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2 stars covered in unusual elements have a puzzling origin story

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Astronomers have discovered a pair of strange stars, unlike any they have ever seen, that is covered in a perplexing accumulation of odd compounds. These cosmic oddballs could have been created by an extremely uncommon sort of union between two stars at opposite ends of their life cycles.

Researchers discovered the two stars, dubbed PG1654+322 and PG1528+025 while examining the sky with data from China's Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) and Arizona's Large Binocular Telescope Observatory.

They are a pre-white dwarf star, which is a star that is slightly too massive to be classified as a white dwarf — the dense leftovers left behind when medium and tiny stars run out of fuel to burn. Unlike previous known pre-white dwarf stars, however, these new stars, which have yet to be given a proper name, have a large quantity of carbon and oxygen near their surface.

These compounds are formed by the nuclear fusion of helium at the cores of stars, hence signs of these elements are not uncommon in some stars. However, these elements typically reach the stellar surface only when a star is towards the end of its life after it has depleted the majority of its helium. However, the size and luminosity of these young stars indicate that their cores are still helium-rich. According to the researchers' calculations, they have been burning long enough to produce the quantities of carbon and oxygen detected by astronomers.

"Normally, we would expect stars with these surface components to have finished burning helium in their cores and to be on their way to becoming white dwarfs," said lead author Klaus Werner, an astronomer at the Germany's University of Tübingen. "These newly discovered stars pose a significant challenge to our knowledge of stellar evolution."

A different set of researchers suggested a possible explanation for the intriguing genesis of these new stars in a separate paper, also published Jan. 7 in the journal Monthly Notices of the Royal Astronomical Society – a stellar merger, in which two stars collide to become a single star.

"We believe the stars discovered by our German colleagues evolved in a very uncommon type of stellar merger event between two white dwarf stars," said lead author Marcelo Miller Bertolami, an astronomer from the Argentina's National University of La Plata.

Star mergers can occur in star clusters, which are large groups of thousands or millions of stars that are closely packed together, or, more commonly, in binary systems, in which two stars circle each other in an orbit that eventually destabilises, causing the plasma of the smaller star to be sucked up by the larger. However, the star merger required to form the new sort of pre-white dwarfs would have had to be exceedingly particular.

"Usually, white dwarf mergers do not result in the formation of stars enriched in carbon and oxygen," Miller Bertolami said in a statement, "but we believe that, for binary systems formed with very specific masses, a carbon- and oxygen-rich white dwarf may be disrupted and end up on top of a helium-rich one, resulting in the formation of these stars."

In this scenario, carbon and oxygen from an older and smaller helium-depleted white dwarf form a shell around a much younger and larger helium-rich white dwarf as the latter steadily sucks in the former's contents. According to the statement, this form of a binary merger involving two white dwarfs at different stages of their life cycle is highly rare.

This is the most recent best guess for how the newly discovered stars were created. According to the statement, the researchers still need to build a clear and improved model for the type of binary system and merger they have anticipated to irrefutably establish their idea. According to the announcement, further research "may also provide a deeper understanding into the late evolution of binary systems and how their stars trade mass as they grow."