Rubin’s Galaxy

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Rubin’s Galaxy

UGC 2885 is the largest spiral galaxy yet seen in our cosmic neighborhood. It’s several times the diameter of our home galaxy, the Milky Way, and it contains several times as many stars. Yet the impact of all those stars is dwarfed by the part of the galaxy we can’t see: dark matter. In fact, it’s one of the galaxies that helped convince astronomers that dark matter exists.

Dark matter produces no energy at all, but its gravity pulls on the visible matter around it. That’s especially easy to notice in spiral galaxies like UGC 2885. Astronomers measure the motions of stars at various locations within the galaxy. They see that something is causing stars in the galaxy’s rim to orbit much faster than expected. That “something” is dark matter. It’s several times as massive as the visible stars and gas.

Vera Rubin and her colleagues paid special attention to UGC 2885 in the 1980s. Careful measurements showed that it’s dominated by a dark-matter halo — strong evidence that dark matter really does exist. So in her honor, UGC 2885 is also known as Rubin’s Galaxy.

This beautiful cosmic pinwheel is about 230 million light-years away, in Perseus. The constellation is in the northeast during the evening. Rubin’s Galaxy is at the edge of the stick-figure outline of the hero, to the upper right of the bright star Capella. But you need a good-sized telescope to pick it out.

Script by Damond Benningfield

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