# Introduction to Astronomy

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```					           Announcements
• 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
•   Our solar system       You are

•   The stars                here

•   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 . . . It’s the difference between night and day!
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   Background
long as the background stars are many times farther        stars
away, the difference is negligible.
Measuring distances by parallax

“Big Circle Problem”:
•   How many baselines
would fit around the
Baseline

circle?

Nearby      •   What’s the circle’s
star           circumference?
•   What’s the circle’s
Measuring distances by parallax

Example (nearest star):
•   Angle = 1.5 seconds
of arc (1/2400 degree),
2 A.U.

so 864,000 baselines
would fit around circle
Nearest        (360 x 2400).
star
•   Circumference = 1.7
million A.U.
(Baseline is earth’s            (circumference / 6.28)
orbit around sun!)
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.
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 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.
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.
Parallax and Proper Motion

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