Measuring Properties of Stars

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Measuring Properties of Stars
Important Slides marked with a
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Measurable Properties of Stars

• Position
• Apparent Brightness
• Color
• Distance
• many methods
• Velocity
• Doppler Shift, proper motion
• Luminosity
• Distance Apparent Brightness

• Temperature
• Color
• Composition
• Spectral Lines
• Size (radius)
• Stephan-Boltzmann Law
• Mass
• Binary star orbit
• Age

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Measuring Distance

• Parallax
• Angular Diameter Method
• Standard Candle Method

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Measuring Distance - Parallax
Parallax is the change in position of a star,
relative to other stars, based on change in the
observers position.
One parsec (pc) distance of an object with a
parallax of one arc-second 3.26 ly
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Measuring Distance - Parallax
Only good out to 300 ly
Or d (pc)1/p(arcsec)
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Measuring Distance - Angular Diameter Method
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Measuring Distance - Angular Diameter Method

• Angular diameter is easy to measure
• D (distance) L (real size) x A (angular
  diameter, in radians)
• If you somehow know real size, you can calculate
  distance
• Does not work for stars (too small)
• Humans judge long distances with this method

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Measuring Distance - Standard Candle Method

• Apparent Brightness
• How bright something looks from the Earth
• Easily measured
• Measured as Apparent Magnitude
• Luminosity
• How bright something really is
• Units of power (energy/sec emitted, or watts)
• Also called Absolute Magnitude (App Mag at 10pc)

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Measuring Distance - Standard Candle Method

• Distance relates Apparent Brightness and
  Luminosity
• Inverse-square Law shows this
• Apparent Brightness-Luminosity-Distance
• Any two yield the third

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You should review pages 365-366 Explorations,
Third Edition by Arny Class Text
(2) Astronomy Magazine Sept. 2002 issue defines
the faintest naked eye star at 6.5 apparent
magnitude.
Apparent Magnitude was defined by Hipparachus
in 150 BC. He devised a magnitude scale based on
Magnitude Constellation Star 1
(Orion) Betelgeuse 2
Big Dipper various 6
stars just barely seen
However, he underestimated the magnitudes.
Therefore, many very bright stars today have
negative magnitudes.
Magnitude Difference is based on the idea that
the difference between the magnitude of a first
magnitude star to a 6th magnitude star is a
factor of 100. Thus a 1st mag star is 100 times
brighter than a 6th mag star. This represents a
range of 5 so that 2.512 the fifth root of 100.
Thus the table hierarchy is the following.
Absolute Magnitude is defined as how bright a
star would appear if it were of certain apparent
magnitude but only 10 parsecs distance.
Magnitude Difference of 1 is 2.5121, 2 is
2.51221 or 6.311, 3 is 2.5123 15.851 etc.
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Images courtesy of Nick Strobel's Astronomy
Notes. Go to his site at www.astronomynotes.com
for the updated and corrected version.
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Images courtesy of Nick Strobel's Astronomy
Notes. Go to his site at www.astronomynotes.com
for the updated and corrected version.
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Measuring Distance - Standard Candle Method

• Standard Candle
• An object with a luminosity that can be found
  without knowing its distance
• We will cover many types over the semester
• Examples 75W light bulb, second brightest
  galaxy in a cluster, main sequence star spectral
  type, Cepheid Variables

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F sT 4
If luminosity and temperature are known, we can
find radius
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Spectral Types

• Around 1901 Annie Jump Cannon finished Henry
  Drapers work categorizing stars by the strength
  of their Hydrogen lines
• Categories A,B,C,D,,M,N,O,P
• A-gt strong lines, P-gt weak/no lines
• No physical basis for these categories (stamp
  collecting)

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Spectral Types

• Around 1920 M. Saha realized temperature
  determines strength of lines
• All stars made of Hydrogen
• Spectral Types -gt temperature
• Cecilia Payne made the modern system
• Spectral Sequence OBAFGKM
• Hottest to coolest
• Letters still represent strength of H-lines

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Other Properties

• Composition
• Spectral lines
• Velocity
• Doppler-shifts and proper motion
• Mass
• Weigh the star with a form of Keplers 3rd Law

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Summary of Spectral Lines
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Putting it Together - The Herztsprung-Russell
Diagram

• Measure surface temperature from color or
  spectral-type
• Measure distance from parallax (or other method)
• Calculate luminosity from apparent brightness and
  distance
• Plot luminosity vs. surface temperature

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Radius From Stephan-Boltzmann Law
Shows three groups of stars.
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Masses come from binary stars
LuminosityMass3 This was empirical and is now
supported by computer models.
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Herztsprung-Russell Diagram

• Plot Luminosity vs. Temperature
• Showed three classes of stars
• Main Sequence, Red Giants, White Dwarfs
• For MS stars Temperature -gt Luminosity
• MS stars are standard candles
• Binaries show variations between Main Sequence
  stars are due to mass

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Binary Systems

• At least 70 of all stars are binaries or
  multiple-star systems
• Optical Doubles Not actual binaries
• Visual binaries Two stars seen as binaries
• Spectroscopic binaries
• Two sets of spectral lines seen, or
• One set of Doppler-shifting lines
• Eclipsing binaries Stars eclipse each other

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Algol - An Eclipsing Binary

• This blue star turns red every 3 days
• Arabic Algol
• Demons Head
• Chinese Tseih-She
• Piled-up corpses
• Hebrew Rosh he Satan
• Satans Head
• Hebrew Lillith
• Adams first, demonic wife

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