Evidence for Supernovas Near Earth
 
August 26, 2014:  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.
 
http://www.youtube.com/watch?v=OPxgBPKwYc0&feature=youtu.be
 
A new ScienceCast video examines evidence that our solar system is inside a
bubble of hot gas created by supernova explosions.  Play 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?  
http://tinyurl.com/qznruvf
 
Click to view a diagram of the local Galactic neighborhood including the
Sun and the Local Bubble.
 
To find out, an international team 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 Dec. 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 outside the solar system.
 
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.
 
Credits:
Production editor: Dr. Tony Phillips | Credit: Science{at}NASA
 
More information:
 
How did DXL distinguish between X-rays from charge exhange in the solar
system vs. X-rays from hot gas in the Local Bubble?
 
Answer: Basically, there is a stream of interstellar helium atoms that
flows through the solar system.  You can read about it here. Every year in
December, Earth passes through the "helium focusing cone," a
region where this neutral helium is concentrated by the gravitational
influence of the sun.  The researchers figured the helium focusing cone was
probably the strongest source of charge exchange x-rays in the solar
system.  Using the sounding rocket, they measured the X-ray glow of the
helium and found that it could not account for all of the X-rays
astronomers had been seeing.  There must be a Local Bubble of hot gas to
account for the difference.
 
 
Regards,
 
Roger

--- D'Bridge 3.99