In space, there is an elusive object which has posed an enigma to scientists; it may look like a black hole, but it has a crucial difference from them: these are black holes without an event horizon, which means that if for some reason we get trapped on one of these, we could escape its gravitational clutches if we tried hard enough. These are the Buchdahl stars, the densest object in the Universe, which technically does not become a black hole.
What are Black Holes
Most scientists share the idea of the existence of black holes, as there are different proofs of their existence, such as the emission of gravitational waves when they collide and the spectacular shadows they create on the surrounding materials. It has even been possible to photograph the first black hole and the Sagittarius A*, the supermassive hole that lives in the heart of the Milky Way.
[Photo: NASA]
Something that is also known about them is their formation: they are remnants of the catastrophic gravitational collapse of massive stars. When giant stars die, no force of nature can support their own weight, so they continue drifting ad infinitum. But like everything else in the Universe, there are still things that have not been understood in their entirety, such as how far an object can be compressed without becoming a black hole.
We are aware of white dwarf stars, which contain the mass of the Sun in a volume equivalent to the Earth, or neutron stars that may be able to compact their volume and more into the volume of a city. However, there is still something we do not know anything about: if there exists something even smaller that may be able to avoid the fate of becoming a black hole.
Stars That May Transform into Black Holes without an Event Horizon
Hans Adolf Buchdahl, a German-Australian physicist, studied the behavior of an idealized star in 1959, which was represented as a perfectly spherical bubble material, compressed to its maximum. As this bubble became smaller each time, its density increased, making its own gravitational pull even stronger.
Buchdahl’s limit is important as it defines the densest possible object that can avoid becoming a black hole. Below that limit, the bubble material will always become a black hole, at least in the Theory of Relativity.
Although Buchdahl’s stars are interesting, the truth is that they are hypothetical since no one has been able to observe any of them. Consequently, doubt invades, and scientists wonder if they really exist or not. Recently, Narres Dadhich, a physicist at the Inter-University Center for Astronomy and Astrophysics in Pune (India), may have discovered a new property of these stars that may be able to help them know if they are real.
[Photo: USRA]
Dadhich, who calls Buchdahl’s stars a type of mimicry black holes since their observable properties would almost be identical, has studied what happens to the energy of a hypothetical star when it begins to collapse and becomes a Buchdahl star.
“A star is formed from the collapse of a self-gravitating dispersed system, and so as collapse proceeds, gravitational energy, which is negative, keeps on increasing in the exterior,” Dadhich explained. At the same time, the interior of the star gains kinetic energy, and that is because all of its particles are forced to push against each other in a volume that is each time smaller.
In this new study, Dadhich has discovered that when the star reaches Buchdahl’s limit, its total kinetic energy becomes equal to half of the potential energy. This relationship is known as the Virial Theorem.
This result indicates that a Buchdahl star could theoretically exist as a stable object with known and well-understood properties, suggesting that they could indeed be out there and could lead to insights into the inner workings of black holes.
References: Dadhich, N. Equation of state for Buchdahl star and black holw. The Astrophysical Journal (2023). DOI
Story originally published in Spanish in Ecoosfera
