Categories: White dwarf

Strange Green Star is the Result of a Merger Between two White Dwarfs

A white dwarf isn’t your typical kind of star. While main sequence stars such as our Sun fuse nuclear material in their cores to keep themselves 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 squeeze electrons into the same state, they exert a degeneracy pressure that keeps the white dwarf from collapsing.

But there is a limit to how much mass a white dwarf can have. Subrahmanyan Chandrasekhar made a detailed calculation of this limit in 1930 and found that if a white dwarf has more mass than about 1.4 Suns, gravity will crush the star into a neutron star or black hole. But the Chandrasekhar limit is based upon a rather simple model. One where the star is in equilibrium and isn’t rotating. Real white dwarfs are more complex, particularly when they undergo collisions.

A new type of star that has never been seen before in X-ray light. Credit: ESA/XMM-Newton, L. Oskinova/Univ. Potsdam, Germany

Binary white dwarfs are fairly common in the universe. Many Sun-like stars and red dwarfs are part of a binary system. When these stars reach they reach the end of their main-sequence life they become a binary system of white dwarfs. Over time their orbits can decay, eventually causing the two white dwarfs to collide. What happens next depends upon the situation. Often they can explode as a nova or supernova, creating a remnant neutron star, but sometimes they can form something more unusual, as a recent paper in Astronomy & Astrophysics shows.

In 2019, an x-ray source was discovered that looked similar to a white dwarf but was too bright to be caused by a white dwarf. It was suggested that the object could be an unstable merger of two white dwarfs. In this new study, a team used the XMM-Newton X-ray telescope to capture an image of the object, seen above. They confirmed that the object has a mass greater than the Chandrasekar limit. The super-Chandrasekar object is surrounded by a remnant nebula with high wind speeds. The nebula is mostly made of neon, seen as green in the image above. This is consistent with the object having been created by a white dwarf merger. It likely has a high rotation, which prevents the object from collapsing into a neutron star.

Eventually, this object will collapse to become a neutron star within the next 10,000 years. It will likely create a supernova in the process. It seems a white dwarf can break the Chandrasekhar limit, but only for a while.

Reference: Oskinova, Lidia M., et al. “X-rays observations of a super-Chandrasekhar object reveal an ONe and a CO white dwarf merger product embedded in a putative SN Iax remnant.” Astronomy & Astrophysics 644 (2020): L8.

Reference: Gvaramadze, Vasilii V., et al. “A massive white-dwarf merger product before final collapse.” Nature 569.7758 (2019): 684-687.

Brian Koberlein

Brian Koberlein is an astrophysicist and science writer with the National Radio Astronomy Observatory. He writes about astronomy and astrophysics on his blog. You can follow him on YouTube, and on Twitter @BrianKoberlein.

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