Evidence for Supernovas near Earth.
Presented by Science at NASA.
Once every 50 years, more or less, a massive star explodes
somewhere in the Milky Way.
The resulting blast is terrifyingly powerful, pumping out
more energy in a split second than the sun emits
in a million years.
At its peak, a supernova can outshine the entire Milky Way.
It seems obvious that you wouldn't want a supernova
exploding near Earth.
Yet there is growing evidence that one did,
actually, more than one.
About 10 million years ago, a nearby cluster of supernovas
went off like popcorn.
We know because the explosions blew an enormous bubble in
the interstellar medium, and we're inside it.
Astronomers call it "the Local Bubble." It is
peanut-shaped, about 300 light years long,
and filled with almost nothing.
Gas inside the bubble is very thin (0.001 atoms per cubic
centimeter) and very hot (roughly a million degrees)
a sharp departure from ordinary interstellar material.
The Local Bubble was discovered gradually
in the 1970s and 1980s.
Optical and radio astronomers looked carefully for
interstellar gas in our part of the galaxy, but couldn't
find much in Earth's neighborhood.
Meanwhile, x-ray astronomers were getting their first look
at the sky using sounding rockets and orbiting satellites,
which revealed a million-degree x-ray glow
coming from all directions.
It all added up to Earth being inside a bubble of hot gas
blown by exploding stars.
However, not all researchers agreed.
"Within the last decade, some scientists have been
challenging the supernova interpretation, suggesting that
much or all of the soft X-ray diffuse background is instead
a result of charge exchange," says F. Scott Porter
of the Goddard Space Flight Center.
"Charge exchange": Basically, it happens when the
electrically-charged solar wind comes into contact
with a neutral gas.
The solar wind can steal electrons from the neutral gas
resulting in an X-ray glow that looks
a lot like the glow from an old supernova.
Charge exchange has been observed many times in comets.
So, is the X-ray glow that fills the sky a sign of peaceful
"charge exchange" in the solar system or evidence of
terrifying explosions in the distant past?
To find out, an international team of researchers including
Porter and led by physics professor Massimiliano Galeazzi
at the University of Miami in Coral Gables, developed an
X-ray detector that could distinguish between
the two possibilities.
The device was named DXL, for Diffuse X-ray emission
from the Local Galaxy.
On December 12, 2012, DXL launched from
White Sands Missile Range in New Mexico
atop a NASA Black Brant IX sounding rocket,
reaching a peak altitude of 160 miles and spending five
minutes above Earth's atmosphere.
That was all the time they needed to measure the amount of
"charge exchange" X-rays inside the solar system.
The results, published online in the journal Nature on July
27, indicate that only about 40 percent of the soft X-ray
background originates within the solar system.
The rest must come from a Local Bubble of hot gas, the
relic of ancient supernovas.
Obviously, those supernovas were not close enough to
exterminate life on Earth-but they were close enough to
wrap our solar system in a bubble of hot gas that persists
millions of years later.
"This is a significant discovery,' said Galeazzi.
"It affects our understanding of the area of the galaxy
close to the sun, and can, therefore, be used as a
foundation for future models of the galaxy structure."
Galeazzi and collaborators are already planning the next
flight of DXL, which will include additional instruments to
better characterize the emission.
The launch is currently planned for December 2015.
For updates from the Local Bubble, stay tuned to
science.nasa.gov.