Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: figuring out stars' red shifts

Learn more physics!

Most recent answer: 07/28/2012
Q:
Could you give me a sense of how light from the spectroscopic analysis of a star is analyzed given that there the color "fingerprints" are from potentially many different elements and the light has been red or blue shifted. Computers have to do the job? The difficulty it seems to me is that you would not know how fast the star was moving and thus how much of a shift has occurred, and thereby the colors we associate with helium , for example, would not look like Helium. So , again, how do astronomers figure this out?
- shaun kelly (age 46)
plymouth,ma,usa
A:
It's actually not very hard. There are some common elements in stars, starting with hydrogen and helium, each with a particular pattern of characteristic frequencies. You agree that these patterns should be easy to spot if un-shifted. Now the beauty of the red shifts or blue shifts is that they have a very simple mathematical form: they multiply every one of the frequencies by some single number. If someone gave you a familiar detailed picture, but stretched or compressed horizontally, I bet you could easily figure out the stretch/compression factor.

One way to see this for the spectra is to plot all the spectral lines on a logarithmic scale. Then all the pictures look alike, just shifted left or right depending on the red or blue shift.

Mike W.

(published on 07/28/2012)

Follow-up on this answer.