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4

The short answer is that the redshift of stars that are close enough to put into H-R diagrams (or "color magnitude diagrams", if we're being precise, given that you're talking about using colors) is so small that the effects are minimal. The correction to color you're thinking of (called "K correction") depends on the redshift: the ...

3

The de Broglie relation suggests the "wavelength of a particle" is proportional to the reciprocal of its momentum. Like all wavelengths, the de Broglie wavelength is effectively stretched by a factor $(1+z)$, where $z$ is the redshift. The net effect is to increase the wavelength and hence reduce the momentum when measured in the co-moving rest ...

6

Yes, there is a "retardation of the co-moving velocity" of particles. It is important to take it into account to understand the time history of peculiar velocities of galaxies and for determining the energy distribution of cosmic rays coming from other galaxies. Peculiar velocities, the difference between an object's velocity and the local rest ...

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