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Stars
 

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
    •