Sun, Moon, and Stars: Astronomy for Beginners · Part III, Ch. VI · Part 3: Part III: The stars
30. Reading the light: the spectroscope
Goal: Your child can make a spectrum, name its colors in order, and explain how dark lines in starlight tell what a star is made of and whether it is coming or going.
You'll need
- The book, open to Part III, Chapter VI
- A clear glass of water and a sheet of white paper (for a sunny window)
- An old CD or DVD
- A lamp
- A TV remote and a phone camera (optional)
What's in the book
The spectroscope splits light into its colors. The chapter explains how heat turns metals to gas, how Newton's prism spreads white light into red, orange, yellow, green, blue, indigo and violet, how a narrow slit shows dark lines in sunlight, and how each gas makes its own lines, so we can tell that iron, sodium and hydrogen are in the Sun and stars. A shift in the lines shows a star rushing toward or away from us.
The lesson, step by step
Read together
Read the chapter aloud.
The spectroscope may be looked upon as twin-sister to the telescope.
Make a rainbow (the safe way)
Set a full glass of water on a sunny windowsill and put white paper on the floor or table where the light comes through. Move the paper until a little rainbow appears. Look at the paper, not at the Sun.
Name the colors in order together: red, orange, yellow, green, blue, indigo, violet. Just as the book says, red is bent the least and violet the most.
A spectrum from a CD
Hold a CD under a lamp and tilt it until bands of color appear. Try different lights in the house: an old-style bulb gives a smooth rainbow, while some LED and fluorescent lights give broken bands. Say: Different lights have different 'fingerprints'. That is the whole idea of the spectroscope.
Light we cannot see
The book says there are rays below the red that we can feel as warmth but cannot see. Point a TV remote at a phone camera and press a button. Many cameras show a flicker of light your eyes cannot see: that is infrared, below the red.
Coming and going
SayWhen a siren comes toward you, its sound is higher; going away, lower. The book says starlight does something like this: the waves are pressed together when a star comes toward us and stretched when it goes away. Listen for a passing car or siren this week.
What we know now
In 1868 astronomers found lines in the Sun's light that matched no known element. They named it helium, after the Greek word for the Sun, and found it on Earth only years later.
The spectroscope showed that stars are made mostly of hydrogen and helium. And the shifting of lines in the light of distant galaxies showed that the universe is expanding.
Tiny wobbles in a star's lines, caused by a planet tugging it, have helped find many of the planets now known around other stars.
Tell it back
Ask your child to explain, in their own words, how astronomers can tell there is iron in the Sun without going there.
Check questions
Who first found that white light is made of many colors?
Which color is bent least by a prism? Which most?
Why are moonlight's dark lines the same as sunlight's?
Tip
Do the glass-of-water rainbow when the Sun is low (morning or late afternoon), so the light streams in sideways. Remind your child to keep their back to the window and look only at the paper.
Answers to the book's problems
Newton. Red is bent least, violet most. Because moonlight is really sunlight reflected from the Moon.
Worked out for this site from the scan. If a number in your copy differs, trust the book.