope and X-ray observations from the Chandra X-ray Observatory in 2024. The Webb data reveals an extremely red source, with strong blue-shifted absorption from molecular gas (CO, CO2, H2O and SO2) and dust. Modeling the dust confirms continued fading of the central source down to about 7% of the progenitor star luminosity, surrounded by a dust shell out to a scale comparable to the planetary system around the Sun. The molecular gas includes about a tenth of a solar mass of gas expanding at a hundred kilometers per second near the inner edge of the dust shell. No X-ray source is detected.
The data are consistent with a theoretical model in which the hydrogen envelope of the progenitor star was ejected while the core collapsed to a black hole. The central black hole is currently accreting only a small amount of loosely bound fallback material, about 0.1% of the original envelope mass, and produces faint radiation. The analysis fits well the fading of the star M31–2014-DS1 and provides the first insights into black hole formation through low-energy explosions and long-term fallback of massive stars.
M31–2014-DS1 is a star that failed to explode in a supernova, owing to its strong gravity. The remnant black hole is enshrouded in an opaque dust shell and has become progressively redder since 2022. The surrounding shell of molecular gas likely originated from the hydrogen-rich envelope of the progenitor star. Its outward motion is consistent with expanding ejecta produced by the eruption that enshrouded the remnant. The source continues to fade dramatically with a remnant black hole powered by fallback accretion with a low radiative efficiency, ~0.5%, for converting rest-mass to radiation.
The life of massive stars resembles the experience of Hollywood movie stars: they shine brightly for a short while and then fade into darkness in silence.
ABOUT THE AUTHOR
Avi Loeb is the head of the Galileo Project, founding director of Harvard University’s — Black Hole Initiative, director of the Institute for Theory and Computation at the Harvard-Smithsonian Center for Astrophysics, and the former chair of the astronomy department at Harvard University (2011–2020). He is a former member of the President’s Council of Advisors on Science and Technology and a former chair of the Board on Physics and Astronomy of the National Academies. He is the bestselling author of “Extraterrestrial: The First Sign of Intelligent Life Beyond Earth” and a co-author of the textbook “Life in the Cosmos”, both published in 2021. The paperback edition of his new book, titled “Interstellar”, was published in August 2024.
https://avi-loeb.medium.com/
https://www.youtube.com/@ProfessorAviLoeb
https://x.com/ProfAviLoeb
