Asked by Claire Armstrong
Most of the galaxies in the Universe are moving away from us and as a result, the light that they emit is shifted to the red end of the spectrum due to an increase in wavelength as the Universe expands. When a galaxy moves towards us, for example in the case of the Andromeda Galaxy (M31), the wavelength of the light emitted is shortened thanks to the Doppler Effect (the same effect that causes the siren of an ambulance to rise and fall in pitch as it drives by), shifting the light to the blue end of the spectrum. The galaxy is then said to be blue-shifted.
Andromeda is not the only galaxy to be moving towards us. With the help of galaxy surveys, astronomers have found that around 100 galaxies are moving towards us. Compared to the numbers of galaxies that we know of (hundreds of billions), blue-shifted galaxies are seemingly quite rare. Those that are moving towards us are either part of our Local Group, which means that we are gravitationally connected to each other, or they are found in the Virgo Cluster which everything in our Local Group is moving towards. The galaxies M90, M86 and M98 are all in the Virgo Cluster and all show blue shifts.
Answered by All About Space contributor Gemma Lavender
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