As astronomers have long expected, exploding stars called supernovas
can accelerate particles up to almost the speed of light, a new study shows.
The discovery helps explain where the extremely energetic
cosmic rays we find near Earth come from.
Cosmic rays are charged particles, mostly protons, that come
swooping through space from beyond the solar system. They carry such an
energetic punch they can knock out electronics systems on Earth if they
manage to make it past our atmosphere.
Until now, scientists couldn't be sure how cosmic rays
acquire their energy and speed.
"It has long
been thought that the super-accelerators that produce these cosmic rays in
the Milky Way are the expanding envelopes created by exploded stars, but our
observations reveal the smoking gun that proves it," said Eveline Helder of
the Astronomical Institute Utrecht of Utrecht University in the Netherlands, leader
of the new study.
When a star dies
in a supernova, the blast releases a huge amount of energy. Much of that
energy is used to heat up a bubble of gas that expands around the remnant of
the star. Some energy, though, goes toward speeding up the particles that
become cosmic rays, the researchers determined.
"When a star
explodes in what we call a supernova a large part of the explosion energy
is used for accelerating some particles up to extremely high energies,"
Helder said. "The energy that is used for particle acceleration is at the
expense of heating the gas, which is therefore much colder than theory predicts."
Helder and team looked at the leftovers from a supernova
called RCW 86 with the European Southern Observatory's Very Large Telescope.
The star exploded about 8200 light-years away in AD 185, and was recorded by
Chinese astronomers.
The modern researchers measured the temperature and speed of
the gas behind the shock wave created by the stellar explosion. They found that
the gas, at 54 million degrees Fahrenheit (30 million degrees Celsius), was
much lower than would be expected given the shock wave's velocity.
Rather than heat up the gas, some of the supernova's energy
went toward speeding up particles to near the velocity of light, the astronomers
concluded.
"The missing energy is what drives the cosmic rays,"
said collaborator Jacco Vink, also from the Astronomical Institute Utrecht.
Helder and team describe their findings in the June 26 issue
of the journal Science.
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