The Measure of Stars

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The Measure of Stars

• Astronomy 112
• Stars Galaxies
• Chapter 13

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The Measure of Stars
Thats the trouble with science. Youve got a
bunch of empiricists trying to describe things of
unimaginable wonder.
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The Measure of Stars
Twinkle, twinkle little star, I dont wonder what
you are, For by the spectroscopic ken I know that
you are hydrogen.
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Happy Valentines Day ?
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Distances!

• Distance measurements are essential!
• Luminosity rate a star emits energy
• A physical property of the star
• Brightness rate we receive energy
• Depends on luminosity distance
• Is it dim near, or bright far?

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Distances!

• Only direct method is parallax
• Change in apparent position caused by change in
  observers position

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Brightness Distance

• From distance brightness, we get luminosity
• Usually expressed in terms of solar luminosities
• Most luminous stars are 106 L?
• Most luminous stars are 10-4 L?

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Stellar Temperature

• We can measure temperatures from Wiens Law

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Stellar Temperature

• We can measure temperatures from Wiens Law
• Hotter is bluer
• Shorter wavelength
• Higher energy

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Stellar Temperature

• Or the color
• Measure brightness at two wavelengths/bands
• Like blue B and visible V
• Ratio of brightness bB/bV is called color (or
  color index)
• Larger ratio is a hotter star
• Most stars are cooler than

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Color
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Color

• Color
• Measure brightness at two wavelengths/bands
• Like blue B and visible V
• Ratio of brightness bB/bV is called color (or
  color index)
• Larger ratio is a hotter star
• Most stars are cooler than
• Or color index
• A ratio in brightness is a difference in
  magnitudes
• B V
• Sun (yellow) B V 0.65
• Rigel (bluish) B V -0.03

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Stellar Size

• With luminosity temperature we can calculate
  size

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Stellar Spectra

• Spectrum the amount of light emitted as a
  function of wavelength.
• Some light leaving the star is absorbed by atoms
  or molecules in the stars atmosphere.
• This makes absorption lines in the spectrum.
• Sometimes one may also see emission lines.

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Stellar Spectra

• Absorption spectra

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Stellar Spectra

• Emission spectra

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Stellar Spectra

• Absorption lines depend (mainly) on temperature
• Hottest stars have weak absorption by hydrogen
  helium
• Type O
• Middle temperatures have strong hydrogen
  absorption
• type A
• Cool stars see absorption by heavy elements
  molecules
• Type M

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Stellar Spectra
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Stellar Composition

• Strength of absorption lines depend (mainly) on
  temperature
• but we can also determine composition
• All stars are mostly hydrogen helium (H He)
• Sun 74.5 H, 23.7 He, rest are heavy elements
• Other stars have 0 to 2 heavy elements
  (metallicity)

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Stellar Mass

• Binary Stars
• At least half the stars in the sky are members of
  a binary system
• Albireo
• b Cyg
• 3rd brightest star in Cygnus
• Head of the swan

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Stellar Mass

• Binary stars

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Stellar Mass

• Binary Stars
• Albireo
• b Cyg
• Parallax 8.46 mas
• Two stars
• Yellow magnitude 3.18 (MV -2.18)
• b Cyg A is itself a close binary
• Blue magnitude 5.82 (MV 0.46)
• Separated by 34
• Period 213.9 years
• Masses 5 3.2
• Stellar classification K3 III B0 V

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Stellar Mass

• Binary stars

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Mass-Luminosity relation

• Massive stars tend to be more luminous and hotter
• Gave rise to early theory
• Massive stars are young with lots of fuel
• As fuel is exhausted, mass luminosity decrease
• O stars are early type
• M stars are late type
• Theory now entirely discredited

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Hertzsprung Russell

• 1905 Ejnar Hertzsprung
• Showed corellation between absolute magnitude
  (luminosity) and (Harvard) spectral type
• Stars of type G later had range of magnitudes
• Termed brighter stars giants

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Hertzsprung Russell

• 1913 Henry Russell
• Independently graphed same data
• Reached same conclusions
• Termed stars giants dwarfs

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Hertzsprung Russell

• 1943 Atlas of Stellar Spectra (MKK Atlas)
• William Morgan Phillip Keenan (Edith Kellman)
• Luminosity classes
• I supergiants
• II bright giants
• III giants
• IV subgiants
• V main sequence (dwarf)
• VI subdwarfs

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Hertzsprung-Russell Diagram
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H-R Diagram
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H-R Diagram

• Luminosity Brightness
• Brightest stars are mostly luminous stars,
  visible from long distances
• Most nearby stars are cool and are not luminous,
  and only visible because they are nearby
• This implies most stars are cool and
  low-luminosity

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H-R Diagram

• Luminosity Mass
• ON THE MAIN SEQUENCE (!!)
• Massive stars are luminous hot
• And have large radii
• Low-mass stars are cool and low-luminosity
• And have small radii

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H-R Diagram
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Stars on the Main Sequence

• Main Sequence stars

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Stars on the Main Sequence
Class Radius Mass Luminosity Temperature
O2 16 158 2,000,000 54,000
O5 14 58 800,000 46,000
B0 5.7 16 16,000 29,000
B5 3.7 5.4 750 15,200
A0 2.3 2.6 63 9,600
A5 1.8 1.9 24 8,700
F0 1.5 1.6 9.0 7,200
F5 1.2 1.35 4.0 6,400
G0 1.05 1.08 1.45 6,000
G2 1.0 1.0 1.0 5,700
G5 0.98 0.95 0.70 5,500
K0 0.89 0.83 0.36 5,150
K5 0.75 0.62 0.18 4,450
M0 0.64 0.47 0.075 3,850
M5 0.36 0.25 0.013 3,200
M8 0.15 0.10 0.0008 2,500
M9.5 0.10 0.08 0.0001 1,900
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Stars on the Main Sequence

• Naos
• ? Pup
• O5 Ia
• 42,400 K
• 59 solar masses
• 20 solar radii
• Blue stars are never very large
• 1400 ly, yet precise parameters are unknown

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Stars on the Main Sequence

• Naos
• ? Pup
• 335 pc (1093 ly) 4
• O5 Ia
• 42,400 K
• 40 solar masses
• Would cast shadows from the distance of Sirius
• About brightness of crescent moon
• Replacing the Sun, Earth would melt into a ball
  of lava
• Explode within 2 million years
• will be brighter than full moon

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Masses on the Main Sequence

• Maximum mass
• 50 100 solar masses

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Masses on the Main Sequence

• Maximum mass
• 50 100 solar masses

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Masses on the Main Sequence

• Maximum mass
• 50 100 solar masses

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Masses on the Main Sequence

• Maximum mass
• 50 100 solar masses
• ? Car
• 100-150 solar masses
• Variable
• 1677 4th magnitude
• 1730 one of Carinas brightest
• 1782 4th magnitude
• 1843 magnitude 0.8
• 2nd-brightest in sky
• But 700-800 ly away

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Masses on the Main Sequence

• Maximum mass
• 50 100 solar masses
• Minimum mass
• 0.08 solar masses
• Gliese 229B
• Brown dwarf

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Light from stars tells us stuff

• LOTS OF STUFF!

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and we get the HR diagram