Michael Braukus
Headquarters, Washington, D.C.                                                 
February 25, 1993
(Phone:  202/358-1547)

Michael Finneran
Goddard Space Flight Center, Greenbelt, Md.
(Phone:  310/286-5565)


RELEASE:  93-036

SUPERNOVA MAY HAVE CAUSED HUGE VOID AROUND SOLAR SYSTEM

     A supernova that shone in the ancient sky like a second moon 
is the probable cause of a huge void known as the "Local Bubble" 
that envelopes the solar system and many nearby stars, a NASA 
scientist reported in today's issue of the British journal 
"Nature."

     The bubble is an area about 300 light-years across that, 
compared to other parts of space, is relatively empty of gases 
except for super-hot hydrogen.  The bubble's origin has been the 
subject of intense speculation for the last 20 years.  

     Now, researchers at NASA's Goddard Space Flight Center, 
Greenbelt, Md., say evidence suggests it was formed by the 
supernova or explosion of a star known as Geminga about 340,000 
years ago.

     "This is a supernova we didn't know about until last year.  
We put this new knowledge together with some other information 
about the Local Bubble, and we were able to say we think we know 
what happened here," said Dr. Neil Gehrels, of Goddard.  Gehrels 
wrote the paper with Dr. Wan Chen, of Universities Space Research 
Association.

     A supernova is an internal explosion that blows away a star's 
outer layers, leaving a dense, collapsed, rapidly spinning core 
that emits energy in pulses.  Stars that have undergone a 
supernova are known as pulsars.  The Local Bubble would have been 
formed by the force of the supernova blasting most of the gases 
out of the surrounding interstellar medium, according to the 
authors.
 
     "This must have been the brightest supernova ever seen by 
Homo sapiens," said Gehrels.  "Everybody would have immediately 
noticed it.  It would have been quite a spectacular and 
frightening event, though not one that would have threatened the 
planet's existence."
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     Supernovas always create such bubbles, Gehrels said.  Similar 
bubbles, including a cluster of several around the Local Bubble, 
probably were formed around our solar system millions of years ago 
by supernovas just as close as Geminga.  But those would have 
collapsed and dissipated by now, leaving little or no trace of 
their existence, he said.

     Astrophysicists have long speculated that the current Local 
Bubble was created by a supernova.  But for that to have happened, 
the star would had to have been relatively nearby.  Until 
recently, the evidence for that was scant, said Gehrels.

     In the last year, however, several discoveries have given 
weight to the theory.  Scientists at Goddard and Columbia 
University in New York City determined that Geminga is a pulsar 
and therefore had to have undergone a supernova at some point.  
That finding was made using x-ray data from the Roentgen Satellite 
and was confirmed by NASA's Compton Gamma Ray Observatory.

     From Geminga's pulsations, scientists calculated that the 
star is no more than 340,000 years old and was about 180 light 
years away -- close enough to have created the bubble -- when the 
supernova occurred.  The authors estimate Geminga has since moved 
to about 400 light-years away.

     Further evidence recently presented by a team of Italian 
astronomers helped Gehrels and Chen place Geminga more precisely.  
Calculations based on its speed suggested the star was born in a 
position to have created the bubble.  The Italian team also 
reported that the swiftness with which Geminga crosses the sky 
indicates it is not far away.

     "So we now know the position for this nearby supernova," 
Gehrels said. "It is right near the current middle of the Local 
Bubble."

     Gehrels said the supernova that created the bubble would have 
been a unique experience for anyone on Earth who witnessed it.

     According to Gehrels, observers would have seen the sudden 
appearance of a star emitting as much light as the moon, visible 
even in daylight.  With a full moon in the sky, the nights would 
have been twice as bright as usual.  Geminga would have lingered 
like a beacon for several months before fading.  Currently, it 
cannot be seen with the naked eye.

     At the time of the supernova, intense x-rays and gamma rays 
from the explosion likely would have depleted the Earth's ozone 
layer by 10 to 20 percent globally, Gehrels said in remarks not 
included in the "Nature" paper.  The resulting increase in 
ultraviolet radiation, he said, would have been small enough not 
to disrupt life on Earth but may have been noticed by early 
humans.

     "Whomever was here back then would have experienced a sunburn 
for a year or two," Gehrels said.  "One can only wonder if anybody 
at that time figured out that Geminga was the cause."
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