# Red - blue shifts and stellar rotation

1. Jun 27, 2008

### Libmr2bs

When considering the speed of objects revolving around a galactic boundary and the gravitational confluence at the center of the galaxy, it would appear that all objects would be spiraling toward the center. If an object is spiraling toward that center, then it would appear that light from these objects would experience a red shift as the object would have a relative motion away from any external static frame. What is interesting is that the motion toward the center should cause a red shift from any observable direction except parallel to the axis of rotation. Additionally the observer's field of vision should encompass a larger field of red shift on the near side of the galaxy and the blue shift should encompass a larger field on the far side of the galaxy. The farther away a galaxy the more this shift should be observable.

Does anyone know a reference or simply know if this occurs and how the compensation is made when measuring the expansion of space?

2. Jun 28, 2008

### mathman

This is not true. Stars in general are in an elliptic orbit around the center.

3. Jun 29, 2008

### yuiop

I think Mathman is correct that stars are not generally spiralling inwards in a galaxy. However your post prompts another similar thought. We are situated at somewhere mid way between the centre of the Milky Way and the perimiter. Should there not be a detectable blue shift from stars further out towards the perimeter than us and a corresponding red shift from stars nearer the centre simply due to their location in the gravitational well of the galaxy as a whole? I imagine this effect would be very slight and difficult to detect due to the individual motions of stars in orbit around the galaxy centre.

Does anyone know if the long axis of the individual eliptical orbits around the galaxy are generally parallel or randomly orientated?

4. Jun 30, 2008

### Libmr2bs

Spiraling toward the center was intended to be general. Each object will have an orbit that most probably would be ellipical but the gravititational fields that it would encounter from other near objects in non-parallel orbits would cause a "bumpy" ride.

For an object to be in orbit requires a velocity with a radial orientated vector that retains it in orbit. It's this radial component that I'm interested in and how it affects the shift. I imagined that it would be only slight but it would appear that a red shift would always be larger than a blue shift due to this radial velocity from any reference frame except parallel to a central axis of the galaxy. This appears to be what is happening and I've been unable to find a reference that considers this vector or attempts to measure it.

5. Jun 30, 2008

### Wallace

The red and blue shift in the 21cm HI radio line is routinely used to measure the rotation of our and other galaxies. So yes, this effect is known and utilised. However, the magnitude of this shift in frequency is relatively small, so for distant galaxies the cosmological redshift is much greater.

When taking the spectrum of a distant galaxy to determine its redshift, the light from the whole galaxy is considered. Indeed this does mean that some of that light is a little more redshifted and some a little more blueshifted due to the rotation of the galaxy. The effect of this, when the light is considered together, is to smear out each line in the spectrum a little bit, since the redshift is not all exactly the same. As I say though, this effect is relatively small and this smearing not a huge problem. It doesn't alter the centre of the peak in frequency and hence doesn't affect out measurement of redshift.

6. Jun 30, 2008

### Libmr2bs

Can you supply any references. I ran across some information that basically says that we can't see the center of our galaxy. This would imply that our measurements of other galaxies would be based on the outer regions - those nearest to the reference frame. In face-on spiral galaxies, calculations of the hubble constant appear to be less than those calculated from side view (axis perpendicular to path). Just wondering whether there was a link and I find references to angluar velocities but no radial velocities.

7. Jun 30, 2008

### Wallace

We can't see the centre of our galaxy because we sit inside the dusty disk of our own galaxy. For other galaxies, the centre is typically the brightest region. For a galaxy seen edge on, then indeed we can't get a good look at the centre. However as I said, the extra redshift from one side of the galaxy is guaranteed to balance the extra blueshift from the other side such that the centroid of the spectral line in unchanged. In any case as I said this effect is small compared to the cosmological redshift.

8. Jul 8, 2008

### Libmr2bs

I understand the reason why we can't see the center of our galaxy. But the effect is not limited to our galaxy.

There simply does not appear to be anyone who has access to a large telescope that has scanned across a galaxy to verify experimentally what you say. If there is I haven't found a reference.

Related to my search there is a question that if space is expanding and galaxies are moving apart, how could they collide.

9. Jul 8, 2008

### Wallace

Have a look http://hubblesite.org/gallery/album/galaxy_collection/" for many beautiful pictures of galaxies taken with Hubble. As you will see, the centre is almost always the brightest part, except in cases where a disk galaxy is seen edge on.

Mapping the rotation curves of galaxies (i.e. scanning across a galaxy measuring the relative red and blueshift) has been done routinely for about 70 years or so. A quick search over at ADS yielded something over 80 thousand papers. http://adsabs.harvard.edu/cgi-bin/n...xt_wgt=YES&ttl_sco=YES&txt_sco=YES&version=1" is the search result.

Just for kicks, have a look at http://adsabs.harvard.edu/abs/1914LowOB...2...66S" paper from 1914 that describes mapping the rotation, using spectroscopy (i.e. to look for the doppler shift), of the "Virgo Nebula', now known to be a galaxy. When this measurement was made we didn't even know there were other galaxies than our own.

As for your last question, space isn't expanding in the literal sense of the words. It's perfectly acceptable to think of the expanding universe as galaxies flying apart, just remember that everything is moving away from everything else, not from any central point. Indeed, observations show that galaxy mergers were much more common in the past than today, since the Universe used to be a lot more crowded. In any case, the Universe is not completely smooth, and there are many regions, such as clusters of galaxies, where there is no expansion at all. The local overdensity of gravity has captured the galaxies such that they are now a bound system. Within these clusters, galaxies fly around roughly orbiting the centre of the cluster. Sometimes they collide with other galaxies in the same cluster.

Last edited by a moderator: Apr 23, 2017