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Brady Yoon
2004-Jun-21, 06:18 PM
I have a couple questions about the Doppler effect.

Firstly, when astronomers say a galaxy has a redshift or blueshift, how do they find the original characteristics of it? I think I understand how it works with stars. My idea is that they use radiation laws to determine what its spectra should be, then see if the observed spectra deviates from the theoretical. But how does it work with galaxies, which have not only stars, but interstellar dust and nebulae?

The second question is, when an object is blueshifted, is only the visual aspect of radiation changed? In other words, if light is blueshifted toward gamma rays, can they kill you?

Thanks for your time. :)
Brady Yoon.

Kaptain K
2004-Jun-21, 06:26 PM
I have a couple questions about the Doppler effect.

Firstly, when astronomers say a galaxy has a redshift or blueshift, how do they find the original characteristics of it? I think I understand how it works with stars. My idea is that they use radiation laws to determine what its spectra should be, then see if the observed spectra deviates from the theoretical. But how does it work with galaxies, which have not only stars, but interstellar dust and nebulae?
It doesn't matter what is emiting (or absorbing) light, each element has its own unique spectrum. You obtain a spectrum and compare it to a known spectrum, sliding it up and down in frequency until you get a match. That gives you the amount of red (or blue) shift.


The second question is, when an object is blueshifted, is only the visual aspect of radiation changed? In other words, if light is blueshifted toward gamma rays, can they kill you?
Yes, light that is blue-shifted to gamma rays are gamma rays and in sufficient quantities can kill!

Brady Yoon
2004-Jun-21, 06:56 PM
Thanks. So it's only comparing the observed frequencies for an element to the theoretical frequencies and finding the difference (if any)?

Kaptain K
2004-Jun-21, 07:02 PM
Actually, you take the whole spectrum. Multiple elements gives redundancy and verifies the unique match => the unique doppler shift.

milli360
2004-Jun-21, 07:15 PM
As Kaptain K says. The shift can shift a line for one element to where it should appear for another element--the only way to recognize the difference is to notice that a whole pattern has shifted.

tofu
2004-Jun-21, 07:23 PM
Thanks. So it's only comparing the observed frequencies for an element to the theoretical frequencies and finding the difference (if any)?

Except for the part about theoretical. I don't think there's anything theoretical about it. You can burn H2 in a lab and observe the spectrum. You can then compare that to the spectrum you see in a galaxy. There might be lots of other elements mixed in there, but the hydrogen pattern will stand out. You can't miss it. The pattern will be there but it will be shifted up or down.

Here is a picture showing what a few elements' spectrums look like:
http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/text_images/CH07/FG07_14.JPG

the patterns are easily recognized and it's pretty easy to see if they are shifted up or down. So see, there isn't really anything theoretical about this. :)

Here's a neat article for you to read:
http://cfa-www.harvard.edu/seuforum/galSpeed/

George
2004-Jun-21, 08:55 PM
You might also like this one.... here (http://skyserver.sdss.org/dr1/en/proj/advanced/hubble/redshifts.asp)

Hale_Bopp
2004-Jun-21, 11:16 PM
Nice to see you like the SDSS web site, George. I did a lot of work on the educational components of the site.

You can find a neat little applet that illustrates finding the redshift to a galaxy at

SDSS Red Shift applet (http://skyserver.sdss.org/dr1/en/proj/basic/universe/redfinder/xcor.htm)

This is one of the exercises for student to see how redshifts can be calculated without getting into lots of nasty math.

Rob

nakayama
2007-Jan-16, 01:24 AM
In outer space, an observer observes a star. When observer jets out gas and moves(in different uniform motions) along the light path, frequency of star light changes. But if light speed is constant(to observer), why frequency changes ? Speed of light is speed of light wave.
* Change of wave length(it may be inevitable outcome from the theory of constancy of light speed) of the light path(from observer to the star) is unimaginable.

http://www.geocities.co.jp/Technopolis/2561/eng.html

P.S. Sorry, I can't receive E-mail. I don't have PC.

