1. Announcements I’m enjoying reading your projects! Turn in homework 8 by 5:00 today Pick up Homework 9. Test this week (W, Th, F before class) To put things in perspective, try Zoomer! (linked at bottom of course web page) Tonight is a great time to start the Phases of the Moon project!

2. Course Outline Naked-eye astronomy Crash course in physics Our solar system The stars Structure and history of the universe

3. Course Outline Naked-eye astronomy Crash course in physics Our solar system The stars Structure and history of the universe You are here

4. Measuring the Stars 23 October 2006

5. Today: How we determine the properties of stars: distance, motion, brightness A survey of the stars in our neighborhood

6. How far away are the stars? A clue: Compare brightness of our sun to brightness of stars... It’s the difference between night and day!

7. Measuring distances by parallax Baseline Nearby star Background stars A B Angle = ? Approximation: Measure the angle between A and B from the baseline instead of from the nearby star. As long as the background stars are many times farther away, the difference is negligible.

8. Measuring distances by parallax Baseline Nearby star “Big Circle Problem”: How many baselines would fit around the circle? What’s the circle’s circumference? What’s the circle’s radius?

9. Measuring distances by parallax 2 A.U. Nearest star Example (nearest star): Angle = 1.5 seconds of arc (1/2400 degree), so 864,000 baselines would fit around circle (360 x 2400). Circumference = 1.7 million A.U. Radius = 275,000 A.U. (circumference / 6.28) (Baseline is earth’s orbit around sun!)

10. The Nearest Star Alpha Centauri

11. How far is 275,000 A.U.? Light-travel time: Light takes 8.3 minutes to travel 1 A.U., so to reach us from the nearest star, it takes 2.3 million minutes, or 4.3 years. In our scale model, 1 A.U. is 100 feet. Then the nearest star is 27.5 million feet away, or 5200 miles. So far, humans have traveled 1/400 A.U.; robotic spacecraft have traveled 100 A.U.; interstellar travel will not be practical any time soon!

12. Space is Big. “Space is big. Really big. You just won't believe how vastly hugely mind-bogglingly big it is. I mean, you may think it's a long way down the road to the chemist, but that's just peanuts to space…” To be fair though, when confronted by the sheer enormity of the distances between the stars, better minds than the one responsible for the Guide's introduction have faltered. Some invite you to consider for a moment a peanut in Reading and a small walnut in Johannesburg, and other such dizzying concepts. The simple truth is that interstellar distances will not fit into the human imagination. --Douglas Adams The Hitchhiker's Guide to the Galaxy

13. History of Parallax Measurements 1838: First successful stellar parallax measurement by Friedrich Wilhelm Bessel 1990: A few hundred stellar parallaxes measured by now, out to about 100 light- years. Atmospheric blurring makes further measurements from earth’s surface virtually impossible. 1990’s: “Hipparcos” satellite, launched by European Space Agency, measures more than 100,000 parallaxes, out to more than 1000 light-years.

14. Stellar Motion “Radial motion” (toward or away from us): Measured by Doppler shift of spectral lines. “Proper motion” (across the sky): Measured by observing changes in a star’s position compared to background stars. Typical speeds of nearby stars (relative to us): a few tens of kilometers per second, or roughly 1/10,000 the speed of light. Directions are mostly random.

15. Parallax and Proper Motion