Transcript day10spectral_analysi+

Spectral analysis of starlight can tell us about:
composition (by matching spectra).
temperature (compare to blackbody curve).
(line-of-sight) velocity (Doppler shift or “red shift”).
object’s rotation rate (broadening of spectral lines).
pressure of the gas (broadening of spectral lines).
magnetic field (splitting of lines into two lines).
Let’s go over the first three topics:
Spectral analysis of visible light:
The visible spectrum is seen in the familiar
rainbow colors that appear due to refraction
in falling droplets of rain.
The spectrum of light emitted from atoms
has only a small number of wavelengths
of light compared to the so-called
Blackbody Spectrum, which has all the
wavelengths, depending on temperature.
Spectrum of a “blackbody” is continuous.
All objects emit electromagnetic radiation,
called blackbody radiation.
As the temperature increases, the wavelength
of the peak of the emission shifts to shorter
wavelength (which has higher frequency).
At several hundred degrees, objects emit
some visible light, and the emission spreads
across the spectrum toward the blue,
as the temperature increases.
The spectrum of a “blackbody” is continuous,
and at high temperature all the colors will be seen:
Spectral analysis of starlight can tell us about:
composition (by matching spectra).
temperature (compare to blackbody curve).
(line-of-sight) velocity (Doppler shift or “red shift”).
object’s rotation rate (broadening of spectral lines).
pressure of the gas (broadening of spectral lines).
magnetic field (splitting of lines into two lines).
The Solar Spectrum has dark absorption lines that can be
compared to known lines from gases in the laboratory.
This can be used to determine the composition of the Sun.
The Doppler Effect occurs when a source of sound
(or light) is in motion relative to an observer
Doppler Effect for a moving source.
Simulations show this more clearly (link).
If the source moves faster than the speed
of sound, shock waves are created. (link2)
Doppler Shift of spectral lines can be used
to measure a star’s velocity toward or away from us
We use spectrometers to separate the wavelengths.
Look in detail at the red line on the next slide.
Doppler Shift of H-alpha spectral line for hydrogen in a
cloud of hydrogen gas that is receding from us at 320 km/s.
This is called a “red shift” since the line moves more
to the red end of the spectrum (longer wavelength).
Red shift
• Red shift is the movement of a spectral
line toward the red end of the spectrum.
• In other words, the wavelength gets
longer, or the frequency gets lower.
• This is due to a star moving AWAY from
us, and the Doppler effect changes the
frequency of the light we detect.