Astronomers have located an unusual anomaly: the lowest mass
white dwarf known in our galaxy. It is a Saturn-sized ball of helium comprising
only about one-fifth the mass of the Sun. When a Sun-like star ages, it becomes a white dwarf. This
newfound one, designated SDSS J091709.55+463821.8, lies about 7,400 light-years
from Earth near the border of the constellations Lynx and Ursa Major.
Further, the source of the white dwarf’s radical weight loss
has been identified. An unseen companion, likely another white dwarf, has
sucked away much of the tiny white dwarf’s material.
“This star
is bizarre,” said Warren Brown of the Harvard-Smithsonian Center for
Astrophysics. “It takes extraordinary circumstances to make such a low mass
white dwarf.”
Researchers speculate that this binary
system began with one star about twice the mass of the Sun and a second
star slightly less massive than the Sun. The more massive star was the first to
evolve, becoming a white dwarf weighing perhaps one-third as much as the Sun.
Ten billion years later, its companion became another white dwarf. In each
step, the outer layers of the evolving star enveloped the companion, causing
friction that moved the two stars closer together. They now orbit each other
every 7.6 hours at a distance of about 650,000 miles and a stunning speed of
335,000 miles per hour.
“The relation between our white dwarf and its companion is
like a cosmic marriage in which both people have to give a lot,” said Mukremin
Kilic of Ohio State University. “Two stars start out close to each other. One
of them engulfs the other (like a hug) and gives continuously (losing mass),
and they get closer. Then the other star evolves and becomes a giant and
engulfs the first star (hugging back) and now it has to give a lot, or lose a
lot of mass. They get closer and closer and end up dancing continuously.”
The astronomers predict that the two white dwarfs eventually
will merge, in 10 billion years or more.
--Harvard-Smithsonian Center for Astrophysics and Space.com Staff
Credit: David
A. Aguilar (CfA)
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