Sun, Moon, and Stars: Astronomy for Beginners · Part III, Ch. II · Part 3: Part III: The stars
26. Some particular suns: 61 Cygni, Alpha Centauri and Sirius
Goal: Your child can tell one story about each of three famous stars, including how Sirius' hidden companion was predicted before it was seen.
You'll need
- The book, open to Part III, Chapter II
- Two balls of different sizes and a short string (to show a pair of stars)
- The star notebook
What's in the book
Three neighbors of the Sun: 61 Cygni, a faint double star whose distance was measured first; Alpha Centauri, the nearest known, a pair of suns circling each other in about eighty-five years; and Sirius, the brightest star, whose dim companion was predicted from its wobbling path and then seen eighteen years later. The chapter ends with what the spectroscope tells of Sirius' make.
The lesson, step by step
A word about 'billions'
This book uses 'billion' in the old British way, meaning a million millions (what Americans call a trillion). So when it says 61 Cygni is 'forty billions of miles away', it means forty trillion miles.
Read together
Read the chapter aloud. At Alpha Centauri, read:
Of all the stars shining in the heavens round our earth, two only — Sirius and Canopus — show greater brilliancy than Alpha Centauri.
Two suns dancing
Tie a small ball and a big ball together with a short string. Hold the string near the big ball and twirl them gently on a table: they swing around a point between them, closer to the big one. Say: That is how a double star moves. The small star swings around more widely and faster.
The hidden companion
Retell the Sirius story: astronomers noticed Sirius wobbling in its path, worked out that an unseen companion must be tugging it, and eighteen years later someone actually saw it.
Ask: Which earlier story in this book is a lot like this one? (The discovery of Neptune.)
What we know now
Sirius' companion is a white dwarf: a burned-out star about the size of the Earth but as heavy as the Sun. It is not cooler than Sirius, as the book guesses; it is actually hotter, but so small that it is very faint.
Light from Sirius takes about 8.6 years to reach us (the book says twenty years here, but nearly nine in Part I; the nine is right). Sirius is also drifting slowly toward us, not away.
Alpha Centauri has a third, tiny red star, Proxima Centauri, which is the nearest star to the Sun of all. Planets have been found circling Proxima.
61 Cygni is about 11 light-years away, a little farther than the book thought.
Tell it back
Ask your child to choose one of the three stars and tell everything the book says about it.
Check questions
Why did astronomers think the faint star 61 Cygni might be near?
Can you see Alpha Centauri from England or most of the United States?
How did astronomers know Sirius had a companion before they saw it?
Notice it this week
In winter, find Sirius: follow the three stars of Orion's belt down and to the left. Notice how it twinkles in many colors when low in the sky. That is our air shaking its light, not the star changing.
Answers to the book's problems
Because it moved across the sky much faster than most stars, which suggested it was close. No, it is in the southern sky, near the Southern Cross. Because Sirius' movement had strange irregularities, now faster, now slower, which could be explained only by a heavy body circling it and pulling on it.
Worked out for this site from the scan. If a number in your copy differs, trust the book.