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.

Recent Posts

A Worldwide Search for Dark Matter Fails to Turn up a Signal for This Mysterious Particle

A global search for dark matter turns up nothing, but than in itself is useful.

3 hours ago

With Webb Safely Launched, Focus Shifts to the Ariane 6

Last month, an Ariane 5 rocket carried the James Webb Space Telescope (JWST) safely to…

4 hours ago

Curiosity Sees a Strong Carbon Signature in a Bed of Rocks

Carbon is critical to life, as far as we know. So anytime we detect a…

2 days ago

Bad Weather Postpones Ingenuity’s 19th Flight on Mars

The first flight of 2022 for the Ingenuity Helicopter has been delayed due to a…

2 days ago

If Launched by 2028, a Spacecraft Could Catch up With Oumuamua in 26 Years

A new study by the Institute of Interstellar Studies (i4is) shows that with the right…

3 days ago

The Moon’s Crust was Formed From a Frozen Slushy Magma

Scientists' detailed study of the Moon dates back to the Apollo missions when astronauts brought…

3 days ago