Download Introduction to Astronomy

Announcements
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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
Luminosity
Calculate from apparent brightness and
distance
Temperature
From overall color or spectral class
Composition
From detailed analysis of spectral lines
Size
Calculate from temperature and
luminosity
Mass
From binary star orbits, Newton’s laws
Course Outline
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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
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27 “open” clusters
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29 globular clusters
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6 diffuse nebulae
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4 planetary nebulae
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1 supernova remnant
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2 small groups of stars
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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 mainsequence 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