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Bill S

NGC 4550 A Multi-Spin Galaxy

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I always think that a lot of the pleasure of observing astronomical objects is learning about them as well as seeing them. Sometimes it's reading about them that is the spur to having a look.

In most galaxies almost all the stars are rotating in the same direction. There are some galaxies where this is not the case. NGC 4550 is a lenticular (i.e. lens-shaped) S0 galaxy. Approximately half its stars are rotating one way and half the other.

This paper talks about the discovery of this by Vera Rubin and her co-workers.


Over the years it was suggested that two galaxies rotating in opposite directions merged to give this multi-spin result. It is not believed this is the case. Rather the galaxy originally consisted of stars rotating in just one direction and then a stream of gas rotating the other way merged with it and new stars formed as a separate population in this counter rotating part.

Mike Merrifield discussed this in this YouTube video:


Anyway, this is what it looked like to me:


NGC 4550 is the one in the middle. The bright galaxy up and to the left in this snapshot is NGC 4551, which is an E2 elliptical galaxy.

Best regards



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That is a fascinating object. Thanks for the back-story! It amazes me how so much variety can emerge from a few simple physical principles.


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Hi Bill, I never knew a galaxy could have stars orbiting in two directions. Very much enjoyed the video - very well explained. Mike

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