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Scientists Just Made A Big Discovery About Black Holes
In 2021, two objects believed to be black holes were discovered. However, researchers recently noticed that the objects, named Giraffe and Unicorn, are actually double-star systems.
Each system consists of two stars, one of which is an older red giant with a puffy atmosphere, while the other is a much smaller sub-giant star in its late stages of stellar life. The distance between the two stars is small enough that the sub-giant appears to be sucking materials from the accompanying red giant due to gravitational pull, resulting in the sub-giant spinning faster.
According to the recent study, Giraffe and Unicorn star systems offer a glimpse into a rare phase of binary star evolution that eventually gives birth to a compact binary containing two helium white dwarfs. The study reveals that the giants in each system underwent the carbon-nitrogen-oxygen cycle, which is the sequence of thermonuclear reactions that acts as the source of energy radiated by hotter stars.
The datasets suggest that the two giants in the star systems started having their envelopes stripped and that both the stars in each system had equal mass — or, maybe, the sub-giant companion was inflated temporarily due to quick accretion. In the Giraffe system, the companion star circles the red giant on an 81-day circular orbit, while the Unicorn follows a 60-day orbit.
Given the current rate and nature of mass transfer, which involves Unicorn and Giraffe having their atmosphere stripped away, the sub-giants will eventually evolve into helium white dwarfs, but there is also a small possibility they will turn into sdOB stars, which is a type of helium-rich, low-mass hot star.