Cosmic Rays

The ability to reduce everything to simple fundamental laws does not imply the ability to start from those laws and reconstruct the universe.
Philip W. Anderson

Cosmic Rays in the News
Cosmic Ray Activities

Particles that bombard the Earth from anywhere beyond its atmosphere are known as cosmic rays. Cosmic rays don't take pretty pictures, but studying the quantity and type of these particles helps us to understand the acceleration processes involved and to measure the composition of the Sun, as well as sources at the far distant reaches of the galaxy.

Cosmic rays include:

• Galactic Cosmic Rays -- coming from outside the solar system
• Anomalous Cosmic Rays -- coming from the interstellar space at the edge of the heliopause
• Solar Energetic Particles -- associated with solar flares and other energetic solar events
  
  
• The term "cosmic rays" used to include X-rays and gamma rays, which are well covered in Imagine the Universe!

The relative numbers of different isotopes found in the galaxy are established by the life cycle of massive stars. Star formation, evolution and explosion results in the creation of many of the heavier isotopes found in space. The process is shown in the figure below.

This stellar evolution image is courtesy of Drs. R. Mewaldt, E. Stone, and M. Wiedenbeck of the California Institute of Technology (Caltech).

In a part of the galaxy where the composition of the interstellar gas is much like that of our own solar system (a), a cloud of gas collapses under the influence of its own gravity, and creates a new star (b). Inside the star (c), fusion converts some of the original hydrogen and helium into particles like carbon-12 and oxygen-16. At the same time, the carbon, nitrogen, and oxygen nuclei that were originally present in the star's fuel are converted into heavier, neutron-rich nuclei, like neon-22 and magnesium-25.

When this burning has exhausted all of the nuclear fuel in the core of the star, the star explodes as a supernova (d). The shock wave generated by the explosion produces additional heavy nuclei and ejects most of these products of nucleosynthesis back into the interstellar gas.

Repetition of these events in each generation of stars steadily enriches the interstellar gas in carbon, nitrogen, and oxygen, and in heavy nuclei with an excess of neutrons.

Some of the nuclei in the gas are accelerated to cosmic ray speeds, possibly by the shock waves from supernovae (e). Cosmic ray acceleration could also occur directly as the supernova is ejecting matter into interstellar space, as in (d).

To learn a lot more about cosmic rays, energetic particles, and plasma, check out our question and answer page on cosmic rays in our "Ask Us" feature.

Even more about cosmic rays:

More about the interstellar medium:

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Cosmic Rays in the News:

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Cosmic Ray Activities: