What is the recession speed of a galaxy based on the wavelengths of two stars?

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Discussion Overview

The discussion revolves around determining the recession speed of a galaxy based on the wavelengths of two stars, utilizing concepts such as Doppler shift and redshift. Participants explore the implications of star measurements for estimating the galaxy's recession speed and distance.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant calculates the speeds of two stars using Doppler shift, yielding values of 4011 km/s and 4999 km/s.
  • Another participant suggests that the recession speed of the galaxy will be close to the speeds of the stars but requires more measurements for accuracy.
  • A participant notes that the spatial arrangement of the stars within the galaxy could affect the estimation of the recession speed, advocating for the use of mean speeds if the stars are positioned on opposite sides.
  • One participant proposes calculating redshift and multiplying by the speed of light to estimate recession speed, arriving at a value of around 900 km/s.
  • Another participant questions the discrepancy between the 900 km/s estimate and the previously calculated star speeds, suggesting averaging the redshifts instead.
  • A later reply indicates that using the mean speed of the stars, given their positions, would yield a recession speed of 4505 km/s and a distance of 60.1 Mpc.
  • Participants express curiosity about the published distance of the galaxy and whether the calculated distance aligns with existing data.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the best method to determine the galaxy's recession speed, with multiple approaches and estimates presented. The discussion remains unresolved regarding the most accurate calculation method and the implications of the star positions.

Contextual Notes

Participants acknowledge the need for additional information about the galaxy's structure and the positions of the stars to refine their estimates. There are also unresolved questions regarding the accuracy of the redshift calculations and the relationship between the different speed estimates.

NebulaBilly
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I have been having a conversation with a few friends, and we have been discussing the speed of stars in a galaxy base of the wave lengths. We have 2 stars measuring wave lengths 494.6nm and 494.2nm. At rest the wavelength is 486.1nm. From this we managed to deduce that the speed of both stars using Doppler shift is 4011 km/s and 4999 km/s. We then wanted to take this further and calculate the distance of this galaxy but to do this we need to know the recession speed of the galaxy, and we are just not sure how to get that answer, hopefully somebody here can shed abit of light onto it.
 
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The recession speed of the galaxy will be somewhat close to the speed of these stars. A better estimate would need more measurements of stars at different points of the galaxy.
 
If you know where those 2 stars lie in the galaxy, and you know that the galaxy is a disk, that could help you make a better estimate. (If it's not a disk, it's not clear that anything other than the average is a reasonable estimate, without a lot more information about it.)

For example, if they lie on opposite sides of its center, by equal apparent distances, then their mean is probably as good an estimate as you can hope for. Conversely, if one of them is much closer to the center, then that star's speed alone may be a better estimate than their average.
 
Hi thanks for your reply, i thought maybe a good way would be to calculate the redshift, then x it by speed of light, this gave me around 900km/s
 
NebulaBilly said:
Hi thanks for your reply, i thought maybe a good way would be to calculate the redshift, then x it by speed of light
Yes, that is what you presumably did in your OP. But you asked about the galaxy's recession speed (and then its distance), so my answer was to help you get the galaxy's speed from the stars' speeds. (You could just as well average the redshifts themselves by the procedure I described, and multiply afterward by the speed of light -- the distributivity property of multiplication. ;-)

this gave me around 900km/s
Why 900 here vs. 4011 & 4999 in your OP?
 
Hi JMz yeah 900 but i think its because we did it incorrectly. If i do the mean like you suggest as the stars are opposite side of galaxy so mean would work, the recession speed would be 4,505km/s and then we would get a distance of 60.1 MPC
 
Souds good. I imagine this galaxy already has a published distance somewhere. Did you check if 60 Mpc is about right?
 
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All looks good atleast can see how they got there now, sometimes the simplest way seems the most difficult.
 

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