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

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 a white dwarf.

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

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

The binary star system VFTS 243 is located in the Large Magellanic Cloud dwarf galaxy on the outskirts of the Milky Way. The Magellanic Clouds are satellite galaxies of the Milky Way. These dwarf galaxies that orbit the galactic center are only visible from the southern hemisphere. Here they are seen above the auxiliary telescopes of ESO’s Very Large Telescope (VLT) in Paranal, Chile. Credit: JC Muñoz/ESO

A new study by astrophysicists at the University of Copenhagen’s Niels Bohr Institute presents the strongest evidence yet that very massive stars can succumb with much more stealth and discretion than as supernovae. In fact, their research suggests that, given enough mass, a star’s gravitational pull can be so strong that an explosion does not occur after its death. Instead, the star may go through what is known as total collapse.

“We believe that the core of a star can collapse under its own weight, as happens with massive stars in the last phase of their lives. But instead of the contraction culminating in a bright supernova explosion that will outshine its own galaxy, expected for stars more than eight times more massive than the Sun, the collapse continues until the star becomes Black holeexplains first author Alejandro Viña-Gómez, who was a postdoctoral fellow at the Niels Bohr Institute when this research began.

Facts and Myths: Vanishing Stars

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

“Study for Nothing,” led by astrophysicist Chris Kochanek, is an example of a research effort that actively seeks out disappearing stars and explanations for their disappearance.

The inquisitive reader can also delve into the historical descriptions. They are often associated with suddenly glowing stars that disappear in accordance with supernova scenarios. But there are other stories of stars suddenly disappearing, such as the Greek myth related to the Pleiades star cluster 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 human and went into hiding, 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 may provide both a clear example and a plausible scientific explanation for phenomena of this kind.

“If one stood watching a visible star undergoing total collapse, it might, at just the right moment, be like watching a star suddenly extinguish and disappear from the heavens. The collapse is so complete that no explosion occurs, nothing comes out, and one would not see a 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 the connection, but the results we obtained from the analysis of VFTS 243 brought us much closer to a plausible explanation,” says Alejandro Viña-Gómez.

Webb Space Telescope view of the Tarantula Nebula, home to VTFS 243. Courtesy: NASA, ESA, CSA and STScI

An unusual star system with no signs of an explosion

This 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 more massive than our Sun orbit each other.

Scientists have known about the existence of such binary star systems in Milky Way for decades, where one of the stars has turned into 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 black holes. As such, they cannot be observed directly. However, some black holes can be identified due to the large amounts of energy emitted by the gases swirling around them. Others, as in the case of VFTS 243, can be observed by 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 of the VFTS 243 system – form when stars with more than eight times the mass of the Sun collapse. Scientists believe there could 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 the 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. A number of credible candidates have emerged in recent years.

There are also theories that describe other types of black holes that have yet to be discovered. One of these, so-called primordial black holes, is thought 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 has collapsed into a black hole, traces of an explosion are nowhere to be found. The VFTS 243 is an exceptional system. The system’s orbit has hardly changed since the collapse of the star into a black hole,” says Alejandro Viña-Gómez.

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

The system shows no sign of a significant “natal kick,” the acceleration of orbiting objects. It is also very symmetrical, almost perfectly circular in its orbit, and the remaining signatures of the energy release during the collapse of the former star’s core point to a type of energy consistent with a complete collapse.

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

A benchmark system for future research

According to Professor Tambora, the VFTS 243 system opens the possibility to finally compare a number 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 observable case yet for the theory of stellar black holes formed by total collapse where a supernova explosion fails, 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 crucial benchmark for future studies of the evolution and collapse of stars,” says the professor.

Further information: The missing “Natal Shock” and other (missing) signs of a supernova

The “natal kick” is gone

The violent forces of a supernova directly affect the newborn neutron stars or black holes left by it, due to the asymmetric emission of matter during the explosion. This is what researchers call the “natal kick”. This kick causes the compact object to accelerate. The birth kick typically gives neutron stars a measurable velocity of 100-1000 km per second. The velocity is expected to be smaller for black holes, but still significant.

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

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

“The VFTS orbit is nearly circular, and our analysis shows no signs of large asymmetries during collapse. This again shows the lack of explosion,” says Alejandro Viña Gómez.

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 weaker shock delivered during stellar collapse is not due to baryonic matter, which includes neutrons and protons, but rather to so-called neutrinos. Neutrinos have very little mass and interact very weakly. This is another indication that the system has not exploded.

Reference: “Constraints on Neutrino Natal Shocks from Black Hole Binary VFTS 243” by Alejandro Viña-Gómez, Reinhold Wilcox, Irene Tambora, Ilya Mandel, Matthew Renzo, Tom Waag, Hans-Thomas Janka, Daniel Kresse, Julia Bodensteiner, Tomer Schenar and Thomas M. Tauris, 9 May 2024, Physical examination letters.
DOI: 10.1103/PhysRevLett.132.191403

The following researchers contributed to the study:

Alejandro Viña-Gómez, Irene Tambora, Hans-Thomas Janka, Daniel Kresse, Reinhold Wilcox, Ilya Mandel, Matthew Renzo, Tom Wagg, Julia Bodensteiner, Tomer Schenar, 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
• Department of Astronomy, University of WashingtonSeattle, USA
• Technical University of Munich, TUM School of Natural Sciences, Faculty of Physics, Garching, Germany
• European Southern Observatory, Garching, Germany
• School of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel
• Aalborg University, Aalborg, Denmark