Did This Black Hole Form Without a Supernova?

An international team of astronomers has revealed a black hole that seems to have formed without the usual supernova explosion. By probing exactly how this happened, they've helped cement a long-held suspicion that this mechanism is responsible for a host of disappearing stars.

When a massive star reaches the end of its life, it usually erupts in a cataclysmic celestial fireworks display. These supernovae can be so bright that they temporarily match the brightness of an entire galaxy — in mere seconds they can release as much energy as the Sun will during its multi-billion-year existence.

But do massive stars always detonate like this? Not according to a team of astronomers led by Alejandro Vigna-Gómez (Max Planck Institute for Astrophysics, Germany). Their work centers on a binary system in the Large Magellanic Cloud known as VFTS 243. “The system . . . is remarkable,” Vigna-Gómez says. It consists of an ordinary star some 25 times more massive than the Sun, accompanied by a black hole tipping the scales at 10 solar masses. The team's findings are published in Physical Review Letters.

When a star dies in a supernova, the material is usually ejected asymmetrically, leading to a kick that sends the stellar remnant surging off across space. Astronomers have seen many neutron stars that have been boosted in this way. Yet the two objects in the VFTS 243 system remain in an almost perfectly circular orbit around one another. If the black hole experienced a kick when it formed, then it must have been only a small one.

Vigna-Gómez’s team used this as an important constraint to help them model the black hole’s formation. “Combining advanced numerical models of stellar collapse with the principles of supernovae in binary star systems allowed us to obtain crucial insights into the complete collapse scenario,” Vigna-Gómez says.

The team’s result points toward neutrinos as a particularly important factor. “It allowed us to conclude, for the first time, that neutrinos are emitted nearly equally in all directions when the massive progenitor collapsed to form the black hole,” says team member Daniel Kresse (also at Max Planck Institute for Astrophysics). No large asymmetry, no big kick.

In fact, the team concludes that the original star lost only a small amount of mass — up to a third of a Sun — when its dense core collapsed. Rather than casting off its outer layers in a supernova, the collapse released energy primarily through a symmetrical ejection of neutrinos. The resulting kick to the newly formed black hole was just 4 kilometers per second.

By contrast, kicks observed for newborn neutron stars (also formed via core-collapse supernovae) are typically hundreds of kilometers per second, with some even exceeding 1,000 kilometers per second. According to Vigna-Gómez, the result for VFTS 243 demonstrates that massive black holes can form without a supernova explosion.

“The case . . . seems pretty convincing,” says Christopher Kochanek (Ohio State University), who was not involved in the research. However, Kochanek does point out one possible alternative: If the two objects started out in an elliptical orbit before a supernova explosion, there could have been some significant asymmetrical mass loss, with the resulting kick pushing them into a circular orbit.

Although not an open-and-shut case just yet, the direct collapse of black holes could help solve an enduring astronomical mystery. Previous efforts such as the Vanishing and Appearing Sources during a Century of Observations (VASCO) project have identified thousands of stars that disappeared from sky surveys conducted over the last 70 years. Some of these stars might have collapsed directly into black holes, without supernova fanfare — explaining their vanishing act. However if, unlike VFTS 243, they aren't in a binary system, then those complete-collapse black holes would be considerably more difficult to find.

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Editorial note (June 3, 2024): Changes to the story note that the stars have disappeared from sky surveys, not from the sky itself. Most of these "vanishing acts" are probably variable stars that have changed from bright to dim, and so are no longer picked up by surveys. However, some could be complete-collapse black holes.