Sun, Moon, and Stars: Astronomy for Beginners · Part III, Ch. V · Part 3: Part III: The stars
29. The Milky Way
Goal: Your child can describe the Milky Way as our home galaxy, explain why Herschel's map was mistaken, and picture how big it is with a scale model.
You'll need
- The book, open to Part III, Chapter V
- Two small beads or peppercorns
- A map app or a road map of your area
- A dark-sky outing, if you can arrange one
What's in the book
The Milky Way, a soft band of light round the whole sky, is made of countless suns, with dark gaps like the Coal-sack. The chapter tells how Herschel counted stars in different directions to sketch its shape, why his rules could mislead, how bright stars gather along it, and imagines a model with the Sun and Alpha Centauri just one yard apart.
The lesson, step by step
Read together
Read the chapter aloud.
There are little suns as well as great suns in the Milky Way.
Herschel's mistake
SayHerschel assumed all stars are about the same, so dim stars must be far away. The book points out the flaw: 61 Cygni is dim, Sirius is bright, yet Sirius is the farther of the two.
Ask: What other mistake could make him think he had reached the edge, when he saw a dark patch?
The one-yard model
Do the book's model. Put two beads one yard apart: the Sun and Alpha Centauri. Say: In this model, the Sun and the Earth would be closer together than the width of a hair.
Then look at a map. At this scale, our whole Milky Way would be about 13 miles across. Find a town about 13 miles away. The Andromeda Galaxy would be about 325 miles away. Find a city that far.
What we know now
The Milky Way is a flat, spiral-shaped galaxy about 100,000 light-years across, with hundreds of billions of stars. The Sun is not near the middle, as Herschel thought; it is about halfway out, in one of the spiral arms.
Herschel was fooled partly by dust: dark clouds of dust block the light of stars behind them. The Coal-sack is one of these dust clouds.
The book's guess that little suns may outnumber great ones was right: small red stars are the most common of all.
Tell it back
Ask your child to describe what the Milky Way is and where we are in it, using a dinner plate as the galaxy and a crumb as the Sun.
Check questions
What is the dark gap in the southern Milky Way called?
About how many stars can be seen with the naked eye in the whole sky, according to the book?
Which way is the Sun traveling, according to the book?
Notice it this week
The Milky Way needs a dark sky, far from city lights, on a night without a bright Moon. Summer and early autumn evenings show its brightest part, toward Sagittarius in the south. Many parks hold dark-sky nights.
Answers to the book's problems
The Coal-sack. Not more than six thousand. Toward the neighborhood of the constellation Hercules.
Worked out for this site from the scan. If a number in your copy differs, trust the book.