Stars
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Stars

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Stars Stars Presentation Transcript

  • stars Parallax and Magnitude
  • 2p 1AU 1AU 61Cygni To distant star p p
    • A Star with a parallax of 1 arc-second is
    • 1 parsec from the Earth
    • 1 par ( allax) sec ( ond )
    • Distance ( pc ) = 1 .
    • Parallax ( s )
  • 61 Cygni is 0.3 seconds of arc
    • Distance = 1
    • 0.3
    • = 3.33 pc from Earth.
  • Consider a star 1 parsec from Earth
  • 1AU CIRCUMFERENCE=60 X 60 X 360 AU
  • 1 PARSEC CIRCUMFERENCE = 2 π parsec
    • So 2 π parsecs = 60 x 60 x 360 AU
    • 1 parsec = 5 x 10 AU
    • = 3.26 LIGHT YEARS
    • 61 Cygni is 3.33 x 3.26 Light Years
  • About 11 Light Years away
  • Looking At Brightness
    • Using the inverse square law
    • Observed Brightness 1 .
    • Area
    • 1 .
    • distance ²
    • Magnitude
  • Ancient Greeks
    • Brightest stars = Magnitude 1
    • Dimmest stars = Magnitude 6
  • 19 th CENTURY LIGHT from a magnitude 1 star was 100 times brighter than from a magnitude 6 star.
    • A Difference of 5 magnitudes
    • Corresponds to a ratio of 100
    • Since
    • 2.5 x 2.5 x2.5 x 2.5 x 2.5 = 100
    • A Difference of 1 magnitude
    • Corresponds to a ratio of 2.5 in the measured intensity of light
    • The Sun has a magnitude of -26.7
    • The dimmest star using a giant telescope + 25
    • Naked eye +6
  • PROBLEMS
    • How bright a star is ( apparent magnitude)
    • Doesn’t tell us how bright a star really is.
    • Why not ?
    • It may be brighter, but distant
    • It may be dimmer ,but closer.
    • Stars are not all the same!
  • ABSOLUTE MAGNITUDE
    • Defined as equal to the apparent magnitude the star would have at a distance of 10 parsecs from Earth.
    • The absolute magnitude of stars ranges from – 10 to +15
    • Absolute Magnitude = M
    • Apparent magnitude = m
    • Distance = d
    • M = m – 5 log d
    • 10
  • Sirius
    • Apparent magnitude – 1.46
    • distance = 2.65 parsecs from Earth.
    • Absolute Magnitude M = - 1.46 -5 log(0.265)
    • = - 1.46 – 5( -0.58)
    • = - 1.46 + 2.9
    • = 1.44
  •  
    • Period = 5.75 days
    • Magnitude increases at twice the rate it falls.
    • Magnitude changes by 0.7
    • Hence intensity ratio =
    • = 1.9
  •