- 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?
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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.
-
- 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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--------------------- --- Saturday, January 21, 2023 ---------------------------
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