Why do stars mysteriously disappear from the night sky?

Astrophysicists at the University of Copenhagen are helping to explain a mysterious phenomenon where stars suddenly disappear from the night sky. Their study of an unusual binary star system has yielded convincing evidence that massive stars can completely collapse and become 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 shrinks into a small, dense body known as a White dwarf.

However, if the Sun were approximately eight or more times as massive, it would likely explode in a massive explosion – in the form of a supernova. Its collapse will cause an explosion, hurling energy and mass into space with tremendous force, before leaving behind a huge mass of energy. Neutron star Or a black hole in its wake.

While this is basic knowledge about how massive stars die, there is still a lot to understand about the starry sky above and the spectacular death of these stars in particular.

The binary star system VFTS 243 is located in the dwarf galaxy “Large Magellanic Cloud” in the vicinity of the Milky Way. The Magellanic Clouds are satellite galaxies of the Milky Way. These dwarf galaxies, which orbit the galactic center, can only be seen from the Southern Hemisphere. Here, they can be seen above the auxiliary telescopes of the European Southern Observatory's Very Large Telescope (VLT) in Paranal, Chile. Credit: JC Muñoz/ESO

New research by astrophysicists at the Niels Bohr Institute at the University of Copenhagen provides the strongest evidence yet that very massive stars can undergo much greater stealth and secrecy than supernovas. In fact, their research suggests that with enough mass, a star's gravity could be so strong that no explosion would occur when it died. Alternatively, the star could undergo what is known as a complete collapse.

“We believe that the star's core could collapse under its own weight, as happens to massive stars in the final phase of their lives. But instead of the collapse culminating in a bright supernova explosion that outshines its galaxy, stars more than eight times the mass are expected to exist.” Sun, the collapse continues until the star becomes a star. Black hole“Explains first author Alejandro Vigna-Gomez, who was a postdoctoral researcher at the Niels Bohr Institute when this study began.

Facts and Myths: Disappearing stars

In modern times, there have been many observations about Stars that disappear inexplicably.

“A survey about nothing.” Led by astrophysicist Chris Kochanek, it is an example of a research effort that is actively searching for disappearing stars and explanations for their disappearance.

The curious reader can also delve into the historical descriptions. It often has something to do with suddenly bright stars disappearing, consistent with supernova scenarios. But there are other stories about stars that suddenly disappear, such as the Greek myth associated with the Pleiades star cluster, known as the Seven Sisters. The myth of the Pleiades describes the seven daughters of the giant Atlas and the nymph Pleione. According to legend, 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 the disappearance of stars, which has aroused the interest of astronomers in recent years, and may provide a clear example and a reasonable scientific explanation for phenomena of this type.

“If one stood and stared at a visible star undergoing complete collapse, it might, in time, be like watching a star suddenly extinguish and disappear from the sky. The collapse was so complete that no explosion occurred, nothing escaped, and one would see no supernova Bright in the night sky Astronomers have already noticed the sudden disappearance of bright stars recently. “We cannot be sure of a connection, but the results we obtained from the analysis of VFTS 243 have brought us much closer to a reliable explanation,” says Alejandro Vigna-Gómez. .

A view from the Webb Space Telescope of the Tarantula Nebula, where VTFS 243 is located. Image credit: NASA, ESA, CSA, and STScI

An unusual star system with no signs of 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 massive star and a black hole roughly ten times larger than our Sun orbit each other.

Scientists have known that such binary star systems exist in the universe milky way For decades, a star turned into a black hole. But the recent discovery of VFTS 243, beyond the Milky Way in the Large Magellanic Cloud, is truly special.

Facts: Black holes

Not even light can escape from black holes. As such, they cannot be observed directly. However, some black holes are recognizable due to the large amounts of energy released by the gases orbiting them. Others, as in the case of VFTS 243, can be observed by their influence on the stars around which they orbit.

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

Stellar black holes — like the one in VFTS 243 — form when stars with a mass more than eight times the mass of the Sun collapse. Scientists believe 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 exist 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 (IMBH) — 100 to 100,000 times the mass of our Sun — have been the missing link for a long time. In recent years, a number of credible candidates have emerged.

