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@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
 * Origin: NCS BBS - Houma, LoUiSiAna (1:3828/7)