Why do stars mysteriously disappear from the night sky?

Astrophysicists from the University of Copenhagen are helping to explain the mysterious phenomenon where stars suddenly disappear from the night sky. Their study of an unusual binary star system resulted in compelling evidence that massive stars can completely collapse into black holes without going supernova.

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

However, if the Sun were in a weight class of about eight times that or more, it would probably go out with a big bang—like a supernova. Its collapse would culminate in an explosion, ejecting energy and mass into space with tremendous force, before leaving behind neutron star or the black hole behind it.

While this is the basic knowledge of how massive stars die, much remains to be understood about the starry sky above and especially the spectacular deaths of these stars.

New research by astrophysicists at the University of Copenhagen’s Niels Bohr Institute provides the strongest evidence yet that very massive stars can collapse with much more secrecy and discretion than as supernovae. Indeed, their research suggests that, with enough mass, a star’s gravitational pull can be so strong that it doesn’t explode after it dies. Instead, the star may undergo what is known as a total collapse.

“We believe that the star’s core can collapse under its own weight, as happens to massive stars in the final stages of their lives. But instead of the contraction culminating in a brilliant supernova explosion that would eclipse its own galaxy, as expected for stars more than eight times more massive than the Sun, the collapse continues until the star becomes black hole,” explains first author Alejandro Vigna-Gómez, who was a postdoctoral fellow at the Niels Bohr Institute when this study began.

Facts and Myths: Fading Stars

In modern times there have been many sightings of stars that inexplicably disappear.

The “Survey of Nothing” led by astrophysicist Chris Kochanek is an example of a research effort that is actively looking for disappearing stars and explanations for their disappearance.

The curious reader can delve into the historical descriptions. This often refers to sudden bright stars that disappear according to supernova scenarios. But there are other stories about stars that suddenly disappear, such as the Greek myth associated with the star cluster of the Pleiades, known as the Seven Sisters. The myth of the Pleiades describes the seven daughters of the titan Atlas and the nymph Pleione. According to the myth, one of their daughters married a man and hid, which gives a very unscientific but beautiful explanation for why we only see six stars in the Pleiades.

This discovery is related to the phenomenon of disappearing stars, which has interested astronomers in recent years, and can provide both a clear example and a convincing scientific explanation for phenomena of this type.

“If one were to stand and look at a visible star undergoing total collapse, it might, at just the right time, be like watching a star suddenly go out and disappear from the sky. The collapse is so complete that there was no explosion, nothing escaped, and no bright supernova would be visible in the night sky. Astronomers have actually noticed the sudden disappearance of bright stars in recent times. We cannot be sure of the connection, but the results we obtained from the analysis of VFTS 243 brought us much closer to a plausible explanation,” says Alejandro Vigna-Gómez.

An unusual star system with no signs of an explosion

The discovery was prompted by a recent observation of an unusual binary star system at the edge of our galaxy called VFTS 243. Here, a large star and a black hole roughly 10 times more massive than our Sun orbit each other.

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

Facts: Black holes

Not even light can escape from black holes. As such, they cannot be directly observed. However, some black holes can be identified due to the large amount of energy emitted by the gases rotating around them. Others, as in the case of VFTS 243, can be observed according to the influence they have on the stars they orbit.

In general, astronomers believe there are three types of black holes:

Stellar black holes – like those in the VFTS 243 system – are formed when stars more than eight times the mass of the Sun collapse. Scientists believe that there may be as many as 100 million of them in our galaxy alone.

Supermassive black holes – 100,000 to 10 billion times the mass of the Sun – are thought to be at the center of almost all galaxies. Sagittarius A* is a supermassive black hole at the center of our galaxy, the Milky Way.

Intermediate-mass black holes (IMBHs) – 100-100,000 times the mass of our Sun – have long been the missing link. In recent years, a number of credible candidates have emerged.

