Interactive & Demonstration
3D Printing an exploded star
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If you have a 3D printer, you can print out a supernova remnant, Cassiopeia A. Data of Cassiopeia A was captured by NASA's Chandra X-ray Observatory and combined with infrared and visible light to make the first ever 3D model of an exploded star. The model is free. Small prints around 3 in/7.62cm in size take about 2 hours to print, depending on the printer type and speed.
 Stars come in many different sizes. During the part of their lives where they are converting hydrogen to helium at their centers (which is the vast majority of their lives), their surface temperatures are directly related to their masses. Low-mass stars are cooler than high-mass stars. The color of a star is directly related to the temperature. Cool stars are red, hotter stars are yellow, and very hot stars are blue. There are many more low-mass stars than high mass stars (just as there are many more pebbles in a quarry than there are huge rocks).

Materials needed: bouquet of balloons (one large blue, a few medium yellow, many small red)

You can convey this picture by getting a bouquet of balloons. Get one very large blue one, a couple of medium-size yellow ones, and a larger number of small red ones. The biggest stars live the shortest lives; they burn themselves out very quickly, and go out with a bang. You can pop the blue one with a pin to demonstrate the explosion. For the others, yellow first followed by red, you can slowly let the air out, letting them gracefully collapse to their final states.

(Note: One confusing thing is that as massive stars evolve beyond the hydrogen-fusion stage, their outer layers expand and cool, turning red. Thus, while low-mass stars are red, one can also have “red supergiants” like Betelgeuse, which are actually very massive stars at a different stage of evolution.)
This demonstrates what happens to the center of a star during a supernova explosion. Core collapse of a star is the result of losing the pressure support at the center of a star, and having the heavy atmosphere of the star crush the core.

Materials needed: soda can, a Bunsen burner, clear bowl of ice water, tongs, oven mitts, safety glasses

Point out to the students that the Earth’s atmosphere is very heavy, and is pushing down on the can. Lucky for the can, there is also air inside pushing back out, providing a pressure support against gravity’s pull on the atmosphere.

What happens when you remove that air? Place about 1/8th of an inch of water in the can and hold it over the Bunsen burner until it is boiling vigorously. Use tongs to hold the can, and wear safety glasses. The steam heats the air in the can and pushes much of it out. Once there is a steady amount of steam coming out of the can, quickly turn it upside down and dunk it into the bowl of ice water. The rapid cooling of the little air that remains in the can decreases its pressure, and the can is quickly crushed by the atmosphere. This is what happens to the center of a star during a supernova explosion.

(Note: The can gets very hot. Wear oven mitts when holding the tongs. It is important to turn the can upside down and put it in the cold water quickly; otherwise the water will have a chance to flow back into the can to equalize the pressure, and the can won’t collapse. It is best to use a clear bowl for the ice water so that the students can see exactly what is being done. The can will not significantly change the temperature of the ice water, but care should be taken if using a glass bowl simply to ensure that it is not knocked over during the demonstration.)
This demonstrates the ejection of the outer layers of a star in a supernova explosion .

Materials needed: basketball, tennis ball

Place the tennis ball on top of the basketball and then drop them. When the basketball hits the ground, it compresses and then bounces back up. This transfers a lot of momentum to the tennis ball, and it will fly high in the air.

(Note: It can be a bit tricky to balance these so that the tennis ball doesn’t fly off sideways. Practice makes perfect! Also, replacing the tennis ball with a ping-pong ball or a compact “super” ball—a hard rubber ball that bounces very well by itself— works even better.)
Encourage the students to go outside with parents and friends on a clear, dark night. Have them choose a spot where they can see lots of stars. There are numerous things that can be observed, depending in part on location and how dark and clear the skies are.

Encourage interaction between the observers; this is best done in groups.

Possible discucssion questions:
• Who can spot the brightest star?
• How about the faintest?
• Are different stars different colors?
• Do some “twinkle” more than others? If so, are they high in the sky, or close to the horizon?

Don’t be surprised if someone spots a “star” that is moving slowly across the sky. This could be a plane, though that should become obvious after a while (the light may get brighter as the plane approaches, or the lights may blink), but it may also be a satellite in orbit high above the Earth, reflecting light from the Sun (though it has already set)!
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Acknowledgements: Recoloring the Universe with Pencil Code was created by volunteers David Bau (developer of Pencil Code and a Google employee at the time), August Muench (astronomer for the American Astronomical Society), Kim Arcand (visualization lead for NASA’s Chandra X-ray Observatory), and Sydney Pickens and Matthew Dawson (both computer science educators with Google CS First.). Further work has been developed with support from the Chandra X-ray Center, at the Smithsonian Astrophysical Observatory, in Cambridge, MA. Recoloring the Universe is also supported by NASA with funding under contract NAS8-03060.

Chandra AAS CODE Google CS First Pencil Code