Astro 18

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Astro 18 Section Week 2

• EM Spectrum

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ElectroMagnetic Radiation

• Energy moves in waves with electrical and
  magnetic components

• All EMR travels at speed of light in a vacuum

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ElectroMagnetic Radiation

• Energy h? where v c/?

• 21cm line (HI) has what frequency energy?

h 6.626068 10-34 m2 kg / s

• 1427583133.3 Hz
• Or 1.4276 GHz
• E 9.459210-25 J

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ElectroMagnetic Radiation
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ElectroMagnetic Radiation
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Wiens Law

• Distribution of radiated energy from blackbody at
  T1 has same shape as distribution at T2 except
  its displaced on the graph
• ?max b/T
• b 2.897768510-3 mK
• T temp in Kelvin

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Wiens Law - Sun

• First need temperature of sun
• Total power radiated 41026 Watts
• Radius 7108 meters
• Then, ?max b/T b 2.897768510-3 mK

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Wiens Law - Sun
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Wiens Law - Student

• Body temp 98F
• Convert to Kelvin
• ?max b/T b 2.897768510-3 mK

• So ?max 10µm which is far-IR

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Spectral Lines

• 2 Types
• Emission
• Absorption

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Emission Lines

• Cloud of gas, warmer than background

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Emission Lines
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Absorption Lines

• Hot source behind cloud of cold gas

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Absorption Lines

• Useful in planetary atmospheres
• The deeper absorption line at 760nm is caused by
  our atmosphere's oxygen molecule. The two
  absorption lines at 720 and 890nm (from methane)
  appear on Saturn and Titan, but the rings do not
  have them

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Doppler Shift

• Classic sound example
• Because the source of the sound is moving
  towards/away from you

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Doppler Shift

• Same thing occurs with light from stars, etc
• In H, the transition from level 2 ? 1 has a rest
  wavelength of 121.6 nm. Suppose you see this line
  at a wavelength of 121.3 nm in star A and 122.9
  nm in star B. Calculate each stars speed and
  state if its moving towards or away from us.

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Red Shift Distance

• In general everything is moving away from us -
  expanding universe
• Red shift can be used to calculate distance to
  objects (Hubbles Law)

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Redshift Distance

• For this galaxy, the measured wavelength of the
  Ca K line was 3962.0 Å, rest wavelength for Ca K
  is 3933.7 Å

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Redshift Distance

• NGC 1357

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Exoplanet Detection

• Orbit of planet causes star to wobble because
  they share a common center of mass
• Need high resolution and lots of time!
• Jupiter ? 13m/s over 12 years
• Earth ? 0.1m/s over 1 year

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Exoplanet Detection

• star's velocity shows a periodic variance of
  1 m/s, suggesting an orbiting mass that is
  creating a gravitational pull on this star
• Use Keplers third law period of planet (equal
  to period of variation in stars spectrum) used
  to get radius

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Exoplanet Detection

• Kepler
• Orbit Eqn
• Mass Eqn

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Exoplanet Detection

• Planet orbiting 51Peg has an orbital period of
  4.23 days, the stars mass is 1.06Msun. What is
  the planets orbital distance?
• And its mass?

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Exoplanet Detection