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Ask an Astrophysicist: Cosmic Rays

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Cosmic Rays

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Library of Past Questions and Answers

    Cosmic Rays

Why is the study of cosmic rays important? (Submitted January 06, 1999)

The Question

Why do we study cosmic rays? I understand that they might come form exploding stars, but why do we study them and for what purpose would it serve?

The Answer

We study cosmic rays because they are one of our few samples of matter from outside the solar system. Studying the composition of cosmic rays tells us how the galaxy has continued to evolve chemically since the solar system was formed. This helps us to understand how the solar system got the chemical composition that it has, and so tells us about our origin. Studying how cosmic rays have gotten to us tells us about the dynamics of the Milky Way galaxy. Also, historically, many new particles (muons and pions for example) were discovered first in the cosmic rays.

For more information on cosmic rays, you can check the Cosmicopia at:

Thanks for your questions

Eric Christian
for Ask an Astrophysicist

Question ID: 990106a

Who first discovered anti-matter? (Submitted March 30, 1998)

The Question

Could you tell me who created antimatter, and who first studied it?

The Answer

Antiparticles are just a set of sub-atomic particles with some properties opposite to those of another set of particles with which we are more familiar. For example, the positron is the antiparticle equivalent of the electron. When a particle and its antiparticle meet, they annihilate with the release of a lot of energy.

Antiparticles will have been produced in the big bang which initiated the universe. In fact, most models of the big bang produce as much antimatter as ordinary matter, which is not easy to reconcile with the largely matter universe we see around us. Antiparticles are also produced in the transformation and interaction of other particles. For instance, a proton can sometimes produce a neutron and a positron.

The English physicist Paul Dirac first proposed the existence of antiparticles, when he derived equations which had solutions corresponding to both normal and antimatter particles. The American physicist Carl Anderson made the first detection of antimatter when looking at cosmic ray tracks in 1932 and noting that some tracks bent the 'wrong way' in a magnetic field.

Paul Butterworth
for the Ask an Astrophysicist team

Question ID: 980330a

What is the effect of cosmic rays on other bodies? How do they get accelerated? (Submitted December 19, 1996)

The Question

I am currently doing a research paper involving cosmic rays. My question is what are the effects of cosmic radiation to other stellar bodies? Or do stellar bodies affect the behavior of cosmic rays? How do cosmic particles get accelerated to speeds almost the speed of light?

The Answer

Cosmic rays generally have little effect on other stellar bodies. The energy absorbed from a cosmic ray may be re-emitted at a lower energy, but the effect is usually small.

Stellar bodies affect the trajectories of cosmic rays primarily through their magnetic fields. However, the overall magnetic field of the interstellar medium is generally more important.

The acceleration mechanism in one of the key issues in the study of cosmic rays. It is generally believed that cosmic rays are accelerated in the shocks which accompany supernova explosions. Recent evidence for this has been detected in ROSAT data of the supernova remnant SN1006. The paper describing this result appeared in the journal Nature in 1995 (the full reference is Koyama et al, Nature, 1995 vol 378, pp. 255-258). Cosmic rays may also be accelerated by the strong magnetic fields associated with pulsars.

For further discussion about cosmic rays, you might also check the book "High Energy Astrophysics" by Malcom S. Longair (2nd Edition, 1992, Cambridge). Volume 2 contains a discussion of acceleration mechanisms. You may likely find articles at more of a lay person level in back issues of Sky and Telescope or Scientific American.

I hope this is helpful.

Jim Lochner
for Imagine the Universe!

Question ID: 961219a

Could ozone depletion be caused by cosmic rays? (Submitted January 05, 1997)

The Question

Is it possible that the primary cause of ozone depletion are cosmic rays as it enters the atmosphere? And the big "hole" of the ozone layer at Antarctica is the result of the geomagnetic effect on cosmic rays, even though they are isotropic?

