An ancient, near-death star with a disc of metal-rich dust
orbiting around it has recently been discovered by astronomers. The dust's
origin is a mystery, though, as it should have been sucked into the star within
a few hundred years of the star's death.
Even though this anomaly is about 82 light-years away and
several billion years old, astronomers think it could provide a preview of our
solar system's future when the Sun dies five to six billion years from now.
Between three and five billion years ago, GD 362 was a star
much like our Sun, only seven times more massive. It takes a lot of energy to
maintain that much mass, and eventually GD 362's nuclear power plant ran out of
fuel and the star could no longer produce enough heat energy to keep its outer layers
from crashing inward.
Once the fuel ran out, the star lost plenty of mass and heat,
and shriveled into a white dwarf – the final stage of star evolution
where the star burns out and dies. Enough heat gets released during this
process to vaporize any surrounding dust, rocks, and planets, leaving behind a
disc of dust. This dust usually gets sucked into the newly formed white dwarf
and incinerated within a few hundred years.
But when GD 362 went through this process five billion years
ago, the dust never cleared up. And astronomers don't know how a dead star
roughly the size of Earth has been able to sustain a disc the size of Saturn's
rings for so long.
"Where the disc comes from is the $64,000 question,"
Ben Zuckerman of UCLA told SPACE.com.
"However, we believe that it's from a planetary system around this star.
Something – comets, asteroids, or planets – is getting ground up
into dust and being pulled into the star."
The dust surrounding a newly formed white dwarf contains light
elements – such as hydrogen and helium – and heavier elements
– such as calcium, sodium, oxygen, iron, and magnesium.
Over the next few hundred years, the light elements and heavy
elements separate like oil and water – the star's gravity pulls the heavy
elements into its interior, leaving behind a hydrogen and helium atmosphere.
This happened to GD 362 too, but since then heavy elements have
continued to find their way into the disc.
"Something's got to be replenishing these metals,"
Zuckerman said. "The suggestion is maybe this white dwarf had planets
around it, and the system was substantial enough that after billions of years
of grinding, there's still enough material to feed the dust disc around the
white dwarf."
This grinding up of planets or asteroids into dust can work two
ways. Rocky objects zip around very fast near white dwarves. When they collide
they chip off planet confetti – the perfect material for making a heavy
element dust disc.
"Or you could just have a big object tossed in close to
the star. It goes into orbit and it will get torn apart by the gravity of the
star once it gets close enough," Zuckerman said.
This could happen here on Earth if one day the Moon one day
decided to drift closer. It's also probably how Saturn got its rings.
"It's possible they were formed by some object getting tossed in too close
to Saturn and it got torn apart by gravity," Zuckerman said.
Other ideas for how the dust could accumulate include comets
breaking up near the star and aggregation of interstellar dust as the star
travels through space. However, Zuckerman doesn't think comets alone could
sustain the disc and interstellar dust would probably just flow around the
star.
Some astronomers believe these observations – taken in
the mid-infrared spectrum during May, 2005 with the Gemini 8-meter Frederick C.
Gillett Telescope on Hawaii's Mauna Kea – provide a look into the Milky
Way's future.
In five or six billion years, our own Sun will run out of
nuclear fuel and begin turning into a super-hot white dwarf star. At that time,
any of the planets and asteroids revolving around it might get crunched up into
a giant dust disc.
"The parallel to our own solar system's eventual demise is
chilling," said Eric Becklin of UCLA and lead
investigator on the Gemini observations.
This isn't the first time astronomers have spotted a dusty
white dwarf. Back in 1987, Zukerman and his colleague
at UCLA Eric Becklin observed G29-38,
a similarly dust-ringed white dwarf. But it's about 100 times less dusty than
their newest find.
Last summer, Zuckerman observed about 100 white dwarves and found
that about a quarter of them had metal in their atmospheres. Now he plans to
see if any of these have similar dust discs.
"If we don't detect metal rich dust around the other white
dwarves, we might have to revise our thinking [about the dust's origin],"
Zuckerman said. "The comet impact model looked pretty good 10 years ago,
but it doesn't look so good anymore. There's still a
lot of mystery"
These findings are detailed in the recent issue of the Astrophysical Journal.
This article is part of SPACE.com's weekly Mystery Monday series.
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