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 1700’s) 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: They’re 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

• We’re 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 can’t we see the galactic center?

Interstellar gas and “dust” (more like smoke)

Gas and dust in Orion

Looking toward the galactic center

Milky Way in Sagittarius

Toward the galactic center

Milky Way from Cygnus to Sagittarius

Southern Milky Way and Magellanic Clouds

Southern Milky Way, full sky

Full sky composite photo

Infrared light penetrates the dust!

Milky Way at many wavelengths

Cold hydrogen emits 21-cm radiation