The Answer

The ozone layer is the result of an equilibrium between the creation and destruction of ozone by UV light (UV creates ozone by ionizing free oxygen atoms which then combine with oxygen molecules to create ozone). Cosmic rays have been studied since the 1930s and we have no reason to believe that cosmic rays have not always existed. So if cosmic rays destroyed ozone at a significant level, then there would be no ozone layer (or at least a layer significantly smaller than the UV-only interaction picture would predict). More specifically, cosmic rays are made up of high energy particles and photons, which only react well with free electrons and dense material with high atomic numbers (note that high-Z atoms have a lot of electrons which are effectively "free" given the high energy of the cosmic rays). So in order for cosmic rays to have been significant in the creation of ozone holes, the flux of cosmic rays would have had to have increased by orders of magnitude, which has not occurred (to my knowledge).

Andy Ptak
for the Ask an Astrophysicist team

Note added in 2001 August: It is thought that man-made chlorofluorocarbons (CFCs), such as Freon, are the major destroyer of the ozone layer. The prevailing theory is that ultraviolet light breaks down CFCs, releasing active chlorine, which destroys ozone molecules. However, a recent study ( suggests that cosmic rays may also break down CFCs. If this is correct, cosmic rays do play a part in creating ozone holes, but only because there are man-made CFC molecules for them to break down.

Question ID: 970105

What are the different compositions and origins of Cosmic Rays? (Submitted October 30, 2003)

The Question

Are some cosmic rays nuclei of iron atoms, and are they from sources in our own galaxy? I read that the ultrahigh-energy cosmic rays that arrive to our atmosphere at 1020 electron volts (eV) or more, are mainly protons? Have any cosmic rays nuclei of iron ever been detected at this range of energy?

The Answer

Thank you for your question. The cosmic rays that have been identified as iron are believed to originate from supernova explosion shock waves in our galaxy.

We don't yet know the composition of the ultra high energy cosmic rays, since they are not directly detected. Although protons seem the likeliest candidate, there is no real physical evidence to support this.

For more on cosmic rays, see

-- Michael Loewenstein and Amy Fredericks
for "Ask an Astrophysicist"

Question ID: 031030a

Where do extrasolar cosmic rays originate? (Submitted April 18, 2012)

The Question

Where do the extrasolar cosmic rays come from?

The Answer

Cosmic rays can come from a variety of sources, but most of the cosmic rays that originate from outside the solar system probably come from supernovae. Since cosmic rays are actual particles, trajectories are bent between when they left their original source and when they arrive on earth, making it difficult to know the exact origin of an individual cosmic ray. There is a lot of exciting studies of cosmic rays going on at NASA, and we invite you to check out the Goddard cosmic ray website: Hope this helps.

Bret & Antara
for Ask an Astrophysicist

Question ID: 120418a

How do cosmic rays produce Lithium? (Submitted March 21, 2008)

The Question

How do energetic cosmic rays produce lithium from elements such as nitrogen, carbon, and oxygen? If cosmic rays interact with the elements listed above, forming lithium, how?

The Answer

The direct effect is that cosmic rays collide with a nucleus of nitrogen, carbon, or oxygen and split it apart into (usually) two parts - one of which can be lithium.

Cosmic rays that are themselves CNO nuclei can hit interstellar hydrogen and be split into smaller nuclei. That process means that cosmic rays have a much higher proportion of Li, Be, and B than other samples of matter.

See the discussion here (which is part of a larger discussion - use the navigation buttons at the bottom of the page to find the beginning and end):

Jay and Jeff
for Ask an Astrophysicist

Question ID: 080321a

Is there a limit to how energetic cosmic rays can be? (Submitted November 02, 2005)

The Question

What is the upper limit to the maximum energy that a cosmic ray can have? What is the upper limit on how much energy we can expect to see from the maximum energy cosmic rays hitting Earth?

The Answer

Theoretically, cosmic rays with energies above 1020 electron volts should rapidly lose their energies by interacting with the cosmic microwave background photons. Technically, this is called the GZK cut-off. However, several events above the GZK cut-off have been observed. The highest energy event ever recorded has about the same energy as a major league fastball (if you're not familiar with baseball, think the fastest tennis ball at Wimbledon).

