Announcements

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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

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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, Newtons laws
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Course Outline

• Naked-eye astronomy
• Crash course in physics
• Our solar system
• The stars
• Structure and history of the universe

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Course Outline

• Naked-eye astronomy
• Crash course in physics
• Our solar system
• The stars
• Structure and history of the universe

You are here
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Star Clusters

• 13 November 2006

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Today

• Fuzzy objects in deep space
• Two types of star clusters
• Measuring distances beyond the range of parallax
  (variable stars as standard candles)

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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)

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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

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Diffuse Nebulae

• Huge clouds of gas, mostly hydrogen
• Often associated with clusters of young stars
  that formed from the gas and illuminate it

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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)
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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

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A young star cluster (Pleiades)
Main sequence only, no red giants or white dwarfs
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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
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Brightness of Stars

• True brightness (or luminosity) is a stars
  actual rate of energy output, measured (for
  example) in watts. The suns 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 suns apparent brightness from
  earths location is about 1400 watts per square
  meter.

True brightness
Formula Apparent brightness
4p(distance)2
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Brightness of Stars
True brightness
Formula Apparent brightness
4p(distance)2
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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

True brightness
Apparent brightness

4p(distance)2
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Variable stars as standard candles

• Longer period of variation implies greater
  luminosity
• Tricky because there are different types of
  variable stars

Henrietta Leavitt
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The Magellanic Clouds