An ultraviolet view of the spiral galaxy M81 shows young, hot stars in blue and older, cooler stars in yellow. The colors show that most of the galaxy's action takes place in its spiral arms, which wrap around a dense core. Known as a "grand design" spiral galaxy for its perfect form, M81 is about 12 million light-years away. This image is from a small space-based telescope known as GALEX. [NASA/GALEX]
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A perfect spiral galaxy would include a bright, round "bulge" of stars in the middle; fully formed spiral arms wrapping all the way around it; lanes of dark dust swirling through the arms; and bright star clusters sprinkled about like lights on a Christmas tree.
In other words, it would look just like M81, one of the best examples of what's called a "grand design" spiral galaxy. It's about 12 million light-years away, and appears close to the bowl of the Big Dipper. It's a bit smaller and less massive than our home galaxy, the Milky Way.
M81's "bulge" is much larger and brighter than the bulge in the center of the Milky Way. And its central black hole is about 30 times as massive as the Milky Way's.
Its spiral arms are outlined by the galaxy's youngest, hottest, and brightest stars. Over the last billion years or so, at least two bouts of intense starbirth have brightened the arms. They're the result of gravitational interactions between M81 and two companion galaxies. The encounters compress big clouds of gas and dust. The clouds break into small clumps, which collapse under their own gravity to form stars -- stars that help make M81 one of the most beautiful galaxies of all.
Under clear, dark skies, you can spot M81 with binoculars. Find the Big Dipper, which is high in the north at nightfall this month. M81 is to the lower right of the bowl at that hour. It looks like an oval smudge of light that's almost as wide as the Moon.
Script by Damond Benningfield, Copyright 2010
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