Introduction to Astronomy

Announcements Im 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! Course Outline Naked-eye astronomy Crash course in physics Our solar system

The stars Structure and history of the universe Course Outline Naked-eye astronomy Crash course in physics

You are Our solar system here The stars Structure and history of the universe Measuring the Stars 23 October 2006 Today: How we determine the properties of stars: distance, motion, brightness

A survey of the stars in our neighborhood How far away are the stars? A clue: Compare brightness of our sun to brightness of stars . . . Its the difference between night and day! Measuring distances by parallax Baseline A

Angle = ? Nearby star B 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. Background stars

Measuring distances by parallax Baseline Big Circle Problem: Nearby star How many baselines would fit around the

circle? Whats the circles circumference? Whats the circles radius? Measuring distances by parallax

2 A.U. Example (nearest star): Nearest star (Baseline is earths orbit around sun!)

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) Alpha Centauri 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! 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 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 lightyears. Atmospheric blurring makes further measurements from earths surface virtually impossible. 1990s: Hipparcos satellite, launched by European Space

Agency, measures more than 100,000 parallaxes, out to more than 1000 light-years. 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 stars 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. Parallax and Proper Motion

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