Download Intro to EM spectrum and analyzing starlight

The Electromagnetic Spectrum
When a beam of white light passes through a glass prism, the
light is separated or refracted into a rainbow-colored band called
a spectrum. The numbers on the right side of the spectrum
indicate wavelengths in nanometers (1 nm = 10-9 m).
Electromagnetic Waves
We Can’t Always See Energy
The transparency of a material depends on the wavelength of light.
Earth’s atmosphere is relatively transparent to visible light and radio
waves, which are referred to as “windows” through which we can view
space from a ground based telescope.
WHEN FIRST
HEATED THE
POKER GLOWS
DIMMLY RED
AS THE
TEMPERATURE
RISES, IT BECOMES
BRIGHTER ORANGE
AT HIGHER
TEMPERATURES IT
BECOMES BRIGHTER
AND YELLOW
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These stars have roughly the same temperatures as the bars above.
This is a plot of intensity
versus wavelength for
blackbodies at different
temperatures. At higher
temperatures the most
intense wavelengths are
shorter. Since the
observed color depends
on these emitted
wavelengths, blackbodies
at different temperatures
will appear different colors.
Stellar surfaces emit light that is close to an ideal
blackbody. We can estimate the surface temperature of a
star by examining the intensity of emitted light across a
wide range of wavelengths.
When a chemical is burned, the light produced is made of
only specific wavelengths.
Different chemical elements have their own series of
wavelengths.
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When an electron moves from a lower energy to a higher energy
level a photon is absorbed. When an electron moves from a
higher energy to a lower energy level a photon is emitted. The
energy of the photon, and thus its wavelength, are determined by
the energy difference between the two energy levels.
The Doppler Effect
The radial velocity of a star is its motion along our line of sight either
toward or away from us. Using the spectrum we can measure this for
nearly every object in space!
The Doppler Effect
Recall that a the wavelength of light, and therefore the
wavelength of the photons that light contains, is slightly
shifted when the source is traveling toward or away
from the observer, the Doppler Effect.
C. Doppler Effect
• We experience the Doppler
Effect in sound waves
produced by a moving wave
source
moving toward you pitch sounds higher
moving away from you
- pitch sounds lower
http://www.kettering.edu/~drussell/Demos/doppler/carhorn.wavchange
C. Doppler Effect
Stationary source
Moving source
Supersonic source
waves combine to
lower
higher
same frequency
produce a shock
frequenc
frequenc
in all directions
wave called a sonic
y
y
boom
http://www.kettering.edu/~drussell/Demos/doppler/doppler.html
The Doppler Effect
If the object is moving toward you, the waves are compressed, so their
wavelength is shorter. The lines are shifted to shorter (bluer) wavelengths--this is called a blueshift.
If the object is moving away from you, the waves are stretched out, so their
wavelength is longer. The lines are shifted to longer (redder) wavelengths--this is called a redshift.
Doppler Effect
Speed
• The faster the star(object) is moving the greater the spectral shift
1. Wavelength shift: = new - rest. rest is the wavelength measured if
the object is at rest and new is the wavelength measured for
the moving object.
2. Doppler effect: = rest × Vradial/c, where Vradial is the object's
speed along the line of sight and c is the speed of light.
3. If new > rest, the object is moving away (redshift).
4. If new < rest, the object is approaching (blueshift).