# Two Questions on Movement

1. Dec 8, 2006

### stardrop

01. Can a star be moving faster than what Doppler effect calculations indicate? This was a discussion question I heard but I'm not sure I understand. Can anyone provide any sort of information about this?

02. Can someone explain this to me, too? "Hydrogen red line is shifted, but other observations make the star appear to be at rest". It's described as an anamoly, but I don't get it.

Even just seeing links somewhere on the internet would be helpful if you can provide them. I want to understand the concepts behind these questions. Thanks in advance!

I'm sorry if this is the wrong forum to be putting this thread in. These are more theory questions, so I thought it more appropriate to put them here than in the homework help.

2. Dec 8, 2006

### mathman

The Doppler shift is a measure of velocity along the line of sight. Velocity perpendicular to the line of sight is extremely difficult to measure (for anything far away), but it has to be considered for the total velocity (Pythag. theorem).

Question 2 - it is an anomaly.

3. Dec 9, 2006

### SpaceTiger

Staff Emeritus
mathman is correct that the tangential component's of the star's velocity can't be measured with the spectrum. Because of this, one expects the star to be moving faster than indicated by the Doppler effect.

Sorry, this is hard to address without context. There are stars that display redshifted or blue-shifted lines, despite having negligible motion relative to us. This can happen when the star expels gas that creates emission or absorption lines. In that case, the gas could be moving while the star sat stationary. One can also get a redshift from gravitational effects, but in ordinary stars, this effect isn't measurable. Finally, if the redshift is cosmological, then the star could actually be at rest, but appear to moving due to the expansion of the universe.

4. Dec 9, 2006

### stardrop

Thanks you two so much! Both of these concepts are a lot clearer to me now.