Astronomers Observe A White Dwarf Which Breaks Chandrasekhar Limit

Astronomy is one field that leaves no chance of amazing people again and again.

In a new study published in Nature, researchers have used the XMM-Newton X-ray telescope to capture an image of a strange white dwarf like star which was first seen in 2019.

In 2019, an X-ray source was discovered that seemed similar to be a white dwarf but was too bright to be caused by a white dwarf. A suggestion was that the object could be an unstable merger of two white dwarfs. The current study has observed this star again and got some conclusions.

The new research confirmed that the object has a mass greater than the Chandrasekar limit. This supports the previous claim saying that the object having been created by a white dwarf merger.

This object is surrounded by a remnant nebula with high wind speeds.

Some Background

A white dwarf, also called a degenerate dwarf, is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to that of the Sun, while its volume is comparable to that of Earth. 

These stars use quantum degeneracy in order to keep them from collapsing under their own weight. We know no two electrons can have the same quantum state. This means that when one tries to squeeze the electrons into the same state, they exert a degeneracy pressure that keeps the white dwarf from collapsing. 

However, there is a limit to much mass a white dwarf can have. Subrahmanyam Chandrashekar has made a detailed calculation of this limit in 1930 and found that if a white dwarf has a mass more than about 1.4 Suns, its gravity will crush the star into a neutron star or a black hole. 

But here enters something called binary white dwarfs which are fairly common in the universe. The binary system comprises many Sun-like stars and red dwarfs. These stars become a part of the binary system of white dwarfs when they reach their main-sequence life. 

Over a period of time, their orbits can decay, leading to the two white dwarfs to collide.

Depending on the situation, there are two things which can happen
1. They can explode in a supernova, creating a remnant neutron star
2. Sometimes they can form something unusual and the new study has observed this.

How is it the current star stable?
Researchers have suggested that the new object could have a high rotation preventing the object from collapsing into a neutron star. 

This could eventually collapse to become a neutron star in about 10000 years. A supernova could be formed in the process. It seems a white dwarf can break the Chandrasekhar limit, but only for a while.

Journal Reference:
Gvaramadze, V.V., Gräfener, G., Langer, N. et al. A massive white-dwarf merger product before final collapse. Nature 569, 684–687 (2019). DOI: 10.1038/s41586-019-1216-1