There are also theories that describe other types of black holes, which have yet to be discovered. One of these, so-called Primordial Black Holes, is said to have formed in the early universe and could theoretically be microscopic.

“Typically, supernova events in star systems can be measured in a variety of ways after they occur. But despite the fact that VFTS 243 contains a star that collapsed into a black hole, there are no traces of the explosion anywhere. The VFTS 243 is an outstanding system. The system’s orbit has hardly changed since the collapse of the star into a black hole,” says Alejandro Vigna-Gómez.

The researchers analyzed the observational data for a number of signs that might be expected from a star system that has undergone a supernova explosion in the past. In general, they considered the evidence for such an event to be minor and inconclusive.

The system shows no signs of a significant “natal bump,” the acceleration of objects in orbit. It is also highly symmetrical, almost perfectly circular in its orbit, and the remaining signs of energy release during the collapse of the former star’s core indicate a type of energy consistent with a complete collapse.

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

A reference system for future studies

According to Professor Tamborra, the VFTS 243 system opens up the possibility to finally compare a range of astrophysical theories and model calculations with actual observations. She expects the star system to be important for studying the evolution and collapse of stars.

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

Additional information: Missing “natal bump” and other (missing) supernova signs

“Natal bump” does not exist

The violent forces of a supernova directly affect the newborn neutron stars or black holes it leaves behind, due to the asymmetric emission of matter during the explosion. This is what researchers call the “natal bump.” This impact causes the compact object to accelerate. A natal shock will normally give neutron stars a measurable velocity of 100-1000 km per second. The velocity is expected to be lower for black holes, but still significant.

Because the black hole in the VFTS 243 system appears to have only been accelerated to about 4 km/s, it shows no signs of having received a significant natal shock, as would be expected if it had gone through a supernova.

Similarly, the symmetry of a star system’s orbit usually shows signs of having felt the impact of a violent supernova explosion, due to the ejection of matter that occurs. Instead, the researchers found symmetry.

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

A burst of energy

By analyzing the orbit of the binary star system, the team was also able to calculate the amount of mass and energy released during the formation of the black hole.

Their estimates are consistent with a scenario in which the smaller shock produced during the star’s collapse was caused not by baryonic matter, which includes neutrons and protons, but by so-called neutrinos. Neutrinos have a very small mass and interact very weakly. This is another indication that the system has not experienced an explosion.

Reference: “Constraints on Neutrino Natal Shocks from the Black-Hole Binary VFTS 243” by Alejandro Vigna-Gómez, Reinhold Willcox, Irene Tambora, Ilya Mandel, Mathieu Renz, Tom Wagg, Hans-Thomas Janka, Daniel Kresse, Julia Bodensteiner, Tomer Shenara and Thomas M. Tauris, May 9, 2024, Physical Review Letters.
DOI: 10.1103/PhysRevLett.132.191403

The following researchers participated in the research:

Alejandro Vigna-Gómez, Irene Tamborra, Hans-Thomas Janka, Daniel Kresse, Reinhold Willcox, Ilya Mandel, Mathieu Renzo, Tom Wagg, Julia Bodensteiner, Tomer Shenar, Thomas M. Tauris

The researchers are affiliated with several research institutions:

• Niels Bohr Institute, University of Copenhagen – International Academy and DARK
• Max-Planck-Institute for Astrophysics, Garching, Germany
• Institute of Astronomy, KU Leuven, Leuven, Belgium
• School of Physics and Astronomy, Monash University, Clayton, Australia
• ARC Center of Excellence for Gravitational Wave Detection—OzGrav, Australia
• Center for Computational Astrophysics, Flatiron Institute, New York, USA
• Steward Observatory, University of Arizona, Tucson, USA
• Institute of Astronomy, University of WashingtonSeattle, USA
• Technical University of Munich, Faculty of Natural Sciences TUM, Department of Physics, Garching, Germany
• European Southern Observatory, Garching, Germany
• School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel
• Aalborg University, Aalborg, Denmark