Ken G
2007-Jan-16, 01:49 AM
And just to clarify, redshifts of distant galaxies are not generally pictured in scientific circles as being due to Doppler shifts. There are many mechanisms that can redshift light, and the "Doppler shift" generally applies to relative motion within the context of special relativity (where relative motion between distant objects is well defined). In cosmology, it's general relativity, and the coordinates are viewed as arbitrary, and so is relative velocity of distantly separated objects. What is not arbitrary is the change in the curvature of spacetime. Measured in the units of an inverse length, that curvature is decreasing as the universe stretches, and that is actually what causes the redshift. A more physical picture requires choosing a particular coordinate system to describe space and time independently of each other, but is then not unique, as it requires identifying a particular set of observers.

ozark1
2007-Jan-16, 04:22 PM
Except for the part about theoretical. I don't think there's anything theoretical about it. You can burn H2 in a lab and observe the spectrum. You can then compare that to the spectrum you see in a galaxy. There might be lots of other elements mixed in there, but the hydrogen pattern will stand out. You can't miss it. The pattern will be there but it will be shifted up or down.

Here is a picture showing what a few elements' spectrums look like:
http://cwx.prenhall.com/bookbind/pubbooks/hillchem3/medialib/media_portfolio/text_images/CH07/FG07_14.JPG

the patterns are easily recognized and it's pretty easy to see if they are shifted up or down. So see, there isn't really anything theoretical about this. :)

Here's a neat article for you to read:
http://cfa-www.harvard.edu/seuforum/galSpeed/

Except of course Fraunhofer lines are absorption lines (black) on continuous spectra. I don't think it's trivial to unambiguously identify lines as the absorbing gas might not be associated with the object, or be moving at a different relative speed to the emitter. The classic example is a spectroscopic binary when each line is split as one star approaches earth and the other recedes.

Ken G
2007-Jan-16, 04:36 PM
I don't think it's trivial to unambiguously identify lines as the absorbing gas might not be associated with the object, or be moving at a different relative speed to the emitter.

I don't understand-- the absorbing lines are the object. It's up to you to figure out what object you are talking about, but something is being Doppler shifted in this situation.

StupendousMan
2007-Jan-16, 06:13 PM
Except of course Fraunhofer lines are absorption lines (black) on continuous spectra. I don't think it's trivial to unambiguously identify lines as the absorbing gas might not be associated with the object, or be moving at a different relative speed to the emitter. The classic example is a spectroscopic binary when each line is split as one star approaches earth and the other recedes.

Ozark1 brings up a good point. In _some_ situations, as when astronomers are looking at very faint little galaxies at very large redshifts, there are times when a spectrum consists of one or two or three features. In those cases, one may not be able to figure out with confidence just what materials are producing the features, and by how much they are being shifted.

Most of the time, however, spectra show tens or hundreds of lines, and so one can be pretty darn confident that a solution which yields, say, 87 matched wavelengths out of 90 features is probably correct.

DaveC426913
2007-Jan-17, 06:07 AM
In outer space, an observer observes a star. When observer jets out gas and moves(in different uniform motions) along the light path, frequency of star light changes. But if light speed is constant(to observer), why frequency changes ? Speed of light is speed of light wave.
* Change of wave length(it may be inevitable outcome from the theory of constancy of light speed) of the light path(from observer to the star) is unimaginable.

http://www.geocities.co.jp/Technopolis/2561/eng.html

P.S. Sorry, I can't receive E-mail. I don't have PC.

The speed of light is not changing. But when emitted from an object moving towards you, the wavelengths pile up -exactly the same way as sound waves pile up in front of a fast-moving train.

See here: look a the right picture. All waves are travelling at the same speed, yet the frequencies are not the same.
http://www.drphysics.com/syllabus/doppler/doppler.GIF