Announcements

1
Announcements

• Im enjoying reading your projects!
• Turn in homework 8 by 500 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 . . . Its the difference
  between night and day!

7
Measuring distances by parallax
A
Baseline
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
8
Measuring distances by parallax

• Big Circle Problem
• How many baselines would fit around the circle?
• Whats the circles circumference?
• Whats the circles radius?

Baseline
Nearby star
9
Measuring distances by parallax

• 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)

2 A.U.
Nearest star
(Baseline is earths 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 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.

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 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.

15
Parallax and Proper Motion