Finding Visitors from Outer Space or “We Are Made of Star Stuff”

Aim / Overview

This activity lets children see the paths of muons, particles that arise from cosmic rays from outer space. Many children will be amazed to learn that there are particles passing right through their bodies many times a second all the time and even more so to “see” these particles.


  1. Small metal sweet tin or cake tin or baking tray (200g to 1kg)
  2. Black paint
  3. Plasticine or BluTak (enough to roll into about a 45cm long “snake”)
  4. Clear plastic drinking cup: one from a drinking machine will do
  5. Pad of 3mm thick felt to fit into top of clear plastic cup
  6. Dry ice to fill sweet tin
  7. Pure propenol or isopropyl alcohol
  8. Safety glasses, gloves and ladle for demonstrator and also for any older children wishing to do the experiment themselves
  9. Powerful, small bar or button magnet which can fit inside caketin standing upright (with its pole parallel to the lid)
  10. Bright light source: a strong LED torch gripped in a retort stand works excellently for this experiment
  11. Darkened Room

Possible Hazards

Dry Ice and Carbon Dioxide

Dry ice will give a freezing burn, which is painful but not likely to be serious aside from in the extreme cases of dry ice in the eyes or if swallowed. The pain from a small block of dry ice will ensure that any contact is only fleeting and highly unlikely to be an issue. In the unlikelihood of a child’s swallowing a pellet, do NOT induce vomiting,  give plenty of water to drink and get the child to a hospital. In the unlikelihood of contact with eyes, hold eyes open, flood with water for at least fifteen minutes and take the child to a doctor. Read the MSDS for dry ice.

It is not recommended that children younger than 13 should handle dry ice. Protective gloves and eyeglasses should be worn when the demonstrator packs the dry ice into the cloud chamber’s bottom. Do not let children touch the cloud chamber’s base – it will burn them, but again once the ice is inside the container, such a burn will only be small and of no lasting ill.

If building a bigger cloud chamber, do so in a well ventilated area. Theoretically there is an asphyxiation risk, so keep the amount of carbon dioxide below the long term threshold limit value (maximum safely allowable amount for long term exposure i.e. well below any choking risk), which is 9 grams of carbon dioxide for every cubic metre of air. This means that a 5m×4m×4m room even if altogether airtight can safely brook the complete sublimation of nearly a kilogram of dry ice (800 grams), so no particular precautions are needed if less dry ice and / or a bigger room is being used. If working in a smaller room or with more dry ice  than this, leave doors and windows open. Ten kilograms in a moderate sized room such as this example with doors and windows open and a small fan to circulate air is altogether safe (remember, it will take many hours for ten kilograms to sublime, unless you dump it into water to see the spectacular effect. If you do this, you’ll need to be careful to provide enough ventilation).


Powerful magnets such as used here are only dangerous if swallowed (and then only in twos or more). Unless you are dealing with a highly mouthy child, there is therefore minimal risk. In the unlikelihood that a magnet should be swallowed, take the child to a hospital, where doctors can retrieve the magnet before any damage happens.


Alcohol is a very mild hazard: its main risk is skin or eye irritation. Read the Propanol MSDS.


This demonstration does NOT involve radioactivity as it detects cosmic rays which are passing though our bodies all the time anyway (we cannot stop this and have evolved to cope with this). However, if a child is particularly interested in the demonstration and wishes to build their own cloud chamber at home, they will inevitably come across ideas on the internet to try with cloud chambers and most of them involve observing particles from radioactive sources, particularly those from stripped down smoke detectors. I believe the best approach is to ask whether anybody would like to build their own cloud chamber at home (with the help of a grownup, as the handling of dry ice by before-teenage children is probably not a good idea) at the end of the demonstration.  Then the following warnings should be emphasised:

