Dr. Fred Ask Dr. Fred


by Dr. Fred Bortz

Safesurf Rated All Ages

For more than twenty years, Dr. Fred Bortz worked as a scientist, researcher, and teacher. Now he spends most of his time writing books and articles for young readers like you.

He enjoys both science and writing for the same reason: HE LOVES QUESTIONS. He writes for people your age because he knows you love questions, too.

To find hot-links to other "Ask Dr. Fred" questions and learn how to send Dr. Fred your favorite question, go to the main "Ask Dr. Fred" page.

This column is copyrighted material. If you plan to use it in the classroom or for other educational or commercial purpose, please click here.

This question came from a first grader named Jennifer at Allegheny-Clarion Valley School, before Dr. Fred's visit there in May, 2001:

Why do stars sparkle?

Dear Jennifer,

That's a great question! Did you know that stars are really other suns that are very far away? They are so far away that we only see them as points of light.

If we went to the Moon and looked at the stars, we would see the same bright points, but they wouldn't twinkle. Their light would be steady.

The difference is that the Moon has no air and Earth has lots of it. Light bends just a little bit as it passes through the air, and it bends differently in warm air than cool air. You may have noticed that bending on a hot summer day, especially when you look across a paved area or desert sand that has been baking in the hot sunlight. Things in the distance can look wavy.

As you look at a star from Earth, the starlight has to pass through several miles of air before it reaches your eye. That air is always changing. Sometimes the starlight passes through more warm air. Sometimes it passes through less warm air. The means the starlight bends different amounts on its way to your eyes, changing from one instant to the next. It changes just a tiny bit, but enough for you to notice the differences as twinkling.

Twinkle, twinkle, Little Star.
I know exactly what you are.
And thanks to Dr. Fred, I think I'll
Tell my friends: Air makes you twinkle!

Poetically yours,
Dr. Fred

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Text copyright 2001-3 by Alfred B. Bortz, all rights reserved

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