Why Black Holes Do Not Turn into Stars After Losing Mass?
Introduction
Black holes, those mysterious cosmic entities, are often misunderstood. When a massive star goes supernova, conditions for a black hole can be achieved. But why, even as they lose mass, black holes do not turn back into stars? This article delves into how and why this process fails.
The Formation of Black Holes
After a large star exhausts its combustible fuel, it undergoes extreme gravitational collapse, leading to a supernova explosion. If the remaining mass is more than three times that of our Sun, a black hole can form. What happens during this process is fascinating. Initially, the star’s core collapses under its own gravity, and at certain masses, it experiences different forms of degeneracy pressure:
Electron Degeneracy Pressure: This pressure keeps white dwarfs from collapsing further. It can support a white dwarf up to a certain mass limit, known as the Chandrasekhar limit, which is about 1.4 solar masses.
Neutron Degeneracy Pressure: For neutron stars, neutron degeneracy pressure overcomes the mass limit for white dwarfs. This pressure allows a smaller volume to contain a huge amount of mass, but it has its limits.
Gravitational Collapse to Black Holes: When the mass is too great and the volume is compressed to a critical point where pressure can no longer resist gravity, a black hole forms. This point is defined by the Schwarzschild radius, which is zero for a black hole.
Black Holes and the Schwarzschild Radius
An essential characteristic of a black hole is its Schwarzschild radius. This is the distance from the center of the black hole to its event horizon, beyond which no light or matter can escape its gravitational pull. The critical aspect here is that the Schwarzschild radius is the distance over which the mass is concentrated. Once a black hole forms, all its mass is contained within this spherical boundary, independent of its size.
Even if a black hole loses mass over time, its Schwarzschild radius remains at zero. This is because mass and volume are inextricably linked in a black hole. If a black hole were to reduce its mass to the point of the Earth’s, its radius would still be zero. Therefore, it remains fundamentally a black hole, not a star.
The Role of Hawking Radiation
Considering the Hawking radiation theory, which states that black holes can lose mass over time due to quantum mechanical effects, the answer to why a black hole cannot return to a star is mainly due to its mass-to-radius ratio. As a black hole evaporates, its mass decreases in direct proportion to its radius. However, this process never leads to a non-black hole state because the mass-to-radius relationship is constantly maintained.
Therefore, while a black hole can theoretically evaporate over extremely long periods, it will always retain its black hole characteristics. This is due to the fundamental physical principles governing black holes, such as the Schwarzschild radius and Hawking radiation.
Conclusion
The reason black holes do not turn into stars after losing mass is deeply rooted in their unique physical properties. They are not just compact stores of mass but also have a fundamental singularity at their center. This singularity, combined with the absence of a nonzero radius, ensures that a black hole will always remain a black hole, no matter how much mass it loses over time. Understanding this process helps us appreciate the elegance and complexity of the universe's physics.