  1. If you are building a cloud chamber at home, use it only to detect cosmic rays as in the current demonstration.
  2. There is NO level of artificial radioactivity that is safe. Radioactivity is altogether unlike any other poison known to humans insofar that even though allowable radioactivity levels for those people exposed to it in industry have been steadily lowered over many decades, these tightening standards have not yet eliminated radiation begotten sickness. In contrast for many other poisons, a threshold toxin level for concerned workers is typically ultimately discovered below which exposure is extremely unlikely to lead to harm;
  3. Most radioactive sources, quite aside from their radioactivity, are extremely poisonous owing to their weird electronic structure, which wreaks havoc with biology (e.g. uranium, americium);
  4. NEVER EVER take a smoke detector apart. NEVER EVER try to get to and isolate the americium source within (as shockingly advised to do by many cloud chamber YouTube clips);
  5. NEVER EVER take a glow in the dark wristwatch apart (again, as is often blithely advised on the internet). Many older such watches use radium on their faces – one of the most radioactive substances known – and are only “relatively safe” owing to the glass’s being in place (which stops the most dangerous alpha radiation).

The building of a much bigger cloud chamber than that demonstrated (with several kilos of dry ice) is a worthwhile thing to do as it will be much more sensitive to cosmic rays but it calls for a great deal more patience (the system will need half an hour or so to cool down properly so that muon tracks can be seen). Therefore, a child certainly doesn’t need to test for radioactivity to do more exciting cloud chamber experiments. There are also some fairly safe sources, such as uranium absorbed into glass to make “Annagrün” or “Annagelb” fluorescent art glass (also called “Vaseline Glass”) or fluorescent uranium marbles which can be readily bought online, but these are only safe by dint of their being much less radioactive than the natural background and so will not work as sources for a home cloud chamber. So using them is pointless. Moreover, even these are dangerous if shivvered because glass shards lodged in the body long term are a severe radiation hazard from even “less-than-background” sources.

You may get the question that “cosmic rays are radiation too, why are these safe”. The simple answer is that, strictly speaking, they aren’t. However, they are a fact of life and there is nothing that we humans can do about them. We have evolved to cope with them, and indeed it is believed that cosmic rays have played an important role in begetting the genetic mutations that have enabled evolution to happen in the first place. We are shielded from the worst of them by the atmosphere.

Age Group

13 to 206 (if doing the experiment for oneself)

6 to 13 (demonstration)

The Experiment

Watch Dr. Suzy Sheehey explain and do the experiment on

A variation on this experiment is to put a powerful magnet at the bottom of the cloud chamber and see its effect on the muon paths. Magnets curl the paths of charged particles, so you may see curly paths just above the magnet’s poles, even spring shapen spirals. You can indeed prove that the particles have negative charge like electrons do by the direction they curve in. Indeed the muon has the right same charge as the electron and behaves exactly alike one, aside from that it is much (207 times) heavier.

After you’re done, you can make a quite spectacular show by dumping the dry ice into a tub of water. There is no sputtering and this is quite safe to do outside. It’s best not to do inside owing to the asphyxiation risk, even though this is tiny.

Afterthoughts and Discussion

See the Wikipedia page on Cosmic Rays. We now know that most of the muons we see in our cloud chambers are only indirectly visitors from outerspace. Our studies from satellites and from space telescopes (where the cosmic radiation is MUCH stronger than on Earth, owing to the shielding by our atmosphere) show that most cosmic rays are protons – Hydrogen nucleusses with their electrons stripped off. Most of them are thought to have come from supernovas – the colossal explosions that stars undergo at the ends of their lives. The most powerful ones (fastest protons) are roughly forty million times as energetic as the most energetic protons that we humans can make in the Large Hadron Collider. They crash into the gas molecules of our atmosphere and some of the energy of the titanic collision is turned into muons, which then shower us from the sky. So our cloud chamber is not quite seeing visitors from outer space, but it lets us hear them when they ring the doorbell.

Incidently, all elements heavier than hydrogen were made inside stars. Many of the important ones for life such as iron were made only in the last split seconds of the lives of stars. The great astronomer Carl Sagan gave us the immortal quote:

“Our Sun is a second- or third-generation star. All of the rocky and metallic material we stand on, the iron in our blood, the calcium in our teeth, the carbon in our genes were produced billions of years ago in the interior of a red giant star. We are made of star-stuff


“We’re made of star stuff. We are a way for the cosmos to know [ken and understand] itself.”

(the bracketed “ken and understand” is mine – reading Sagan it is very clear that he means both meanings of the word).