When you wish upon a star...

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When you wish upon a star...
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shows what a dweeb you really are ..!
Luminosity and all that

• Luminosity
• Inverse square law
• Magnitudes
• Distance, temperature, composition ..
• H-R diagram

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Getting our bearings
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Luminosity

• Observing apparent brightness.
• Brightness is the amount of energy striking per
  unit area of the human eye or a detector.
• The amount we receive is affected by distance
  according to the inverse square law.

Apparent brightness (energy flux) ?
Luminosity/distance2
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Luminosity and magnitudes

• Apparent brightness.
• Absolute brightness.
• Apparent magnitude
• Absolute magnitude

To compare intrinsic or absolute properties of
stars, use a standard distance of 10 pc.
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Lets make this difficult (actually the ancient
Greeks are to blame)

• Around second century B.C.E., Hipparchus scaled
  naked eye stars into a ranking of 1 to 6 (
  brightest to least bright).
• 1 6 range spans a factor of 100 in apparent
  brightness. ( a 1st magnitude star is 100 X
  brighter than a 6th magnitude star).
• The physiology of the human eye dictates that
  each magnitude change of 1 corresponds to a
  change of 2.5 in apparent brightness.
• Combining both concepts 2.55 ? 100
• A 1 st magnitude star is approximately 100 X
  brighter than a 6 th magnitude star

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But what does it mean?

• Well, lets look at the 10 pc thing

10 pc
Earth
Apparent Mag. gt Absolute Mag.
Apparent Mag. lt Absolute Mag.
Apparent brightness vs. absolute brightness?
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Oh !
Brightness decreases this way !
Graph of apparent magnitudes of some common
things in the sky .
Brightness increases this way !
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Luminosity and magnitude

• We know from
• Apparent brightness ? luminosity/distance2
• And 2.55 ? 100 -gt 1001/5 ? 2.5.
• So for every magnitude change we see with our
  eyes the brightness changes 10X.
• IDEA! We can build a chart to relate luminosity
  to magnitudes

luminosity
magnitude
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Recipe brightness to luminosity

• To determine a stars luminosity
• 1. Determine apparent brightness (use a chart or
  for a new star, measure amount of energy detected
  per unit time).
• 2. Measure the stars distance (parallax method
  for nearby stars).
• 3. Use apparent brightness luminosity/ d2

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Using our recipe for more stuff

• Let m apparent brightness
• Use our recipe luminosity d2 m.
• Star A d 0.707 pc, m 1, Star B d 2.12
  pc, m 1.
• Find luminositys for Star A and Star B

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More luminosity magnitude stuff

• Making things simpler
• Scale luminosities to solar luminosity this way
  we wont have to deal with units
• Let m apparent magnitude, M absolute
  magnitude.
• Throw in the inverse square relationship and some
  math and.

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Tah Dah!!!

• D 10 pc x 10(m M)/5
• We have another formula for distance, D!
• Do we believe it! Lets look at an example.
• (alot like More Precisely ex., page 447)

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Luminosity, temperature, size .

• We know relationship between luminosity and
  magnitude (Table previous slide).
• Using Wiens Law ?(peak emission) ?
  1/temperature
• And Stefans law total energy emitted ?
  temperture4
• Wiens law the hotter the object the bluer is
  its emission.
• Stefans law energy emitted per unit area
  increases as the 4th power of the temperature..
• Luminosity ? radius2 temperture4

Stellar size!
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More tools from what we know

• Knowledge of color/temperature relationship and
  now, luminosity/radius/tem-perature relationship
  combined with emission/absorption spectrum we get
  from certain stars, lets us classify our spectra
  (OBAFGKM) according to temperature.

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H-R Diagram
Sizes, Temperature, Luminosities And
Stellar Lifetime
Star life time 1/(star mass)3
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Features 1. 2. 3. 4.
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H-R Diagram
http//instruct1.cit.cornell.edu/courses/astro101/
java/evolve/evolve.htm

• stellar mass determines lifetime behavior of
  star
• With regards to mass, you may want to note
• size/masses of stars that spend
• all their lives on the main sequence
• some of their lives on the main sequence
• leave main sequence early