Death is not always a straightforward affair for stars. It can actually get bizarre straight. An extremely rare Undead Star born in a fantastical collision detected by astronomers. A star made up of the remains of two dead stars that fused together, reigniting fusion in the core-bringing the new star back to life.
The star in the constellation of Cassiopeia is called J005311, 10,000 light-years away. It caught the attention of astronomers because it sits within a planetary nebula that emits nearly exclusively infrared radiation, and no optical light at all. So, they decided to look closer, using spectroscopy to analyze the strange cloud’s chemical composition. And that’s where it became even more curious. The spectral analysis showed that the object did not contain any hydrogen or helium.
But a clue lay in it as well. Main sequence stars in their centers fuse hydrogen into helium. But a white dwarf – the remaining ‘ dead ‘ left behind at the end of a solar-mass star’s lifespan, with a mass up to 10 times the Sun’s mass-typically has none. During their lifetime, they will have burned through their supply of hydrogen, fusing it into helium. The core contracts when this is all gone and begins to fuse the helium into carbon and oxygen itself.
From this fusion, the radiation pressure causes the outer layers of the star to expand into a red giant. Ultimately, when the helium runs out, these outer layers will be thrown into space. Forming a planetary nebula around the white dwarf’s brilliantly shining but cooling core-too small to fuse the remaining oxygen and carbon.
But for a single white dwarf, J005311 shines too brightly-it’s about as bright as 40,000 Suns. However, we do know that most of the stars in the sky are in binary systems at least. And, as data from the collaboration between LIGO-Virgo is now being revealed, mergers between dead stars may not be all that rare.
Astronomers believe that J005311 is the result of a fusion of two white dwarfs. The two stars have orbited each other in a closer and closer spiral over the course of millions of years, inexorably drawn together. They come together and become one star.
“This is an extremely rare event,” said astronomer Götz Gräfener of the University of Bonn’s Argelander Institute for Astronomy (AIfA). “Probably in the Milky Way there aren’t even half a dozen such objects, and we have found one of them.”
This one star has combined the mass of two stars. Which means that it would now have enough mass to fuse elements heavier than hydrogen or helium. And J005311 is rich in both carbon and oxygen, as follow-up spectroscopy revealed. It also has an extremely powerful 16,000-kilometer stellar wind per second, a stream driven by nuclear fusion radiation.
Fusion alone cannot account for the power of this wind. But an extremely powerful magnetic field is expected to be the product of a white dwarf fusion. This magnetic field would then speed up the stellar wind, producing an effect as seen in J005311.
When white dwarfs wind down, they turn into cold lumps in space, called black dwarfs, at least theoretically. It is thought that this process will take a very long time, and astronomers believe it has not actually happened yet. But J005311 doesn’t have such a quiet fate in store. That’s because it probably puts its new combined mass over something called the Chandrasekhar limit, which is the maximum mass for a stable white dwarf.
Its temperature and wind speed show that the star is close to the endpoint of its current evolutionary stage. When it runs out of burning material, it will probably collapse under its own gravity in a few thousand years. Fusing electrons and protons into neutrons, turning the Franken star into a neutron star of low mass. This extremely rare undead star, born in a fantastical collision detected by astronomers will also produce a neutrino flash and a gamma-ray burst, as well as a very weak Type Ic supernova, the researchers said.
The study was published in Nature.