When our Sun will exhaust its hydrogen fuel billions of years into the future, it would have turned into a Red Giant. In this form, the Sun will expands rapidly to huge proportions, eventually engulfing Earth and other rocky planets in the solar system. Once it runs out of helium too, our faithful Sun will see its final transformation into a dense and relatively cold object known as a white dwarf.

This stellar retirement is the most common fate for most stars. However, exceptionally large stars — at least ten times more massive than the Sun — typically go out with a bang, collapsing into a massive explosion that ejects a lot of energy and particles into space. This explosion is known as a supernova, and in its wake, either a superdense neutron star or a black hole is left behind.

However, exciting new research out of the University of Copenhagen has now found evidence for a novel way a star could die: by directly turning into a black hole, with no intermediate explosion. This event is known as a complete collapse and could explain a very troubling observation that has been puzzling astronomers for decades: the strange phenomenon of disappearing stars.

“Were one to stand gazing up at a visible star going through a total collapse, it might, just at the right time, be like watching a star suddenly extinguish and disappear from the heavens. The collapse is so complete that no explosion occurs, nothing escapes and one wouldn’t see any bright supernova in the night sky. Astronomers have actually observed the sudden disappearance of brightly shining stars in recent times.”

“We cannot be sure of a connection, but the results we have obtained from analysing VFTS 243 has brought us much closer to a credible explanation,” says Alejandro Vigna-Gómez, one of the study’s authors.

Vanishing stars

The discovery was made while observing a peculiar, far-away binary star system known as VFTS 243. This is not your typical system made of a star and some planets, nor your run-off-the-mill Tatoine-like binary star system. Rather, this system is formed of a large star and a freaking black hole about ten times more massive than the Sun, which orbit one another like a dog chasing its tail.

During their observations, the astronomers couldn’t see any convincing evidence of a supernova in VFTS 243, despite the obvious presence of the black hole. For instance, there was no “natal kick“, or what the researchers refer to as the acceleration imparted by the massive explosion on the remaining neutron star or black hole.

Normally, a natal kick would give a black hole a speed of 100 to 1,000 kilometers per second. However, the VFTS 243 black hole seems to have been accelerated to only 4 kilometers per second.

Likewise, a star system that has suffered a supernova event is typically asymmetric in its orbit due to the huge ejection of matter and energy. However, VFTS 243 is symmetric.

“The orbit of VFTS is almost circular and our analysis indicates there are no signs of large asymmetries during collapse. This again indicates the absence of an explosion,” says Alejandro Vigna Gomez.

By carefully monitoring the orbit of the binary star system, the astronomers calculated the estimated mass and energy released during the black hole’s formation. They conclude that the black hole was formed through a total stellar collapse. The small kick following this event was imparted not by neutrons and protons — the baryonic matter typically ejected during a supernova — but rather by neutrinos.

Overall, all these signs point to a direct gravitational collapse of one of the stars in VFTS 243 directly into a black hole. This may also explain the mystery of hundreds of vanishing stars. In 2019, astronomers part of the Vanishing and Appearing Sources during a Century of Observations (VASCO), compared data from the 1950s U.S. Naval Observatory Catalogue with the Pan-STARRS Data Release. They found 150,000 anomalies, with 100 stars appearing only in the older catalog.

“Our analysis unequivocally points to the fact that the black hole in VFTS 243 was most likely formed immediately, with the energy mainly being lost via neutrinos,” says Professor Irene Tamborra from the Niels Bohr Institute, who also participated in the study.

“Our results highlight VFTS 243 as the best observable case so far for the theory of stellar black holes formed through total collapse, where the supernova explosion fails and which our models have shown to be possible. It is an important reality check for these models. And we certainly expect that the system will serve as a crucial benchmark for future research into stellar evolution and collapse,” added Tamborra.

The study is published in the journal Physical Review Letters.