Characteristics of Stars

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Characteristics of Stars

• Luminosity vs. Apparent Brightness
• Luminosity-Distance Formula

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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

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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.

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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!

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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.

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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.

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How Bright Are The Stars?

• Measured using two scales
• Apparent magnitude
• Absolute magnitude

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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.

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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

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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

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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

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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.

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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

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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