A star 2.5 million light years away quietly disappeared and was replaced by a black hole

This image shows a failed supernova that turned directly into a black hole without exploding.

Stars in the universe all perform the cycle of life in their own unique way. Usually, when a star's mass reaches more than eight times that of the sun, it will end its journey heroically at the end of its life in the form of a supernova explosion.

A star that explodes like a supernova can light up its entire galaxy for months, but astronomers have discovered a star that skipped the supernova explosion and collapsed directly into a black hole.

The disappearing star

This unusual star is called M31-2014-DS1 and is located in the Andromeda galaxy, 2.5 million light-years away.

In 2014, the star M31-2014-DS1 was noticed by astronomers as it began to brighten in the mid-infrared. For the next 1,000 days, its brightness remained stable. But then, over another 1,000 days between 2016 and 2019, its brightness dropped dramatically.

By 2023, it had completely disappeared from the telescope's field of view, and could no longer be found using either optical or near-infrared imaging.

The researchers believe that the star was about 20 times more massive than the Sun when it was born, but at the end of its life, it was only about 6.7 times more massive than the Sun. The observations also showed that M31-2014-DS1 is surrounded by a recently ejected dust shell, which is consistent with the characteristics of a supernova explosion, but there is no evidence of an optical burst.

The researchers believe they may have witnessed the collapse of a star known as a "failed supernova".

The Andromeda Galaxy, 2.5 million light-years away

The fate of stars

Why do stars experience such a fate? This starts with the combustion mechanism of stars .

Stars use nuclear fusion reactions to convert hydrogen into helium, releasing enormous amounts of energy. When their hydrogen fuel is exhausted, the helium continues to fuse into heavier elements, all the way to iron.

When the core of a star is filled with unreactive iron, nuclear fusion reactions cannot continue, and the star can no longer generate enough energy to counteract the gravitational pull of its outer layers, causing the star to begin to collapse inward.

In most cases, this collapse will trigger a violent supernova explosion, releasing huge amounts of energy and possibly leaving behind a neutron star or black hole. However, in some cases, it may fail to trigger a supernova explosion and collapse directly into a black hole, which is called a "failed supernova" , as is the case with the star M31-2014-DS1.

The disappearance of M31-2014-DS1 provides a rare opportunity for scientists to peek into this rare phenomenon. The relevant research paper was published on the arXiv "preprint" website on October 18.

Not the only "failed" supernova

Failed or suspected failed supernovae are usually hard to spot because they are so quiet and invisible that nothing seems to happen. Supernovae, on the other hand, are extremely bright and appear suddenly in the sky, making them hard to miss. Ancient astronomers recorded many supernova events.

NGC 6946 (also known as the "Fireworks Galaxy")

However, M31-2014-DS1 is not the only object of this type discovered by astronomers.

In 2009, astronomers discovered another confirmed "failed" supernova. It was a super red giant in the NGC 6946 galaxy (also known as the "Fireworks Galaxy"), named N6946-BH1, with a mass equivalent to about 25 suns. After disappearing, it left only a faint infrared glow. Its brightness once soared to 1 million times the brightness of the sun in 2009, but by 2015, it could no longer be seen through optical observations.

Scientists have used the LBT large binocular telescope to observe 27 nearby galaxies, looking for massive stars that have quietly disappeared. The results show that 20% to 30% of massive stars may end up as "failed" supernovae . However, M31-2014-DS1 and N6946-BH1 are currently the few confirmed observation results.

References

https://www.livescience.com/space/black-holes/extremely-rare-failed-supernova-may-have-erased-a-star-from-the-night-sky-without-a-trace

https://phys.org/news/2024-11-star-andromeda-birth-black-hole.html

Compiled by: Liu Yun

Reviewed by: Li Xin, Research Librarian of Beijing Planetarium