Why do stars mysteriously disappear from the night sky?

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

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

However, if the Sun had a mass class of about eight times that or more, it would probably go out with a huge bang—like a supernova. Its collapse would culminate in an explosion that would eject energy and matter into space with enormous force before leaving behind neutron star or the black hole in his tracks.

While this is a basic understanding 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 Niels Bohr Institute at the University of Copenhagen provides the strongest evidence yet that very massive stars can collapse much more quietly and discreetly than as supernovae. Indeed, their research suggests that with enough mass, the star’s gravitational pull can be so strong that no explosion occurs 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 with massive stars in the final stages of their lives. But instead of the contraction culminating in a bright supernova explosion that would eclipse its own galaxy, expected for stars more than eight times the mass of the Sun, the collapse continues until the star becomes Black hole” explains first author Alejandro Vigna-Gómez, who was a postdoc at the Niels Bohr Institute when this study was set in motion.

Facts and myths: fading stars

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

“A Survey about Nothing,” led by astrophysicist Chris Kochanko, is an example of a research effort actively looking for disappearing stars and explanations for their disappearance.

The curious reader can delve into the historical descriptions. These often have to do with suddenly bright stars that disappear in accordance with supernova scenarios. But there are other stories about stars suddenly disappearing, such as the Greek myth associated with the Pleiades star cluster, commonly known as the Seven Sisters. The Pleiades myth describes the seven daughters of the titan Atlas and the nymph Pleione. According to the myth, one of their daughters married a human and went into hiding, which provides 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 plausible scientific explanation for phenomena of this kind.

“If one were to stand looking at a visible star undergoing total collapse, at the right time it might be like watching a star suddenly go out and disappear from the heavens. The collapse is so complete that there is no explosion, nothing escapes, and one would not see any bright supernova in the night sky. Astronomers have indeed recently observed the sudden disappearance of brightly shining stars. We cannot be sure of the connection, but the results we obtained from the analysis of VFTS 243 brought us much closer to a credible explanation,” says Alejandro Vigna-Gómez.

An unusual star system with no signs of an explosion

The discovery was 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 a black hole roughly 10 times the mass of our Sun are orbiting each other.

Scientists knew of the existence of such binary star systems in Milky Way for 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 by the effect they have on the stars they orbit.

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

Stellar black holes – such as those in the VFTS 243 system – are formed when stars with 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.

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

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

There are also theories describing other types of black holes that have yet to be discovered. One of these, so-called primordial black holes, was supposed to have formed in the early universe and could theoretically be microscopic.

“Normally, 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 has collapsed into a black hole, traces of the explosion are nowhere to be found. VFTS 243 is an extraordinary system. The orbit of the system 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 can be expected from a star system that has experienced a supernova explosion in the past. They generally considered the evidence for such an event to be scant and inconclusive.

The system does not show signs of a significant “birth kick”, i.e. acceleration of orbiting objects. It is also very symmetrical, almost perfectly circular in its orbit, and the remaining signs of energy released during the collapse of the former star’s core indicate the type of energy corresponding to a total collapse.

“Our analysis clearly points to the fact that the black hole in VFTS 243 was most likely created instantaneously, with energy being lost mainly through neutrinos,” says Professor Irene Tamborra of 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 number of astrophysical theories and model calculations with real observations. He expects the star system to be important for studying the evolution and collapse of stars.

“Our results highlight VFTS 243 as the best-observable case to date of the theory of total-collapse stellar black holes failing a supernova explosion that our models have shown to be possible. This is an important reality check for these models. And we certainly expect the system to serve as a crucial benchmark for future research into stellar evolution and collapse,” says the professor.

Further information: Missing “Natal Kick” and other (missing) signs of a supernova

“Natal Kick” is not there

The violent forces of a supernova directly affect the newly born neutron stars or black holes left over from it due to the asymmetric emission of matter during the explosion. This is what scientists refer to as the “birth kick”. This kick causes the compact object to accelerate. A natal kick normally gives 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 suffered a major natal kick, as would be expected if it had gone supernova.

Similarly, the orbital symmetries of a star system usually show signs of having felt the impact of a violent supernova explosion due to the ejection of matter that occurs. Instead, the researchers found a symmetry.

“The VFTS orbit is nearly 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 matter and energy released during the formation of the black hole.

Their estimates are consistent with a scenario in which the smaller kick caused during stellar collapse was not caused by baryonic matter, which includes neutrons and protons, but rather by so-called neutrinos. Neutrinos have very little mass and interact very weakly. This is another sign that the system has not experienced an explosion.

Reference: “Constraints on Neutrino Natality Kicks from the Black-Hole Binary VFTS 243” by Alejandro Vigna-Gómez, Reinhold Willcox, Irene Tamborra, Ilya Mandel, Mathieu Renzo, Tom Wagg, Hans-Thomas Janka, Daniel Kresse, Julia Bodensteiner, Tomer Shenar and Thomas M. Tauris, May 9, 2024, Physical inspection letters.
DOI: 10.1103/PhysRevLett.132.191403

The following researchers contributed to 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 Discovery — OzGrav, Australia
• Center for Computational Astrophysics, Flatiron Institute, New York, USA
• Steward Observatory, University of Arizona, Tucson, USA
• Department of Astronomy, University of WashingtonSeattle, USA
• Technical University of Munich, TUM School of Natural Sciences, Department of Physics, Garching, Germany
• European Southern Observatory, Garching, Germany
• The School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel
• Aalborg University, Aalborg, Denmark