# Astronomy - help!

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Imagine that you have obtained spectra for several galaxies
and have measured the observed wavelength of the H emission line of atomic hydrogen. The rest wavelength of H is 656.3 nanometers
(nm). Your results are as follows:
• Galaxy 1: Observed H wavelength is 664.7 nm
• Galaxy 2: Observed H wavelength is 659.6 nm
• Galaxy 3: Observed H wavelength is 679.2 nm
Put these three galaxies into a list sorted by distance away from the Earth, with the closest
galaxy first in the list and the farthest galaxy last in the list. Explain your reasoning.

• Astronomy - help! - ,

The galaxy that is shifted most to the red (#3) is the one that is moving fastest away from Earth. That is a direct result of the Doppler shift relationship.

Because of the observed "Hubble constant" relationship that results from the "Big Bang" expansion of the universe, galaxies that are moving fastest away from the Earth are also the farthest away. The average velocity of galaxies away fropm us is approximately proportional to their distance.

More recent findings indicate that the "Hubble Constant" is not quite true. The expansion of the universe appears to be accelerating.