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)