Saturday, January 21, 2023

3835 - BUCHDAHL STAR - when a blackhole is a star?

 

     -  3835  -   BUCHDAHL  STAR -  when a blackhole is a star?    An elusive object in space has posed a riddle for scientists. It looks like a black hole. It acts like a black hole. It may even smell like a black hole. But it has one crucial difference: It has no event horizon, meaning that you can escape its gravitational clutches if you try hard enough.


            ---------  3835  -  BUCHDAHL  STAR -  when a blackhole is a star?

            -    It's called a “Buchdahl star”, and it is the densest object that can exist in the universe without becoming a black hole itself.  Is it theory or is it real?

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            -     Astronomers understand how black holes form: They are the remnants of the catastrophic gravitational collapse of massive stars. When giant stars die, no force in nature is capable of sustaining the stars' own weight, so these doomed behemoths just keep crushing themselves to infinity.

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            -    What astronomers currently don't understand, however, is how compressed an object can get without becoming a black hole. We know of white dwarfs, which contain a sun's worth of mass in a volume equivalent to Earth, and we know of neutron stars, which compress all that down even further into the volume of a city. But we don't know if there's anything smaller still that avoids the fate of becoming a black hole.

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            -    In 1959, German-Australian physicist Hans Adolf Buchdahl explored how a highly idealized "star",  represented as a perfectly spherical blob of material,  might behave as it was compressed as much as possible. As the blob got smaller and smaller, its density rose, making its own gravitational pull even more intense. Using the tools of Einstein's general theory of relativity, Buchdahl found an absolute lower limit to the size of that blob.

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            -    That special radius is equal to 9/4 times the mass of the blob, multiplied by Newton's gravitational constant, all divided by the speed of light squared. The Buchdahl limit is important because it defines the densest possible object that can still avoid becoming a black hole.

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            -  Below that limit the blob of material must always become a black hole, at least in the theory of relativity.   Finding exotic objects that come right to the edge of that limit, so-called Buchdahl stars, has become a popular pastime of theorists and observationalists alike.

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            -    Now, Naresh Dadhich, a physicist,  may have discovered a surprising property held by Buchdahl stars.   Dadhich, calls Buchdahl stars "black hole mimics" because their observable properties would be nearly identical, studied what happens to the energy of a hypothetical star as it begins collapsing into a Buchdahl star.

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            -    As the star collapses, it picks up gravitational potential energy, which is negative because gravity is attractive.  At the same time, the interior of the star gains kinetic energy as all the particles are forced to jostle against each other in a smaller volume.

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            -    By the time the star reaches the Buchdahl limit, Dadhich found a surprising yet familiar relationship: 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. This means that a Buchdahl star could theoretically exist as a stable object with known, well-understood properties.

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            -    There has always been attempts to define objects that are as close as possible to black holes.  The event horizon of a black hole blocks our view of what's inside it. But we can interact with a Buchdahl star and study what it's made of, which may give us clues as to what black hole interiors are like.

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            -    Finding a Buchdahl star is another matter. To date, there is no known arrangement of matter that can create a Buchdahl star.  Further research will be needed to discover what other properties these exotic objects might have, and what they might tell us about black holes.

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            January 20, 2022       BUCHDAHL  STAR -  when a blackhole is a star?      3835                                                                                                                             

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