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Characteristics of Stars

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It says that every time you double the distance to a star, the brightness ... Measured using two scales. Apparent magnitude. Absolute magnitude ... – PowerPoint PPT presentation

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Title: Characteristics of Stars


1
Characteristics of Stars
  • Luminosity vs. Apparent Brightness
  • Luminosity-Distance Formula

2
Luminosity
  • Luminosity The total amount of power a star
    radiates into space.
  • This is usually measured in watts.
  • ex Sun 3.8 X 1026 watts

3
Luminosity
  • Problem The amount of light we measure from
    each star depends on where we are in relationship
    to it.
  • Example The brightness of two ceiling lights at
    different distances.

4
Thats called Apparent Brightness
  • Apparent brightness is the amount of light
    reaching us per unit area.
  • Also called flux
  • Because we cant compare stars based on how they
    appear, we have to get creative and of course use
    a lot of math!

5
Inverse Square Law
  • This law relates the decrease in brightness with
    the increase in distance from an object.
  • It says that every time you double the distance
    to a star, the brightness decreases by the square
    of that distance.

6
Luminosity Distance Formula
  • Main Idea
  • We can measure how bright the star appears, and
    we can measure how far the star is away from us.
  • Since we know how brightness appears to change
    with distance, then we can work backwards to find
    out how bright the star actually is.

7
How Bright Are The Stars?
  • Measured using two scales
  • Apparent magnitude
  • Absolute magnitude

8
Apparent Magnitude
  • Apparent magnitude measures the brightness of a
    celestial object as seen with our eyes from
    Earth.
  • An ancient system
  • Developed by Hipparchus in about 150 BCE
  • The brightest stars were called first magnitude
    next would be second magnitude
  • The lower the number, the brighter the object.
  • Negative numbers indicate extreme brightness.
  • The full moon has an apparent magnitude of -12.6
    the Sun's is -26.8. We can see objects up to 6th
    magnitude without a telescope.

9
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10
Absolute Magnitude
  • A more modern way of measuring magnitude
  • Measures how bright an object appears if it were
    a standard distance from Earth (10 parsecs)
  • Example
  • Suns apparent magnitude is -26.8
  • Suns absolute magnitude is 4.8

11
Luminosity Is Still Better
  • Both absolute and apparent magnitude are slightly
    outdated
  • Luminosity-distance formula is more commonly
    used.
  • Shows distance, apparent brightness, and
    luminosity

12
Stellar Surface Temperature
  • Second most important property used in modern
    stellar classification.
  • Only refers to the surface temperature of stars
  • Easier to measure than luminosity
  • Is not affected by distance

13
Surface Temperature and Color
  • Surface temperature determines the color of light
    that the star emits
  • Red stars are
  • Blue stars are
  • Yellow stars are than red stars but ____
    than blue stars
  • How is surface temperature related to wavelength
    and frequency?
  • Hotter stars emit at higher frequencies and
    shorter wavelengths than cooler stars.

14
Spectral Type
  • Emission and Absorption lines are used to provide
    a more accurate way of measuring a stars surface
    temperature.
  • light elements are hot
  • Heavier molecules are cooler
  • O B A F G K M
  • Oh be a fine girl/guy kiss me
  • Hotter stars are the O stars
  • Cooler stars are the M stars
  • Each spectral type can be subdivided 0-9 where
    the larger the number the cooler the star
  • A0 is hotter than a A1 but a B9 is hotter than a
    A0

15
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16
Stellar Masses
  • Most important property of a star is its mass
  • Determines what type of life the star will have
  • Measuring stellar masses
  • Method 1 mathematically
  • Method 2 visual binary stars
  • Method 3 eclipsing binary star
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