Announcements

• Test this week (all about stars)

• Turn in Homework 11 today

• Pick up Homework 12

• Questions on last 3 assignments?

• Second project is due December 1

Summary of Stellar Properties

Distance Measure using parallax (if close enough)

Velocity Proper motion and Doppler shift

LuminosityCalculate from apparent brightness and distance

Temperature From overall color or spectral class

Composition From detailed analysis of spectral lines

SizeCalculate from temperature and luminosity

Mass From binary star orbits, Newton’s laws

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

• The stars

• Structure and history of the universe You are

here

Star Clusters

13 November 2006

Today:

• Fuzzy objects in deep space

• Two types of star clusters

• Measuring distances beyond the range of parallax (variable stars as standard candles)

Fuzzy Objects

• Nebulae, star clusters, and galaxies

• Most famous list was compliled by Charles Messier, to avoid confusion with comets

• William Herschel started what became the NGC list (New General Catalog)

Messier Catalog

• 27 “open” clusters

• 29 globular clusters

• 6 diffuse nebulae

• 4 planetary nebulae

• 1 supernova remnant

• 2 small groups of stars

• 40 other fuzzy things in which no individual stars are visible

Diffuse Nebulae

• Huge clouds of gas, mostly hydrogen

• Often associated with clusters of young stars that formed from the gas and illuminate it

Open Clusters

• Irregular in shape

• Typically contain a few hundred stars

• H-R diagrams indicate that they are fairly young (few or no giant stars)

Double cluster in Perseus

Pleiades (Subaru)

Globular Clusters

• Spherical clusters, each containing 105 - 106 stars

• About 100 known

• H-R diagrams indicate that they are very old (no massive main-sequence stars left)

• Nearly all are on one side of the sky, centered on Sagittarius

• Most are known by Messier numbers (M4, M13, etc.) or NGC numbers

A young star cluster (Pleiades)

Main sequence only, no red giants or white dwarfs

An old star cluster (Messier 3)

Main sequence “cuts off” above a certain point; plenty of red giants and white dwarfs

Oldest known cluster ages are about 12 billion years

Brightness of Stars

• True brightness (or “luminosity”) is a star’s actual rate of energy output, measured (for example) in watts. The sun’s luminosity is about 4 x 1026 watts.

• Apparent brightness is determined by the “intensity” of starlight striking a detector. It is measured (for example) in watts per square meter. The sun’s apparent brightness from earth’s location is about 1400 watts per square meter.

Formula: Apparent brightness = True brightness

4π(distance)2

Brightness of Stars

Formula: Apparent brightness = True brightness

4π(distance)2

Measuring distances beyond parallax

• Principal method is “Standard candles”: Find an object whose true luminosity we can guess, measure its apparent brightness, then calculate distance

• Out to a few million light-years, the best standard candles are pulsating variable stars

Apparent brightness

True brightness

4π(distance)2=

Variable stars as standard candles

• Longer period of variation implies greater luminosity

• Tricky because there are different types of variable stars

Henrietta Leavitt

The Magellanic Clouds