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Paul Kohlmiller

M81 - from Hubble. Courtesy of STSci and NASA.


The new picture of M81 from Hubble is cause to revisit this great galaxy. It is located in Ursa Major. It is thought that M81 collided with nearby galaxy M82 and also with another galaxy NGC 3077. If the galaxies are will placed in the sky, one of my favorite things to do with my goto telescope at a star party is show someone M81 and then slew to M82 while they are still looking through the scope. In the space of a few instants they see two galaxies. If one appears to be vertical the other will appear horizontal. If it is true that M82 and M81 collided, M82 took the worst of it. (

M81 is known for being photogenic. Tony Hallas’s photo of M81 was on the APOD website in April. Check it out at

The galaxy group that M81 belongs to is called the M81 group (so there’s no excuse for forgetting that). This group of galaxies is one of the closest groups next to the Milky Way’s group which is called the Local Group (lots of imagination in these names, huh?).

Recall that one of the strongest indicators for dark matter is that galaxies like the Milky Way rotate as if they had more mass — rotating closer to the way that a solid disk would with the outer parts not slowing down as expected using Keplerian type orbits. But M81 does rotate more like we would expect a galaxy to rotate if it did not have dark matter. Did the collisions with M82 and NGC 3077 strip off large parts of dark matter leading to the starbursts in those galaxies?

M81 is also known as Bode’s Galaxy. Johann Bode discovered M81 and M82 on New Year’s Eve in 1774. It was Bode who offered the name Uranus for the new planet discovered by William Herschel in 1781. Herschel wanted to name it Georgium Sidus to honor his benefactor, the king of England. Fortunately, Bode’s idea won out.


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