Stellar collapse: Why stars sometimes vanish from our skies

In the vast expanse of the universe, stars are born, live, and die in a cosmic dance that has captivated astronomers for centuries. While the life cycle of stars is well understood, there are still many mysteries surrounding their collapse and spectacular deaths.

A recent study by astrophysicists at the University of Copenhagen’s Niels Bohr Institute has shed new light on a perplexing phenomenon: the sudden disappearance of stars from the night sky.

Stars collapse into black holes instead of supernova

In our solar system, the Sun will eventually expand and become unstable, transforming into a white dwarf.

However, if it were eight times heavier, it would explode as a supernova, ejecting energy and mass before becoming a neutron star or black hole.

While this process is well-known for massive stars, there is still much to learn about their spectacular deaths.

What is a supernova?

A supernova is a powerful and luminous explosion that marks the end of a massive star’s life. When a star at least eight times more massive than our Sun exhausts its nuclear fuel, its core collapses under its own gravity.

This collapse triggers a violent explosion that releases an enormous amount of energy, often outshining an entire galaxy for a brief period.

The explosion ejects the star’s outer layers into space at incredible speeds, creating a spectacular light show visible across vast cosmic distances.

The remnant of the star’s core becomes either a neutron star or, if the original star was extremely massive, a black hole. Supernovae play a crucial role in the universe by recycling heavy elements and shaping the evolution of galaxies.

Basics of black holes

Black holes cannot be observed directly, as not even light can escape them. They can be identified by the energy emitted from the gasses around them or by their influence on nearby stars.

There are three types of black holes:

• Stellar black holes: Form from stars more than eight times the mass of the Sun. There may be as many as 100 million in our galaxy.
• Supermassive black holes: Found at the center of nearly all galaxies, including our Milky Way’s Sagittarius A*.
• Intermediate-mass black holes: Recently identified as the missing link between stellar and supermassive black holes.

In this study, scientists have provided strong evidence that very massive stars can collapse completely without a supernova. The findings suggest that a star’s gravitational pull can be so strong that it collapses into a black hole without an explosion.

“We believe that the core of a star can collapse under its own weight. Instead of a bright supernova explosion, the collapse continues until the star becomes a black hole,” said lead author Alejandro Vigna-Gómez, a postdoctoral fellow at the University of Copenhagen.

Rethinking stellar evolution and star collapse

This discovery is linked to the phenomenon of disappearing stars, which has intrigued astronomers for many decades.

“Were one to stand gazing up at a visible star going through a total collapse, it might just 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,” Vigna-Gómez explained.

“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 analyzing VFTS 243 has brought us much closer to a credible explanation,” he continued.

Witnessing a star collapse in VFTS 243

As an unusual binary star system, VFTS 243 features a large star and a black hole roughly 10 times the Sun’s mass. Despite the presence of a black hole, there are no signs of a supernova explosion.

“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,” Vigna-Gómez said.

VFTS 243 is an extraordinary system. The orbit of the system has barely changed since the collapse of the star into a black hole.

The system shows no significant “natal kick,” an acceleration typically caused by a supernova explosion, and the orbit remains symmetrical. Analysis points to the black hole in VFTS 243 forming immediately, with energy lost via neutrinos.

Missing “natal kick” eludes to star collapse

Supernova explosions typically give newborn neutron stars or black holes a “natal kick” due to asymmetric matter emission. This kick accelerates the compact object. In the VFTS 243 system, the black hole’s lack of acceleration suggests it did not undergo a supernova.

The system’s symmetrical orbit further supports the absence of an explosion. The researchers estimate that mass and energy released during the black hole’s formation were consistent with a scenario involving neutrinos rather than baryonic matter.

“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,” added co-author Irene Tamborra, an astrophysicist at the same university.

“This 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,” Tamborra concluded.

Pushing the boundaries of astrophysics

In summary, the discovery of VFTS 243 and the groundbreaking research conducted by the astrophysicists at the Niels Bohr Institute have proposed a new theory for understanding the mysterious phenomenon of disappearing stars and the formation of black holes.

This study challenges our current understanding of stellar evolution and collapse, offering compelling evidence for the existence of complete stellar collapse without the signature supernova explosion.

As we continue to explore the cosmos and unravel its secrets, the lessons learned from VFTS 243 will undoubtedly shape the future of stellar research, guiding us toward a deeper comprehension of the life and death of stars and the awe-inspiring intricacies of the universe.

Did a star vanish from the Pleiades?

Stars inexplicably disappearing have been frequently observed in modern times. For example, “A Survey about Nothing” led by astrophysicist Chris Kochanek actively looks for such stars and explanations for their disappearance.

Curious readers can explore historical accounts of disappearing stars, often tied to supernova scenarios. One example is the Greek myth associated with the Pleiades star cluster, known as the Seven Sisters.

The myth tells of the seven daughters of the titan Atlas and the nymph Pleione. According to the story, one daughter married a mortal and went into hiding, providing a poetic, albeit unscientific, explanation for why only six stars are visible in the Pleiades.

The study is published in the journal Physical Review Letters.

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