How far away are the stars

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How far away are the stars?
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Parallax
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Measuring the Distance to the Stars with Parallax
Remember that 1 parsec 3.26 light years
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Visual magnitude scale

• Originally, the brightest stars in the sky were
  considered first magnitude, the next brightest
  group second magnitude, and so on
  
• Now, magnitude is more precisely defined, so that
  a m1.0 star is 100 times as bright as a m6.0
  star
  
• Each magnitude is a factor of 2.5

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Star Names

• Most of the brightest stars have names (e.g.,
  Betelgeuse, Sirius)
  
• Up to 24 stars in each constellation have a name
  with a Greek letter (e.g., ß Tauri)
  
• The rest may only be known by a catalog number
  (e.g., HDE 226868)

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Absolute magnitude

• Equivalent to apparent magnitude of the star if
  it was at a distance of 10 parsecs
  
• The sun is M4.8

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Luminosity

• Measure of the amount of light a star gives off
• L 3.83 x 1026 W

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The Nature of Stars
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A stars color reveals its surface temperature
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Photometry

• Technique used to accurately measure the color of
  stars using UBV filters
  
• U - ultraviolet wavelengths
• B - blue wavelengths
• V - visible wavelength
• astronomers compare combinations of U minus B
  (U-B) and B minus V (B-V)
  
• these combinations are called color indices

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Stars are classified by their spectra as O, B,
A, F, G, K, and M spectral types
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Stars are classified by their spectra as O, B,
A, F, G, K, and M spectral types

• O B A F G K M
• hottest to coolest
• bluish to reddish
• An important sequence to remember
• Our Best Astronomers Feel Good Knowing More
• Oh Boy, An F Grade Kills Me
• Oh Be a Fine Girl (or Guy), Kiss Me

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The Hertzsprung-Russell diagram identifies a
definite relationship between temperature and
absolute magnitude
Absolute magnitude vs temperature
or
luminosity vs spectral type
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Determining the Sizes of Stars from an HR Diagram

• Main sequence stars are found in a band from the
  upper left to the lower right
  
• Giant and supergiant stars are found in the upper
  right corner
  
• Tiny white dwarf stars are found in the lower
  left corner of the HR diagram

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Luminosity classes

• Class I includes all the supergiants
• Class V includes the main sequence stars
• For example, the Sun is a G2 V

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How do we know the mass of a star?

• Binary star systems
• Recently, also from extrasolar planets

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Binary stars orbit their common center of mass
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animation
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