Measurement of Galaxy Rotation: How Long Does it Take?

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Discussion Overview

The discussion revolves around the measurement of galaxy rotation, specifically how the rotation of galaxy arms is quantified over time. Participants explore the methods used for measurement and the implications of observed velocities of stars within these arms.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant questions how the rotation of galaxy arms can be measured given that it takes many years to observe changes.
  • Another participant suggests that spectroscopy is used to measure the redshift of stars in the galaxy's arms.
  • Concerns are raised about the definition of "fast" in relation to the velocities of stars, particularly in comparison to the Sun's orbital speed.
  • It is noted that while individual stars have varying velocities, most stars in the arms generally share similar velocities, which can be measured through spectroscopy.
  • One participant mentions that stars in the arms are moving faster than predictions made by general relativity, leading to the hypothesis of dark matter as an explanation for this discrepancy.

Areas of Agreement / Disagreement

Participants express differing views on the measurement of galaxy rotation and the implications of star velocities, indicating that multiple competing perspectives exist without a clear consensus.

Contextual Notes

Definitions of speed and the assumptions regarding the velocities of stars in relation to general relativity are not fully resolved, leaving some aspects of the discussion open to interpretation.

jayaramas
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it takes many years to see the change of arms of a galaxy. so how was the rotation of arms measured?
 
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Spectroscopy. You check the red shift of stars in the various arms of a galaxy.
 
but it is said that stars in the arms are moving at fast rate. the arms are nothing but collection of stars. so, how can arms have a different speed and if so, how can it be measured?
 
jayaramas said:
but it is said that stars in the arms are moving at fast rate.

Define fast. Compared to what? The Sun orbits the galaxy at about 251 km/s.

the arms are nothing but collection of stars. so, how can arms have a different speed and if so, how can it be measured?

While each individual star will have it's own specific velocity at any point in time, in general most stars will have roughly the same velocity as they move around the galaxy. This is measurable through spectroscopy like Chronos said.
 
jayaramas said:
but it is said that stars in the arms are moving at fast rate.
They are indeed going faster than general relativity predicts, hence the assumption that there is hidden (dark) matter.
 

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