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Q & A: Why is there a blue shift of Andromeda?

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Most recent answer: 03/12/2013
Q:
I've been searching online and trying to find an answer to my question of expanding universe. Since we know the universe is expanding because of red shifts in galaxies being observed how could we have the andromeda galaxy have a blue shift? That one galaxy is going the opposite direction of other galaxies! This question came up after I just saw an example they used in this video to explain redshift which makes sense to me. http://youtu.be/8U4O8XB14tg
- Joseph (age 35)
Ft Lauderdale, FL, USA
A:
Good question.   Although a random galaxy very, very, very  far away from own Milky Way will have a red shift,  Andromeda is a fellow member of our local galaxy group.  Andromeda and the Milky Way are currently on a collision trajectory and they will eventually merge in a few billion years.  Since Andromeda is approaching the earth at a high velocity it will have a slight blue shift that is greater than that of the universe expansion rate. 

LeeH

(published on 03/12/2013)

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