Nearly
2,000 years ago, Chinese astronomers spotted a bright light materializing in
the night sky. Turns out the skywatchers had witnessed the spectacular
explosion of a dying star.
That was
the year 185 AD. Today astronomers said they might have identified the remains
from this ancient stellar
explosion, now considered the oldest supernova
on record.
"I think it
is very interesting that we can now say with some confidence, but not absolute
certainty, that RCW 86 is the remnant of AD 185," said Jacco Vink of the
University of Utrecht, the Netherlands.
Shell
shocked
In the
Astrological Annals of the Houhanshu, Chinese astronomers noted the bright
light in the sky twinkled like a star but didn't appear to move, arguing
against the object being a comet. Within eight months, they recorded, the
bright light faded, a phenomenon that astronomers now know is consistent with
supernovas.
When a star
that's more than eight times the mass of our sun
burns out, gravity's inward tug tears apart the star's innards. It collapses
and then rebounds. Called a supernova,
the explosion propels jets of high-energy light and matter out into space. The
hurling remains and the material they encounter are heated to millions of
degrees and can emit intense X-ray radiation for thousands of years.
Astronomers
had suggested the stellar material called RCW 86 could be leftovers from the
185 AD supernova, based on historical records of the object's position in the
sky. But previous work estimated the object was 10,000 years old, which led to
doubts about the link.
Astronomers
led by Vink studied the material in RCW 86 to determine when its progenitor
star originally exploded within the Southern Hemisphere constellation Centaurus.
To do this, they used the X-ray capabilities of NASA's Chandra X-ray Observatory
and the European Space Agency's XMM-Newton Observatory.
RCW 86 is
encased in an expanding shock
wave formed during its transit from the original explosion. By peering into
one part of RCW 86, the astronomers calculated how fast this cosmic bubble was
moving. So with the size and speed of RCW 86, the astronomers could calculate
the time of the explosion, and hence the remnant's age.
"Our new
calculations tell us the remnant is about 2,000 years old," said Aya Bamba, a
coauthor from the Institute of Physical and Chemical Researcher in Japan.
Youthful
star
The new age
estimate matches the supernova spotted in 185 AD. But this calculation means
the remnant
is 8,000 years younger than previously thought. The astronomers said the
difference can be attributed to the irregular shape of the remnant's expanding
bubble. Stellar wind from the progenitor star pushed some of the remnant's
gases in a certain direction, forming a dense pile. "The idea for RCW 86 is
that in some regions the shock has hit this piled-up material. In those regions
the shock will start moving slower," Vink said. And in other regions, the shock
wave is much speedier.
"This could
explain the earlier measurements for lower shock velocities," Vink told SPACE.com.
The previous speeds were used to calculate RCW 86's age.
With more
information on RCW 86, Vink said he could now probe the original supernova to
learn what type of star exploded and how much energy was involved in the stellar
fireworks.
The
research is detailed in the Sept. 1 issue of The Astrophysical Journal
Letters.
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