Questions on the Movement of Stars

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Discussion Overview

The discussion revolves around the movement of stars, specifically addressing questions related to the Doppler effect and anomalies in star motion. Participants explore theoretical implications and observations related to star velocities and redshift phenomena.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants question whether a star can be moving faster than what Doppler effect calculations indicate, noting that the Doppler shift measures velocity only along the line of sight.
  • It is suggested that the tangential components of a star's velocity are difficult to measure, leading to the expectation that stars may be moving faster than indicated by the Doppler effect.
  • Participants discuss an anomaly where the hydrogen red line is shifted, yet other observations suggest the star appears to be at rest, indicating complexities in interpreting redshift data.
  • One explanation offered is that stars can display redshifted or blue-shifted lines due to gas expulsion, which may create emission or absorption lines independent of the star's motion.
  • Another point raised is the potential for gravitational effects to cause redshift, although this is noted as typically unmeasurable in ordinary stars.
  • Cosmological redshift is also mentioned, where a star could appear to be moving due to the universe's expansion while being at rest in its local frame.

Areas of Agreement / Disagreement

Participants express varying levels of understanding and agreement on the complexities of star motion and redshift phenomena. Multiple competing views and interpretations remain, particularly regarding the implications of the Doppler effect and the nature of observed anomalies.

Contextual Notes

Limitations include the dependence on definitions of motion and redshift, as well as the unresolved nature of certain mathematical and observational aspects related to star velocities.

stardrop
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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. :cry:

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. :blushing:
 
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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?
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.
 
stardrop said:
01. Can a star be moving faster than what Doppler effect calculations indicate?

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.


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. :cry:

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

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