Stars

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Stars
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• Composed of 98 H and He
• Fusion in the core supports the star
• Full spectrum of masses

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Key Properties

• Apparent Brightness
• Luminosity
• Temperature / Color
• Mass
• Evolutionary State

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Brightness

• Absolute brightness
• Luminosity
• Power emitted by star into space
• Only depends on star
• Lsun 4 X 1026 Watts
• Apparent brightness
• How bright star appears in the night sky
• Power per unit area
• Depends on stars brightness and distance

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Inverse square law for light

• Apparent brightness measured in watts per square
  meter
• Drops off as square of distance

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Measuring Distance

• Stellar Parallax
• Caused by motion of Earth in its yearly orbit
• d 1/p
• where p is in arcsecs and d is in parsecs
• 1 parsec 3.26 lyrs

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Magnitudes

• Logarithmic
• Large values are dim objects
• Small values are bright objects

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Magnitudes

• Absolute Magnitudes

• Apparent Magnitudes

• A bright a star would appear if it were 10 pc
  away
• Does not depend on distance

• How objects appear from here on Earth
• Depends on distance
• We can only see objects with m6

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Color and Temperature

• Color is the difference between intensity in two
  filters
• B-V color is a good proxy for temperature
• Color is independent of distance

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Spectral Type

• Spectral types are subdivided for intermediate
  temperatures
• Values run from 0-9

• Smaller numbers are hotter
• Larger numbers are cooler
• Eg. B1 is hotter than B7

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Spectral Types

• Order was alphabetical depending on strength of
  Hydrogen line
• Williamina Flemming
• Revised to follow a more natural order
• Annie Cannon

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Measuring Stellar Masses

• Using Binary Systems

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Visual Binaries
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Eclipsing Binaries
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Spectroscopic Binaries
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HR Diagram

• Main Sequence
• Giants
• Supergiants
• White Dwarfs

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HR Diagram

• Luminosity class gives size and luminosity
  information

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Main Sequence

• Mass is the most important property for a star on
  the MS
• Stars spend 90 of their lives here, burning H in
  their cores
• MS lifetime depends on mass

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Main Sequence

• More massive stars live much shorter lives
• Burn fuel very quickly to support such a large
  star
• Less massive stars live longer
• Less fuel, but burn it more slowly

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Life After the Main Sequence

• When stars run out of H in their cores, they
  evolve off the MS
• Giants and Supergiants expand to extremely large
  sizes
• Temperatures are very low
• Luminosity is very high
• White dwarfs are small and hot
• Have no nuclear fusion
• Heated by collapse of gas

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Star Clusters

• All stars in the cluster formed about the same
  distance from Earth
• All stars in the cluster formed at about the same
  time
• Very useful in understanding stellar formation
  and evolution
• Can use them as clocks
• Most of what we know about stars comes from
  studying clusters

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Open Clusters

• Only a few million years old
• Contain lots of luminous blue stars
• Contain several thousand stars
• 30 lyrs across

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Globular Clusters

• Often several billion years old
• Some of the oldest objects in the galaxy
• Contains mostly smaller stars
• Around 105-106 stars concentrated in a relatively
  small volume
• 50-150 lyrs across

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Age of Cluster

• Main Sequence Turnoff (MSTO) more massive stars
  have evolved off of the Main Sequence
• MSTO gives age of cluster
• Lifetime of cluster same as MS lifetime of stars
  at the MSTO

MSTO
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• Young clusters still have their massive stars on
  the MS

• Old clusters are missing the massive blue stars
  on the MS