There are also theories describing other types of black holes, which have not yet been discovered. One of these holes, called primordial black holes, was supposed to have formed at the beginning of the universe and could theoretically be microscopic.

“Normally, supernova events in stellar systems can be measured in different ways after they happen. But despite the fact that VFTS 243 contains a star that collapsed into a black hole, no traces of an explosion have been found anywhere. VFTS 243 is an exceptional system.” “The system's orbit has barely changed since the star collapsed into a black hole,” says Alejandro Vigna-Gomez.

The researchers analyzed observational data for a set of signs that could be expected from a star system that experienced a supernova explosion in the past. In general, they find evidence of such an event simple and unconvincing.

The system shows no sign of a significant “birth kick,” that is, the acceleration of orbital objects. It is also very symmetrical, almost perfectly circular in its orbit, and residual signs of energy release during the previous star's core collapse indicate a type of energy consistent with complete collapse.

“Our analysis points unambiguously to the fact that the black hole in VFTS 243 likely formed instantaneously, with energy lost mainly by neutrinos,” says Professor Irene Tambora from the Niels Bohr Institute, who was also involved in the study.

A reference system for future studies

According to Professor Tambora, VFTS 243 opens the possibility of comparing a range of astrophysical theories and model calculations with actual observations. She expects the stellar system to be important for studying the evolution and collapse of stars.

“Our results highlight VFTS 243 as the best observable case so far of the theory of stellar black holes formed through complete collapse, where the supernova explosion that our models have shown is possible fails. It is an important real-world test for these models,” says the professor. “We certainly expect it to be “The system serves as a critical benchmark for future research on stellar evolution and collapse.”

Additional information: Missing “birth kick” and other (missing) signs of a supernova

The “birth kick” is not there

The violent forces of the supernova directly affect the newborn neutron stars or the black holes left behind, due to the asymmetric emission of matter during the explosion. This is what researchers refer to as the “birth kick.” This kick causes the compressed body to accelerate. The kick of birth typically gives neutron stars a measurable speed of between 100 and 1,000 kilometers per second. The speed is expected to be lower for black holes, but it is still significant.

Because the black hole in VFTS 243 appears to have accelerated to approximately 4 km/s, it shows no sign of having received a major birth kick, as would be expected if it had undergone a supernova.

Likewise, the symmetry of a star system's orbit usually shows signs that it has 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 indicates that there are no signs of significant asymmetry during the collapse. This again indicates that there was no explosion,” says Alejandro Vigna Gomez.

Energy explosion

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 fit a scenario in which the smaller kick that occurred during the stellar collapse was not caused by baryonic matter, which includes neutrons and protons, but by so-called neutrinos. Neutrinos have very little mass and interact very weakly. This is another indication that the system did not suffer an explosion.

Reference: “Constrains on natal neutrino kicks from the binary black hole VFTS 243” by Alejandro Vigna-Gomez, Reinhold Wilcox, Irene Tambora, Ilya Mandel, Matteo Renzo, Tom Waag, Hans-Thomas Janka, Daniel Kress, Julia Bodensteiner, Tomer Shenar and Thomas M. . Torres, May 9, 2024, Physical review letters.
doi: 10.1103/PhysRevLett.132.191403

The following researchers contributed to the research:

Alejandro Vigna-Gomez, Irene Tambora, Hans Thomas Janka, Daniel Kress, Reinhold Wilcox, Elia Mandel, Matteo Renzo, Tom Waag, Julia Bodensteiner, Tomer Shenar, Thomas M. Torres

The researchers belong to several research institutions:

• Niels Bohr Institute, University of Copenhagen – International Academy and Dark
• Max Planck Institute for Astrophysics, Garching, Germany
• Institute of Astronomy, University of Leuven, Leuven, Belgium
• School of Physics and Astronomy, Monash University, Clayton, Australia
• ARC Center of Excellence for Gravitational Wave Detection, Osgraph, 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 Faculty of Natural Sciences, Department of Physics, Garching, Germany
• European Southern Observatory, Garching, Germany
• Faculty of Physics and Astronomy, Tel Aviv University, Tel Aviv, Israel
• Aalborg University, Aalborg, Denmark