A star’s mass at the time of birth determines what it becomes at the end of its life. While a star with a huge mass may end up as a neutron star or a black hole, low to medium mass ones may become a white dwarf. Our Sun is the nearby example; during the end of its life, it will become a white dwarf.
A white dwarf is as massive as our Sun, yet only slightly bigger than the Earth. With such high densities, approximately 200,000 times that of Earth, it has one of the densest collections of matter in our known universe.
A typical star fuses hydrogen into helium in its core; the resulting hot gases create an outwards pressure, while gravity pulls the star inward. A delicate balance between the pressure and the gravity makes the star stable. There is no fusion in a white dwarf to create the internal pressure, and gravity compacts the matter until even the electrons are smashed together.
What is preventing the white dwarfs from collapsing?
According to Pauli’s exclusion principle, identical electrons are not allowed to occupy the same energy level. Since electrons can have only two spins states, only two electrons can occupy the same energy level. This is not a problem in a normal gas, but in white dwarfs, the electrons occupy almost all the energy levels due to extreme densities – a state referred to as a “degenerate” gas. Gravity cannot compress any more as there is no more space available for the electrons to go. This effect is known as quantum degeneracy.
There is a limit to the amount of mass a white dwarf can have. In 1930, Subrahmanyan Chandrasekhar calculated the limit to be about 1.4 Suns.
Anything more will crush the star into a neutron star or black hole. The calculation is based on a simple model assuming the star is in equilibrium and isn’t rotating.
Real white dwarfs are much complex than that, especially when they undergo collisions. Binary star systems are common in the universe; Cataclysmic variables (CV) is an example – a white dwarf and a normal star orbiting around their commons barycenter.
When a binary star system with two white dwarfs eventually collides due to orbital decay, they often explode as a supernova or a nova.
In 2019, an X-ray source discovered was suggested to be from an unstable merger of two white dwarfs. In a new study, published recently in Astronomy & Astrophysics, a team captured the object’s image using the XMM-Newton X-ray telescope. The object is surrounded by a remnant nebula made mostly of neon, giving the bizarre green zombie look consistent with being created by a white dwarf merger.
The team confirmed that the object has a mass greater than the Chandrasekar limit, showing that white dwarfs can exist with a higher mass than originally thought was possible.
As with any other white dwarf, this object will eventually collapse to become a neutron star within the next 10,000 years.
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