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A cluster of massive stars seen with the Hubble
Space Telescope. The cluster is surrounded by clouds of interstellar gas
and dust called a nebula. The nebula, located 20,000 light-years away
in the constellation Carina, contains the central cluster of huge, hot
stars, called NGC 3603. Credits: NASA/U. Virginia/INAF, Bologna, Italy/USRA/Ames/STScI/AURA.Full caption
This is a mosaic image-- one of the largest ever
taken by NASA's Hubble Space Telescope -- of the Crab Nebula, a
six-light-year-wide expanding remnant of a star's supernova explosion. Credits: NASA/ESA/Arizona State University. Full caption
Most of the cosmic rays that we detect at Earth originated relatively
recently in nearby clusters of massive stars, according to new results
from NASA's Advanced Composition Explorer (ACE) spacecraft.
ACE allowed the research team to determine the source of these cosmic
rays by making the first observations of a very rare type of cosmic ray
that acts like a tiny timer, limiting the distance the source can be
"Before the ACE observations, we didn't know if this radiation was
created a long time ago and far, far away, or relatively recently and
nearby," said Eric Christian of NASA's Goddard Space Flight Center in
Greenbelt, Maryland. Christian is co-author of a paper on this research
published April 21 in Science.
Cosmic rays are high-speed atomic nuclei with a wide range of energy
-- the most powerful race at almost the speed of light. Earth's
atmosphere and magnetic field shield us from less-energetic cosmic rays,
which are the most common. However, cosmic rays will present a hazard
to unprotected astronauts traveling beyond Earth's magnetic field
because they can act like microscopic bullets, damaging structures and
breaking apart molecules in living cells. NASA is currently researching
ways to reduce or mitigate the effects of cosmic radiation to protect
astronauts traveling to Mars.
Cosmic rays are produced by a variety of violent events in space.
Most cosmic rays originating within our solar system have relatively low
energy and come from explosive events on the Sun, like flares and
coronal mass ejections. The highest-energy cosmic rays are extremely
rare and are thought to be powered by massive black holes gorging on
matter at the center of other galaxies. The cosmic rays that are the
subject of this study come from outside our solar system but within our
Galaxy and are called galactic cosmic rays. They are thought to be
generated by shock waves from exploding stars called supernovae.
The galactic cosmic rays detected by ACE that allowed the team to
estimate the age of the cosmic rays, and the distance to their source,
contain a radioactive form of iron called Iron-60 (60Fe). It
is created inside massive stars when they explode and then blasted into
space by the shock waves from the supernova. Some 60Fe in the
debris from the destroyed star is accelerated to cosmic-ray speed when
another nearby massive star in the cluster explodes and its shock wave
collides with the remnants of the earlier stellar explosion.
60Fe galactic cosmic rays zip through space at half the
speed of light or more, about 90,000 miles per second. This seems very
fast, but the 60Fe cosmic rays won't travel far on a galactic
scale for two reasons. First, they can't travel in straight lines
because they are electrically charged and respond to magnetic forces.
Therefore they are forced to take convoluted paths along the tangled
magnetic fields in our Galaxy. Second, 60Fe is radioactive
and over a period of about 2.6 million years, half of it will
self-destruct, decaying into other elements (Cobalt-60 and then
Nickel-60). If the 60Fe cosmic rays were created hundreds of
millions of years or more ago, or very far away, eventually there would
be too little left for the ACE spacecraft to detect.
"Our detection of radioactive cosmic-ray iron nuclei is a smoking gun
indicating that there has likely been more than one supernova in the
last few million years in our neighborhood of the Galaxy," said Robert
Binns of Washington University, St. Louis, Missouri, lead author of the
"In 17 years of observing, ACE detected about 300,000 galactic cosmic
rays of ordinary iron, but just 15 of the radioactive Iron-60," said
Christian. "The fact that we see any Iron-60 at all means these cosmic
ray nuclei must have been created fairly recently (within the last few
million years) and that the source must be relatively nearby, within
about 3,000 light years, or approximately the width of the local spiral
arm in our Galaxy." A light year is the distance light travels in a
year, almost six trillion miles. A few thousand light years is
relatively nearby because the vast swarm of hundreds of billions of
stars that make up our Galaxy is about 100,000 light years wide.
There are more than 20 clusters of massive stars within a few
thousand light years, including Upper Scorpius (83 stars), Upper
Centaurus Lupus (134 stars), and Lower Centaurus Crux (97 stars). These
are very likely major contributors to the 60Fe that ACE detected, owing to their size and proximity, according to the research team.
ACE was launched on August 25, 1997 to a point 900,000 miles away
between Earth and the Sun where it has acted as a sentinel, detecting
space radiation from solar storms, the Galaxy, and beyond. This research
was funded by NASA's ACE program.
Additional co-authors on this paper were: Martin Israel and Kelly
Lave at Washington University, St. Louis, Missouri; Alan Cummings, Rick
Leske, Richard Mewaldt and Ed Stone at Caltech in Pasadena, California;
Georgia de Nolfo and Tycho von Rosenvinge at Goddard; and Mark
Wiedenbeck at NASA's Jet Propulsion Laboratory in Pasadena, California.
Karen C. Fox NASA Goddard Space Flight Center, Greenbelt, Maryland 301-286-6284 firstname.lastname@example.org