spectrum by dandanhuanghuang

VIEWS: 4 PAGES: 27

									     The Electromagnetic
          Spectrum

 All black bodies when
 heated start to glow,
emitting radiation, some
  we can see, some
        invisible.
         The Visible Spectrum
• White light (from a light
  bulb or from the sun) is
  actually a mixture of many
  different wavelengths.
• When separated, a rainbow
  or a continuous visible
  spectrum is revealed.
• However, there are many
  other wavelengths that
  human sight cannot detect.
This is the electromagnetic
          spectrum.




Short wave, high   Long wave, low
energy             energy
Wavelength
     • Electromagnetic energy
       is wave energy, similar to
       waves in water.
     • The distance between
       two crests is called the
       wavelength ().
     • The number of waves
       per second is the
       frequency (f).
     • All wavelengths of light
       travel at 186,000 mi/sec
       (known as c).
Gases that glow produce
  emission spectra.

                This is what
                you see when
                you point a
                spectroscope
                toward a neon
                light.
             Energy Levels



• Electrons when energized jump to a higher level.
• When they drop down, they emit a specific wavelength of
  light.
• In a spectroscope, this is seen as a line in an emission
  spectrum.
• Hydrogen has the simplest emission spectrum, since it only
  has one electron.
   Emission spectra are like
fingerprints…each element is
           different!

     Helium tube    Argon tube
  A hot radiating body
surrounded by cool gas
     produces an
 absorption spectra.



 HOT

       COOL
This is the situation for the
   Sun and most stars.
The Hydrogen absorption
 spectrum for the Sun.


           It’s a negative image of
           the emission spectra.
    SUMMARY: Three Spectra
• Stars produce continuous spectra, with no gaps
  between the colors.
• Thin gases emit emission spectra.
• When light passes through cool gas, colors are
  removed, forming an absorption spectrum.
The Effect of Temperature
            • Temperature can be
              measured in Fahrenheit,
              Celsius, or Kelvin scales.
            • In the Celsius scale, 0 °C
              is freezing and 100 °C is
              boiling.
            • The Kelvin scale is
              measured from absolute
              zero, the coldest possible
              temperature (– 273 °C).
            • The higher the temperature
              the more energy is emitted
              from a glowing object.
Spectral Graph for the Sun

                    Most of the
                    sun’s energy is
                    in the visible
                    part of the
                    spectrum as
                    well as the
                    infrared.
                    This indicates a
                    temperature of
                    5800 °K.
      Other Stars,
Different Temperatures
           • The hotter the star, the
             more energy it emits into
             space.
           • The peak of its spectrum
             shifts toward the violet
             and even the ultraviolet.
           • Cool stars have peaks in
             the red.
           • Planets (like Earth) peak
             in the IR part of the
             spectrum.
Wien’s Law
     • Max Plank determined
       that black bodies have a
       wavelength of maximum
       energy.
     • It drops off in both
       directions from that point.
     • Wien’s Law states the
       higher the temperature,
       the shorter that maximum
       wavelength will be and
       the more intense its light
       will be.
            Using Wien’s Law…
• If we know the
  temperature of a star, we
  can determine the
  wavelength of its
  maximum intensity.
• Use the formula on the
  right.
• Temperature must be in
  °K and the wavelength
  will be in nanometers
  (one billionth of a meter).

 max = 2,900,000/12000 =
 242 nm (blue-white star)
Picturing a Nanometer…

           • The average width
             of a human hair is 1
             mm or 100,000
             nanometers.
           • A chlorine atom is
             around 0.2 nm
             across, so a
             nanometer is equal
             to 5 Cl atoms.

  0.2nm
Finding Temperature…
          • This red supergiant star
            has a max of 950 nm.
            What is its surface
            temperature?
          • T= 2,900,000 ÷ 950 =
            3053 °K
          • This version of Wien’s
            Law is much more
            practical, since we can
            directly measure
            wavelength in a
            spectroscope.
   The peak of the sun’s
radiation is in the middle of
the visible spectrum, so…
              The Sun is
              yellow. Its peak
              radiation is about
              500nm, so its
              temperature is
              5800 °K.
Introducing the Doppler Effect
             • The train has a higher pitch
               whistle when approaching
               you.
             • The train has a lower pitch
               when moving away from
               you.
             • This Doppler Effect is caused
               by compression or stretching
               of sound waves.
             • The same phenomenon
               occurs with light, only the
               object must be moving very
               fast to detect it.
         Blue Shifts, Red Shifts
• Light waves moving away
  from an observer are
  stretched.
• They shift toward the red
  end of the spectrum.
• Those waves moving
  toward an observer are
  compressed.
• They shift toward the blue
  end of the spectrum.
• They larger the shift, the
  faster that object is
  moving.
Detecting Rotation…
          • If a galaxy is
            rotating, then one
            end should show a
            blue shift and the
            other a red shift.
          • This principle
            applies to stars with
            planets. They show
            a spectral wobble.
           Magnitude
• Stars differ by brightness, which is
  measured by magnitude.
• The lower the magnitude, the brighter
  the star.
• The Sun has a magnitude of -27, by far
  the brightest object in the sky.
• Without telescopes you cannot objects
  with magnitudes over 6.
If a photograph is
taken of the sky,
the stars appear
as dots. The
larger the dot, the
brighter the star
(the lower the
magnitude).
                      1
When you
decrease the
magnitude by 1,
the brightness is
                          2
2.5 stronger.
 Star Names
• Bright stars have traditional
  names, many from Arabic:
  Betelgeuse, Aldebaran, Sirius,
  Arcturus, etc.
• Today, astronomers name stars
  using Greek letters ()
  followed by the constellation
  name with a Latinized ending.
• Therefore, Arcturus is also
  known as Alpha ( Bootis.
EXAMPLE:
                                  Polaris
The brightest star in the
constellation Ursa Minor is
Polaris. In modern astronomy it
is also called Alpha ( Ursa
Minoris.
The second brightest star
Kocab (on the “bowl” itself) is
known as Ursa Minoris
If the Greek alphabet is used
up, then Roman letters are
used.
             Quick Quiz!
• Short wave radiation is produced by objects
  with…?
• What is an emission spectrum?
• What is an absorption spectrum?
• Why do we know that Hydrogen and Helium are
  found on most stars?
• How do we know that the Sun’s surface
  temperature is 5800 °K?
• What is the Doppler Effect? What does it tell us?
• How would we name the fourth brightest star in the
  constellation Orion?

								
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