a71br
02-22-2009, 03:15 AM
The spectrum from M31 reflects the fact that M31 is moving with respect to the Earth, and the light it emits is therefore Doppler shifted. What does this mean?
http://imagine.gsfc.nasa.gov/YBA/M31-velocity/images/doppler.gif
Johann Christian Doppler
1803-1853
Have you noticed that when an emergency vehicle with its siren blaring passes you that the tone that you hear changes in pitch? This is an example of what is called the Doppler shift, and it is an effect that is associated with any wave phenomena (such as sound waves). It is the same effect that Hubble used to measure the velocities of distant galaxies. This is how it works:
Consider first a case where the fire truck is at rest in the station driveway waiting for the firemen to board the firetruck. The siren is already on, and a listener some distance away to the right will perceive the siren at the same frequency as which it is emitted. Note that another stationary person on the left side of the truck would hear the same tone also.
http://imagine.gsfc.nasa.gov/YBA/M31-velocity/images/doppler_at_rest.gif=<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/Man9ulEYSgk&hl=en&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/Man9ulEYSgk&hl=en&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>
More = http://imagine.gsfc.nasa.gov/YBA/M31-velocity/Doppler-shift-2.html
http://imagine.gsfc.nasa.gov/YBA/M31-velocity/images/doppler.gif
Johann Christian Doppler
1803-1853
Have you noticed that when an emergency vehicle with its siren blaring passes you that the tone that you hear changes in pitch? This is an example of what is called the Doppler shift, and it is an effect that is associated with any wave phenomena (such as sound waves). It is the same effect that Hubble used to measure the velocities of distant galaxies. This is how it works:
Consider first a case where the fire truck is at rest in the station driveway waiting for the firemen to board the firetruck. The siren is already on, and a listener some distance away to the right will perceive the siren at the same frequency as which it is emitted. Note that another stationary person on the left side of the truck would hear the same tone also.
http://imagine.gsfc.nasa.gov/YBA/M31-velocity/images/doppler_at_rest.gif=<object width="425" height="344"><param name="movie" value="http://www.youtube.com/v/Man9ulEYSgk&hl=en&fs=1"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/Man9ulEYSgk&hl=en&fs=1" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="425" height="344"></embed></object>
More = http://imagine.gsfc.nasa.gov/YBA/M31-velocity/Doppler-shift-2.html