A white dwarf is not a typical star. While main sequence stars like our sun fuse nuclear material in their cores to prevent them from collapsing under their own weight, white dwarfs use an effect known as quantum degeneracy. The quantum nature of electrons means that no two electrons can have the same quantum state. When you try to get electrons into the same state, they exert a degenerative pressure that prevents the white dwarf from collapsing.
But there is a limit to how much mass a white dwarf can have. Subrahmanyan Chandrasekhar did a detailed calculation of this limit in 1930 and found that gravity will crush the star into a neutron star or black hole if a white dwarf has mass more than about 1.4 suns. However, the Chandrasekhar limit is based on a fairly simple model. One where the star is balanced and not rotating. True white dwarfs are more complex, especially when exposed to collisions.
A new kind of star that has never been seen in X-rays before. Photo credit: ESA / XMM-Newton, L. Oskinova / Univ. Potsdam, Germany
Binary white dwarfs are quite common in the universe. Many sun-like stars and red dwarfs are part of a binary system. When these stars reach the end of their lives in the main sequence, they become a binary system of white dwarfs. Over time, their orbits can disintegrate and the two white dwarfs collide. What happens next depends on the situation. Often times, they can explode as a nova or supernova and form a remaining neutron star, but sometimes they can form something more unusual, as shown by a recent article in Astronomy & Astrophysics.
In 2019, an X-ray source was discovered that resembled a white dwarf, but was too bright to be caused by a white dwarf. It has been suggested that the object could be an unstable fusion of two white dwarfs. In this new study, a team used the XMM-Newton X-ray telescope to take an image of the object shown above. They confirmed that the object has a mass greater than the Chandrasekar limit. The Super Chandrasekar object is surrounded by residual fog with high wind speeds. The nebula is mostly made up of neon and is shown as green in the image above. This is in line with the object created by a merger of the White Dwarfs. It probably has a high rotation that prevents the object from collapsing into a neutron star.
Eventually, this object will collapse into a neutron star within the next 10,000 years. This will likely create a supernova. It seems that a white dwarf can cross the Chandrasekhar limit, but only for a while.
Reference: Oskinova, Lidia M. et al. “X-ray observations of a super Chandrasekhar object show an ONe and a CO fusion product for white dwarfs embedded in a putative SN Iax residue.” Astronomy & Astrophysics 644 (2020): L8.
Reference: Gvaramadze, Vasilii V. et al. “A massive white dwarf merger before ultimate collapse.” Nature 569,7758 (2019): 684-6. 687.
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