You can find more information at the website of the Pierre Auger Observatory, which is a facility dedicated to the study of highest energy cosmic rays:

Hope this helps,

Koji & Kevin
for "Ask an Astrophysicist"

Question ID: 051102a

Are there any non-solar cosmic-ray cycles? (Submitted April 18, 2006)

The Question

Have we been able to detect periodic cycles in the galactic cosmic ray flux (i.e. independent of the solar wind cycles)?

The Answer

Thanks for your excellent question. The most notable periodicity of galactic cosmic rays, is as you stated, due to the solar wind 11 & 22-year cycles. However, scientists do also see periodicity in the beryllium-10 (and carbon-14) content in deep ocean sediments. Beryllium-10 is produced when cosmic rays interact with the Earth's atmosphere and is later stored in deep ocean sediments or in glaciers. The periodic cycles observed in beryllium-10 abundance are associated with sunspot activity which in turn is correlated with glacial cycles. One theory is that the Sun's solar activity varies over longer time scales than just the 11 & 22 year cycles. Another theory postulates that the Earth experiences small changes in its orbit which in turn can affect the Earth's magnetic field and thus the number of cosmic rays entering the Earth's atmosphere. We recommend a very interesting article on the subject:

Hope this helps,
Georgia & Mike
For "Ask an Astrophysicist"

Question ID: 060418a

Can Cosmic Rays be seen? (Submitted October 30, 2006)

The Question

Can you see cosmic rays? My dad told me about a science project he saw when he was in school that involved dry ice, a fish tank, and an iron, and maybe something else. With this set-up you can actually see cosmic rays. I found this very interesting and want to learn more about it.

The Answer

Thanks for your excellent question. Cosmic rays are generally single atoms or particles that are produced from supernovae events when a star dies. These tiny particles get to very high energies (fractions of the speed of light) and occasionally some of them reach the Earth. Most of these particles are deflected by the Earth's magnetic field or scatter in the Earth's atmosphere, but particles called muons can reach the ground. Hundreds pass through your body per second! You can read more about cosmic rays at our website:

Now almost 100 years ago, scientists discovered that muons leave a trail in a super-cooled, super-saturated mist (water vapor) that can be observed. The device that was first used to produce these trails is called a cloud chamber, see:

The trail that is formed when a charged particle (muon) passed through the cloud chamber is likely what your Dad observed. We don't actually see the particles themselves (much too small), but we can see the trail of ionized gas that is produced as muons pass through the gas chamber. In fact, you or your classroom can actually build a cloud chamber without too much difficulty, see:

Hope this helps,
Georgia and Mike
For "Ask an Astrophysicist"

Question ID: 061030a

What do Cosmic Rays tell us about potentially Earth-destroying black holes? (Submitted August 5, 2008)

The Question

(Submitted August 5, 2008)

My 8 yr old grandson heard something on the TV (I did not hear this) - about scientists attempting to create a black hole, and he is extremely worried that the Earth will be swallowed up and disappear. (to the point he's having trouble sleeping and is really concerned about it). This is a smart child, not normally fearful, gets good grades, etc.

Can you please give us a simple explanation that our World is NOT going to be sucked up in a Black Hole?

Thank you very much.
Helen Broadway

The Answer

There are a few ways of approaching the problem. One, my favorite, is to assure him that scientists are regular people. Most of them have children of their own. They don't want to destroy the Earth any more than anyone else does, in my experience the opposite, actually. They are just smart people that want to find out things. He can be a scientist himself.

What he probably heard about was the new Large Hadron Collider (LHC) at CERN:
Some theories, but not the Standard Model theory, predict that the energy reached by the LHC would be enough to make a micro black hole. If the LHC did produce black holes, they would last for tiny fractions of a second before dissolving in a burst of particles. They would not have time to merge with any additional material.

Nature has been performing this experiment for billions of years through the bombardment of the Earth (and everything else) with high energy cosmic rays. If such collisions did produce black holes, and if micro black holes could destroy the Earth, it would have already happened. The only difference is that CERN is doing it in a controlled way, inside detectors, so we can see what happens. So either the collisions don't make them, or they are not capable of destroying the Earth, or (probably) both.

Jay and Jeff
for Ask an Astrophysicist

Question ID: 080805b

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