Introduction to Astronomy - Weber State University

Introduction to Astronomy - Weber State University

Announcements Tests are graded! (Homework, too) Take the test today, tomorrow, or Friday before class (all about stars) Sunspot viewing at end of class today!

The Milky Way 15 November 2006

Today: Observing the Milky Way; light pollution Finding our place in the Milky Way Measuring distances beyond the range of

parallax Interstellar dust and how to see through it Observing the Milky Way

Light Pollution Light Pollution

Light Pollution: Solutions Full cut-off fixtures (no light comes out above a horizontal

plane) Motion sensors on security lights Low-pressure sodium lights Good old-fashioned moderation! All these steps will also save

energy and money How far do the stars go?

Herschel (late 1700s) counted stars in all directions and found more along the Milky Way than elsewhere

(surprise!)

But no big differences between different parts of the Milky Way William Herschel

Structure of our star system Using variable stars as standard candles, we can

measure distances to globular clusters Result: Theyre centered on a point about 25,000 light-years away, behind

the constellation Sagittarius--the center of the Milky Way galaxy!

Structure of our star system Huge disk, 100,000 light-years across Were about 25,000 light-years out from center.

Globular clusters (and some stars) are not confined to disk, but are in random orbits about the center, going around in all directions.

Estimated to contain about 100 billion stars (1011) Spiral arms are hard to discern from within.

Why cant we see the galactic center? Interstellar gas and dust (more like smoke)

Gas and dust in Orion Looking toward the galactic center Toward the galactic center

Milky Way from Cygnus to Sagittarius Full sky composite photo

Infrared light penetrates the dust! Milky Way at many wavelengths

Cold hydrogen emits 21-cm radiation

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