Announcements

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Announcements

• Pick up graded homework
• Lecture slides on web site have been updated
• Take the test W, Th, or F before class
• Physics seminar today, 100, SL 121 The Utah
  Science Center, Joe Andrade

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Properties of Stars

• 25 October 2006

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Today

• More properties of stars brightness,
  temperature, size
• A survey of the stars in our neighborhood

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Brightness of Stars

• True brightness (or luminosity) is a stars
  actual rate of energy output, measured (for
  example) in watts. The suns luminosity is about
  4 x 1026 watts.
• Apparent brightness is determined by the
  intensity of starlight striking a detector. It
  is measured (for example) in watts per square
  meter. The suns apparent brightness from
  earths location is about 1400 watts per square
  meter.

True brightness
Formula Apparent brightness
4p(distance)2
5
Brightness of Stars
True brightness
Formula Apparent brightness
4p(distance)2
6
How far away are the stars?

• A clue Compare brightness of our sun to
  brightness of stars . . . Its the difference
  between night and day!

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Magnitude system for brightness

• Smaller numbers imply brighter stars.
• Apparent magnitude is a measure of apparent
  brightness. Antares has mag. 1 Polaris has mag.
  2 naked eye limit is about 6. Sirius has mag.
  1.5.
• Absolute magnitude is a measure of true
  brightness. Its what the apparent magnitude
  would be if the star were 33 light-years away.
  Suns absolute magnitude is about 5.
• The formulas that relate magnitudes to
  brightnesses (in watts or W/m2) are complicated
  and not so important.

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Names of Stars

• Brightest stars have actual names, like Sirius,
  Canopus, Betelgeuse, Rigel, Vega, Polaris.
• Prominent stars in each constellation have
  Greek-letter designations, like Alpha Centauri,
  Epsilon Eridani, Tau Ceti.
• For dimmer stars, must resort to various
  numbering schemes.

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The Nearest Stars
10
The Brightest Stars(as viewed from earth)
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Sirius, Canopus, Orion
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Star colors and temperatures
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Calculating sizes of stars
Luminosity (constant) x (4pR2) x (temperature)4
Suppose two stars (A and B) have the same
temperature, but A is 100 times more luminous
than B. How do their sizes compare? A must have
100 times as much surface area, but this means
its radius (or diameter) is only 10 times greater.
A
B