Yes, astronauts do have stars in their eyes
29 Jun 2009
During the Apollo flights of the 1960s and 1970s, astronauts reported seeing mysterious, occasional flashes of light, visible even when their eyes were closed.
We've since learned that the cause was cosmic rays—extremely energetic atomic particles that come from outside the Solar System. These particles constantly bombard Earth's atmosphere and have sufficient energy to cause glitches in electronic components (and flashes in astronauts' eyes).
But there has long been a mystery about how these particles obtain their extreme levels of energy and huge velocities. Scientists have been searching for the cosmic "particle accelerators" responsible.
Now, thanks to a study that combines data from the European Southern Observatory's (ESO) Very Large Telescope in Chile and NASA's Chandra X-ray Observatory satellite, astronomers think they've solved the mystery. They show in a paper published in Science Express that cosmic rays are very efficiently accelerated within the remnants of exploded stars.
The cosmic rays come from somewhere inside our home galaxy, the Milky Way, and consist mostly of protons moving at close to the speed of light, the "ultimate speed limit" in the Universe. These protons have been accelerated to energies far exceeding those that even CERN's Large Hadron Collider will be able to achieve.
"It has long been thought that the super-accelerators that produce these cosmic rays in the Milky Way are the expanding [gas] envelopes created by exploded stars, but our observations reveal the smoking gun that proves it", says Eveline Helder from the Astronomical Institute Utrecht of Utrecht University in the Netherlands, the first author of the new study.
"You could even say that we have now confirmed the calibre of the gun used to accelerate cosmic rays to their tremendous energies", adds collaborator Jacco Vink, also from the Astronomical Institute Utrecht.
A smoking celestial gun
For the first time Helder, Vink and colleagues have come up with a measurement that solves the quandary of whether or not stellar explosions produce enough accelerated particles to explain the number of cosmic rays that reach Earth.
The study indicates that they indeed do, by directly telling us how much energy is lost from the gas in the stellar explosion that accelerates the particles.
"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", says Helder. "The energy that is used for particle acceleration is at the expense of heating the gas, which is therefore much colder than theory predicts".
RCW 86 is the remnant of a stellar explosion that was spotted by Chinese astronomers in CE 185.
The researchers looked at the remnant of a star that exploded in the year CE 185, as recorded by Chinese astronomers. The remnant, called RCW 86, is located about 8,200 light-years away in the direction of the constellation Circinus. It is probably the oldest recorded explosion of a star.
Using ESO's Very Large Telescope, the team measured the temperature of the gas right behind the shock wave created by the stellar explosion. They measured the speed of the shock wave as well, using images taken with NASA's X-ray Observatory Chandra three years apart. They found it to be moving at between 10 and 30 million kilometres per hour, which is between one and three percent the speed of light.
The temperature of the gas turned out to be 30 million degrees Celsius. This is extraordinarily hot compared to everyday standards, but much lower than expected, given the measured shock wave's velocity, which should have heated the gas up to at least half a billion degrees.
"The missing energy is what drives the cosmic rays", concludes Vink.
Adapted from information issued by ESO.
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