The Universe in the Classroom

Stellar Evolution (a la Chez Stella)

Everything is faster now. You try to stop him to ask questions but you have no chance. He's talking faster and faster and going through stars one after another.

Stars number 4, 5, and 6 behave a lot like the first three, though they are not as big or as bright. They still turn red and puff up at the end of their lives, but they don't get nearly as large. None of them die in a flash. Instead they create huge shells of gas and dust that slowly expand. All they leave behind are tiny white dwarfs with no fuel left to burn. These will hang endlessly in space to cool into dark lumps of matter.

"I don't understand," you ask when the last one is finished. "They seemed to change color, brightness, and size all at the same time. Are those things related?"

But there is no answer. You look around. The waiter is no longer there. You start to call out, but, looking around, you change your mind. The table is gone too, and the stars, and the warm wind. You are hot and it is very quiet. Something seems very familiar. You open your eyes and blink.

The classroom is empty. The clock shows 4:00 p.m.

"Rats!" You say to yourself. Your notebook is open and you can see in your own handwriting scribbles on Kepler and planet orbits and... That is where they stop. "I'll never learn any astronomy this way," you think as you begin to pack up your books to leave.

That is when you notice the menu. Tucked neatly beneath your notes is a familiar menu. On top it says "Chez Stella -- Serving the Stars for 15 Billion Years." On the left is a description of every star you saw. The right side is labeled "Tasting Notes." It contains three carefully drawn graphs labeled "Evolution of the Sun," "Evolution of a 25 Solar Mass Star," and "Temperature Changes in Six Stars." Attached is a note written in the waiter's neat hand.

Bonjour -

I regret there was so little time to answer the many excellent questions you had. Some dreams are like that. I believe you'll find the answer to your and many other questions in the graphs given under "Tasting Notes." As you look them over, you may want to remember that the color of a star depends entirely on its temperature. The hotter a star is the bluer it looks, and the cooler it is the redder it looks.

A Bientot, M. Poisson

P.S. Should any other questions occur to you, remember you are always welcome at Chez Stella.

MICHAEL CHABIN is a programmer who specializes in web-based interactive software. Currently he is collaborating with three others to develop a new school concept. Called The Urban School, you can find more about that at Michael can be reached at


The stellar models used here came from two separate sources. Both are available via the web from the Strasbourg (France) Data Center.

Schaller, G., et al. 1992, Astronomy and Astrophysics Supplements Series, vol. 96, pg. 269. (at URL

Charbonnel, C. et al, 1996, Astronomy and Astrophysics Supplements Series, vol. 115, pg. 339 (at URL

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