Sun, Moon, and Stars: Astronomy for Beginners · Part I, Ch. IX, part 1 · Part 1: Part I: Our family in the sky, and its neighbors
10. Neighboring families: how many stars, and how far?
Goal: Your child knows that star 'magnitude' means brightness, and can show with their thumb how astronomers use a shift in view to measure distance.
You'll need
- The book, open to Part I, Chapter IX
- Just your thumb
- A notebook for a star count
What's in the book
Our Sun is just one star among millions. The chapter tells how many stars the eye can see (two or three thousand at once), what magnitudes mean, and how astronomers measure distance with a base-line, shown by a tree across a river. It ends with the first star distances: Alpha Centauri, 61 Cygni and Sirius.
The lesson, step by step
Read together
Read from the start of the chapter to the steeple seen through two window panes. Ask: How many stars do you think you can see on a clear night? Then read what the book says.
Magnitude means brightness
SayAstronomers call the brightest stars 'first magnitude'. The faintest you can see without a telescope are 'sixth magnitude'. Bigger number, fainter star.
So when we speak of different magnitudes we really mean different brightnesses.
The thumb jump
Hold your thumb up at arm's length in front of a far-off object, like a door frame or a tree. Close your left eye, then switch to the right eye. Your thumb seems to jump.
Now bring your thumb close to your face and do it again. It jumps much more.
SayThe space between your eyes is your base-line. Near things jump a lot, far things jump a little. Astronomers look at a star from opposite sides of the Earth's orbit, six months apart, and measure its tiny jump.
Why it was so hard
Read the book's steeple picture again: the stars are so far that even the Earth's whole orbit is like two window panes looking at a steeple twenty miles off. Most stars hardly jump at all.
What we know now
The book says fewer than a hundred star distances were known. Today a space telescope called Gaia has measured the distances of more than a billion stars this same way.
The book thinks other suns may have planets. Since the 1990s astronomers have found thousands of planets circling other stars, more than 5,000 so far.
Our galaxy, the Milky Way, holds hundreds of billions of stars, not sixty million.
Tell it back
Ask your child to explain the river and tree example, and then do the thumb trick for someone else, explaining it.
Check questions
About how many stars can a person see at once on a clear night?
What does 'first magnitude' mean?
What is the nearest star whose distance the book gives?
Notice it this week
On a clear night, hold up a paper towel tube and count the stars you can see through it in one patch of sky. Try again from a darker spot, or on a night with no Moon, and compare.
Answers to the book's problems
About two to three thousand (five or six thousand round the whole world). The brightest stars, about twenty of them; magnitude means brightness, not size. Alpha Centauri.
Worked out for this site from the scan. If a number in your copy differs, trust the book.