Scientists are puzzled by a strange object in the cosmos that appears to be a black hole, behaves like a black hole, and may even have similar characteristics to a black hole, but it has a key difference: there is no event horizon, meaning it is possible to escape its gravitational pull if enough effort is made.
This object, known as a Buchdahl star, is the densest object that can exist in the universe without turning into a black hole. Despite its theoretical existence, no one has ever observed one, sparking debate on whether these objects exist. A physicist may have recently discovered a new property of Buchdahl stars that could provide answers.
The existence of black holes is widely accepted by astronomers due to various forms of evidence, such as the detection of gravitational waves during collisions and the distinct shadows they cast on surrounding matter. It is also understood that black holes form from the catastrophic collapse of massive stars at the end of their life, following a Verve Times report.
Becoming Black Hole
However, there is still a lack of understanding of the limit of compression an object can endure before becoming a black hole. White dwarfs, containing the sun's mass in the Earth's volume, and neutron stars, which compress even further to the size of a city, are known to exist. But it remains unclear if other smaller objects can exist without becoming black holes.
In 1959, Hans Adolf Buchdahl, a German-Australian physicist, studied the behavior of a highly idealized "star" represented as a perfectly spherical blob of matter, as it is compressed as much as possible. As the blob becomes smaller, its density increases, making its gravitational pull stronger. Using the principles of Einstein's general theory of relativity, Buchdahl determined an absolute lower limit for the size of the blob.
This special radius is calculated as 9/4 times the mass of the blob, multiplied by Newton's gravitational constant, divided by the speed of light squared. The Buchdahl limit is significant as it defines the densest possible object that can exist without becoming a black hole. According to the theory of relativity, any object below this limit must always become a black hole. Naresh Dadhich, a physicist at the Inter-University Centre for Astronomy and Astrophysics in Pune, India, has discovered a new property held by Buchdahl stars. He calls Buchdahl stars "black hole mimics" as their observable properties would be nearly identical.
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Peculiar Buchdahl Star
He studied a hypothetical star's energy as it collapsed into a Buchdahl star. He found that as the star collapses, it picks up gravitational potential energy, which is negative because gravity is attractive. At the same time, the star's interior gains kinetic energy as all the particles are forced to jostle against each other in a smaller volume. When the star reached the Buchdahl limit, Dadhich found that the total kinetic energy was equal to half the potential energy.
This relationship is known as the virial theorem, and it applies to numerous situations in astronomy where the force of gravity is in balance with other forces, which means that a Buchdahl star could theoretically exist as a stable object with known, well-understood properties. This finding suggests that theoretical Buchdahl stars may exist in reality and could lead to a better understanding of the internal workings of black holes. Dadhich said in an email to Live Science that there have always been attempts to define objects as close as possible to black holes.
Furthermore, the event horizon of a black hole blocks our view of what's inside it. Also, humans can interact with a Buchdahl star and study its composition, which may give scientists hints as to what black hole internal compositions are like. However, currently, there is no known way for matter to form a Buchdahl star. But Dadhich's work provides a starting point for understanding how they may exist. Further research is needed to determine what other properties these exotic objects might have and what they could reveal about black holes.
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