Black dwarf

A black dwarf is a theoretical stellar remnant. It represents the final stage of a cooling white dwarf. A white dwarf is the dense core left behind after a medium-mass star dies. As a white dwarf loses its heat, it eventually stops emitting significant light or warmth. This dark, cold object is called a black dwarf. Because these objects are defined by their lack of light, they are purely hypothetical. Scientists believe they will exist in the very distant future.

The process of formation begins with a main sequence star. These stars must have a mass below approximately 9 to 10 solar masses. When such a star runs out of fuel, it can no longer perform nuclear fusion. This is the process where atoms join together to release energy. The star then expels its outer layers. What remains is a dense sphere of electron-degenerate matter. This matter cools very slowly through a process called thermal radiation. Thermal radiation is the release of heat as light or infrared waves.

Black dwarfs are expected to have a specific chemical makeup. They would be composed mainly of carbon and oxygen. However, they might also contain trace amounts of other elements. These elements include neon and magnesium. The term "black dwarf" is sometimes used for a different type of object as well. It can refer to late-stage, cooled brown dwarfs. These are substellar objects that never had enough mass to start hydrogen-burning nuclear fusion.

Scientists use mathematical models to estimate how long this cooling takes. Barrow and Tipler estimate it takes 10 to the 15th power years for a white dwarf to cool. This is a massive amount of time. However, some theories suggest the process could be much slower. If weakly interacting massive particles, or WIMPs, exist, they might interact with the star. These interactions could keep white dwarfs warmer for about 10 to the 25th power years. Another factor is proton decay. This is a theoretical process where protons, the building blocks of atoms, break down. Adams and Laughlin calculated that proton decay could keep a white dwarf warm for 10 to the 37th power years.

We know that no black dwarfs exist in the universe today. The current age of the universe is 13.79 billion years. The time required to form a black dwarf is much greater than this age. In 2012, astronomers used the MDM Observatory's 2.4 meter telescope to find something interesting. They discovered white dwarfs that had cooled below the M0 spectral class. These stars are estimated to be 11 to 12 billion years old. These findings help scientists set observational limits on the age of the universe.

There is a theory that some massive black dwarfs might eventually explode. This could happen through pycnonuclear fusion. Pycnonuclear fusion is a type of fusion driven by high density rather than temperature. This process could turn much of the star into nickel-56. This nickel then decays into iron by emitting a positron. This change could lower the Chandrasekhar limit. The Chandrasekhar limit is the maximum mass a stable white dwarf can have. If the star's mass exceeds this new limit, it could collapse. This collapse would trigger runaway nuclear fusion, resulting in a supernova.

Our own Sun will eventually follow this lifecycle. In about 8 billion years, the Sun will stop fusing helium in its core. It will then eject its layers as a planetary nebula. The remaining core will become a white dwarf. Over trillions of years, it will cool into a black dwarf. At that point, the Sun will no longer be visible to the naked human eye. We would only be able to detect it through its gravitational influence.