Complete Stellar Collapse: unusual star system proves that stars can die quietly

One day, the star at the center of our own solar system, the Sun, will begin to expand until it engulfs Earth. It will then become increasingly unstable until it eventually contracts into a small and dense object known as a white dwarf.

However, if the Sun were of a weight class roughly eight times greater or more, it would probably go out with a huge bang – as a supernova. Its collapse would culminate into an explosion, ejecting energy and mass into space with enormous force, prior to leaving behind a neutron star or a black hole in its wake.

While this is basic knowledge about how massive stars die, there remains plenty to understand about the starry skies above and the spectacular death of these stars in particular.

New research by astrophysicists at the University of Copenhagen’s Niels Bohr Institute presents the strongest evidence to date that very massive stars can succumb with far more stealth and discretion than as supernovae. Indeed, their investigation suggests that, with enough mass, a star’s gravitational pull can be so strong that no explosion takes place upon its death. Instead, the star can go through what is known as a complete collapse.

 "We believe that the core of a star can collapse under its own weight, as happens to massive stars in the final phase of their lives. But instead of the contraction culminating into a bright supernova explosion that would outshine its own galaxy, expected for stars more than eight times as massive as the Sun, the collapse continues until the star becomes a black hole," explains first author Alejandro Vigna-Gómez, who was a postdoc at the Niels Bohr Institute when this study set in motion.

This discovery is linked to the phenomenon of disappearing stars, which has interested astronomers in recent years, and it may provide both a clear-cut example as well as a plausible scientific explanation for phenomena of this kind.

"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.

An unusual star system with no signs of an explosion

This discovery has been prompted by the recent observation of an unusual binary star system at the edge of our galaxy called VFTS 243. Here, a large star and black hole roughly 10 times more massive than our Sun orbit one another.

 Scientists have known about the existence of such binary star systems in the Milky Way for decades, where one of the stars has become a black hole. But the recent discovery of VFTS 243, just beyond the Milky Way in the Large Magellanic Cloud, is something truly special.

 "Normally, supernova events in star systems can be measured in various ways after they occur. But despite the fact that VFTS 243 contains a star that has collapsed into a black hole, the traces of an explosion are nowhere to be found. VFTS 243 is an extraordinary system. The orbit of the system has barely changed since the collapse of the star into a black hole," says Alejandro Vigna-Gómez.

The researchers have analysed the observational data for a range of signs that would be expected from a star system having undergone a supernova-explosion in the past. Generally, they found evidence of such an event minor and unconvincing.

The system does not show sign of a significant “natal kick”, an acceleration of the orbiting objects. It is also very symmetrical, almost perfectly circular in it’s orbit, and remaining signs from the energy release during the core collapse of the former star points to a type of energy consistent with complete collapse.

“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.

A benchmark system for future studies

According to Professor Tamborra, the VFTS 243 system opens the possibility for finally comparing a range of astrophysics theories and model calculations with actual observations. She expects that the star system will be important for studying stellar evolution and collapse.

"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," says the professor.