This image combines data from four different space telescopes to create a multi-wavelength view of all that remains of the oldest documented example of a supernova, called RCW 86. Infrared data from NASA’s Spitzer Space Telescope, as well as NASA’s Wide-Field Infrared Survey Explorer (WISE) are shown in yellow and red, and reveal dust radiating at a temperature of several hundred degrees below zero, warm by comparison to normal dust in our Milky Way galaxy. Image: NASA/ESA/JPL-Caltech/UCLA/CXC/SAO |
A
mystery that began nearly 2,000 years ago, when Chinese astronomers
witnessed what would turn out to be an exploding star in the sky, has
been solved. New infrared observations from NASA’s Spitzer Space
Telescope and Wide-field Infrared Survey Explorer, or WISE, reveal how
the first supernova ever recorded occurred and how its shattered remains
ultimately spread out to great distances.
The
findings show that the stellar explosion took place in a hollowed-out
cavity, allowing material expelled by the star to travel much faster and
farther than it would have otherwise.
“This
supernova remnant got really big, really fast,” said Brian J. Williams,
an astronomer at North Carolina State University in Raleigh. Williams
is lead author of a new study detailing the findings online in the
Astrophysical Journal. “It’s two to three times bigger than we would
expect for a supernova that was witnessed exploding nearly 2,000 years
ago. Now, we’ve been able to finally pinpoint the cause.”
In
185 A.D., Chinese astronomers noted a “guest star” that mysteriously
appeared in the sky and stayed for about 8 months. By the 1960s,
scientists had determined that the mysterious object was the first
documented supernova. Later, they pinpointed RCW 86 as a supernova
remnant located about 8,000 light-years away. But a puzzle persisted.
The star’s spherical remains are larger than expected. If they could be
seen in the sky today in infrared light, they’d take up more space than
our full moon.
The
solution arrived through new infrared observations made with Spitzer
and WISE, and previous data from NASA’s Chandra X-ray Observatory and
the European Space Agency’s XMM-Newton Observatory.
The
findings reveal that the event is a “Type Ia” supernova, created by the
relatively peaceful death of a star like our sun, which then shrank
into a dense star called a white dwarf. The white dwarf is thought to
have later blown up in a supernova after siphoning matter, or fuel, from
a nearby star.
“A white dwarf is like a smoking cinder from a burnt-out fire,” Williams said. “If you pour gasoline on it, it will explode.”
The
observations also show for the first time that a white dwarf can create
a cavity around it before blowing up in a Type Ia event. A cavity would
explain why the remains of RCW 86 are so big. When the explosion
occurred, the ejected material would have traveled unimpeded by gas and
dust and spread out quickly.
Spitzer
and WISE allowed the team to measure the temperature of the dust making
up the RCW 86 remnant at about minus 325 F, or minus 200 C. They then
calculated how much gas must be present within the remnant to heat the
dust to those temperatures. The results point to a low-density
environment for much of the life of the remnant, essentially a cavity.
Scientists
initially suspected that RCW 86 was the result of a core-collapse
supernova, the most powerful type of stellar blast. They had seen hints
of a cavity around the remnant, and, at that time, such cavities were
only associated with core-collapse supernovae. In those events, massive
stars blow material away from them before they blow up, carving out
holes around them.
But
other evidence argued against a core-collapse supernova. X-ray data
from Chandra and XMM-Newton indicated that the object consisted of high
amounts of iron, a telltale sign of a Type Ia blast. Together with the
infrared observations, a picture of a Type Ia explosion into a cavity
emerged.
“Modern
astronomers unveiled one secret of a two-millennia-old cosmic mystery
only to reveal another,” said Bill Danchi, Spitzer and WISE program
scientist at NASA Headquarters in Washington. “Now, with multiple
observatories extending our senses in space, we can fully appreciate the
remarkable physics behind this star’s death throes, yet still be as in
awe of the cosmos as the ancient astronomers.”
