Black hole

A black hole is a geometrically defined region of spacetime exhibiting such strong gravitational effects that nothing—including particles and electromagnetic radiation such as light—can escape from inside it. The theory of general relativity predicts that a sufficiently compact mass can deform spacetime to form a black hole. The boundary of the region from which no escape is possible is called the event horizon. Although crossing the event horizon has enormous effect on the fate of the object crossing it, it appears to have no locally detectable features. In many ways a black hole acts like an ideal black body, as it reflects no light. Moreover, quantum field theory in curved spacetime predicts that event horizons emit Hawking radiation, with the same spectrum as a black body of a temperature inversely proportional to its mass. This temperature is on the order of billionths of a kelvin for black holes of stellar mass, making it essentially impossible to observe.

Objects whose gravitational fields are too strong for light to escape were first considered in the 18th century by John Michell and Pierre-Simon Laplace. The first modern solution of general relativity that would characterize a black hole was found by Karl Schwarzschild in 1916, although its interpretation as a region of space from which nothing can escape was first published by David Finkelstein in 1958. Black holes were long considered a mathematical curiosity; it was during the 1960s that theoretical work showed they were a generic prediction of general relativity. The discovery of neutron stars sparked interest in gravitationally collapsed compact objects as a possible astrophysical reality.

Frozen star (hypothetical star)

In astronomy, a Frozen star, besides a disused term for a black hole, is a hypothetical type of star that, according to the astronomers Fred Adams and Gregory P. Laughlin, may appear in the future of the Universe when the metallicity of the interstellar medium is several times the solar value.

Characteristics

Due to opacity effects, as metallicity of the interstellar gas increases both the maximum and minimum masses a star can have will decrease. For the latter case, it's expected that an object with a mass of 0.04 solar masses (40 times the mass of Jupiter), that currently would become a brown dwarf unable to fuse hydrogen, could do so ending in the main sequence with a surface temperature of 0 °C (273 K, thus frozen), much cooler than the dimmest red dwarfs of today, and water ice clouds forming in its atmosphere. The luminosity of these objects would be more than a thousand times smaller than the faintest stars currently existing and their lifetimes would be also sensibly longer.