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Star Waltz with a Dramatic End

Astronomers from the University of Bonn and their colleagues from Moscow have identified an unusual celestial object.

It is most likely the product of the fusion of two long-dead stars. After billions of years around us, these so – called white dwarves have merged and are resurrected from the dead. In the near future, their lives could finally end – with a huge blow. The researchers are now presenting their results in the journal Nature.

The extremely rare fusion product was discovered by scientists from the Moscow University. On images taken by the Wide-field Infrared Survey Explorer (WISE) satellite, they found a gas nebula with a bright star in the center. Surprisingly, the nebula emitted almost exclusively infrared radiation and no visible light. "Our colleagues in Moscow realized that this already pleaded for an unusual origin," says Dr. Götz Gräfener of the Argelander Institute for Astronomy (AIfA) of the University of Bonn.

In Bonn, the spectrum of radiation emitted by the nebula and its central star has been analyzed. AIfA researchers were able to demonstrate that the enigmatic celestial object contained neither hydrogen nor helium, a typical feature of white dwarf interiors. Stars like our Sun generate their energy by burning hydrogen, nuclear fusion of hydrogen. When hydrogen is consumed, they continue to burn helium. However, they can not fuse even heavier elements – their mass is insufficient to produce the necessary high temperatures. Once all helium is used, they stop burning and cool to turn into so-called white dwarfs.

Usually, their life is over at this point. But not for J005311 – this is how the scientists baptized their new discovery in the constellation Cassiopeia, 10,000 light-years away from Earth. "We assume that two white dwarves have formed nearby each other several billion years ago," says Professor Norbert Langer of AIfa. "They circled themselves, creating exotic distortions of space-time, called gravitational waves." In doing so, they gradually lost energy. In return, the distance that separated them decreased more and more until they merged.

Only five of these objects in the Milky Way

Now, their total mass was sufficient to fuse heavier elements than hydrogen or helium. The stellar furnace is re-lit. "Such an event is extremely rare," says Gräfener. "There is probably not even half a dozen objects of this type in the Milky Way, and we have discovered one."

An extreme stroke of luck. Nevertheless, the researchers are convinced that they are right with their interpretation. On the one hand, the star in the center of the nebula shines 40,000 times brighter than the sun, much brighter than a single white dwarf. In addition, the spectra indicate that J005311 has an extremely strong stellar wind – this is the flow of material that emanates from the stellar surface. Its engine is the radiation generated during the engraving process. Only, at a speed of 16,000 kilometers per second, the wind of J005311 is so fast that this factor alone is not enough to explain it. However, the fused white dwarfs should have a very strong rotating magnetic field. "Our simulations show that this field acts like a turbine, which in turn accelerates the stellar wind," says Gräfener.

Unfortunately, the resurgence of J005311 will not last long. In just a few thousand years, the star will have turned all the elements into iron and faded again. As its mass has increased more than 1.4 times that of the Sun in the fusion process, it will undergo an exceptional destiny. The star will collapse under the influence of its own gravity. At the same time, the electrons and protons that build its material will merge into neutrons. The resulting neutron star has only a fraction of its previous size, measuring only a few kilometers in diameter, while it weighs more than the entire solar system .

J005311, however, will not leave without a last hello. His collapse will be accompanied by a huge blow, a so-called supernova explosion.

Publication: Vasiliy V. Gvaramadze, Götz Gräfener, Norbert Langer, Olga V. Maryeva, Alexei Y. Kniazev, Alexander S. Moskvitin and Olga I. Spiridonova: A massive white dwarf fusion product before the final collapse; Nature, https://doi.org/10.1038/s41586-019-1216-1

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