A new type of star, called an “extremely low-mass white dwarf” (or simply “ELM”) was first observed by astronomers at the Harvard Smithsonian Center for Astrophysics. Such a star was, until then, only the object of theory as it was supposedly physically impossible to exist, as its age would make it older than the universe itself.
The star has a binary character – that is, two stars orbiting a common center, where the primary is brighter and the secondary is less conspicuous. In the case at hand, Kareem El-Badry, postdoctoral researcher at the institution and author of paper which details the discovery, claims that this is “the missing link in binary star formation models that we’ve always been looking for.”
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El-Badry explains that when a star dies, it usually transforms. in an ordinary white dwarf – an extremely dense object that is basically the core of a star after it burns all of its fuel. In extremely rare situations, however, this star can become an ELM, a type of white dwarf with less than a third of the mass of our Sun.
ELMs pose a difficult problem to tackle: according to our understanding of astronomical calculations, every star of this type must be more than 13.8 billion years old – a number that would make them older than the universe itself and, as a consequence, physically inexplicable. .
For this reason, ELMs have always existed in the theoretical field, but never in the practical one. Experts say that an ordinary white dwarf could reach the ELM state at an accelerated rate if it found a companion star to form a binary system. Thus, the gravitational force the partner could “eat around the edges” of the other star, making her an ELM over the years.
But this scenario is not error-free: our sun it has been observed performing accretion movements towards white dwarfs, but ELMs have not been detected from this. Evidently, the Sun is not a binary system, but its gradual approach could generate influence on nearby stars due to its gravitational attraction.
“The universe is simply not old enough to produce such stars through normal stellar evolution,” said El-Badry.
Using new data collected by Gaia space observatory, from ESA, El-Badry then decided to look for signs of the moment of stellar transition – when a white dwarf becomes an ELM (a phase known as “evolved cataclysmic variable”), monitoring the behavior of 21 candidates. What ended up working very well:
“100% of the candidates constituted this kind of ‘pre-ELM star’ that we were looking for,” he said. “They were more bloated and swollen than ELMs, and they were shaped more like an egg because the gravitational pull of their companion stars distorts the spherical shape.”
Breaking down into numbers: Of the 21 binary stars analyzed, 13 were still losing mass to their companions, while eight of them no longer had mass to lose and showed themselves hotter than previous variables.
Now, he will continue to look at the 21 selected stars, as well as look at another 29 selected candidates, to confirm the repetition of the process.
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