Astronomy: A galactic whirlpool

Whirlpool Galaxy, Messier 51, M51

T he Whirlpool Galaxy (Messier 51) is one of the most popular galaxies in Earth's northern hemisphere night sky. It is located near the end star (Alkaid) of the handle of the Big Dipper.

Although it is comparatively large and bright, it is still too dim to see with the naked eye. M51 can be viewed as a dim, fuzzy smudge through binoculars from a dark light pollution-free site. Small telescopes reveal a dual, fuzzy smudge and medium to large scopes display the fine structure of its graceful spiral arms.

The closer you look at this colorful collection of hundreds of billions of stars, the more wonders you see. The first thing you may notice is that this is not just one galaxy! Rather, it is a pair of galaxies interacting with each other.

One outer spiral arm of M51 is being pulled like an enormous cosmic crack-the-whip toward NGC 5195, the smaller galaxy to the left. This gravitational whiplash is causing NGC 5195's disk to splatter in all directions as shown by the haphazard starry haze around it. This is how galaxies collide, not by direct hits, but rather by disruption of their structures much like a whirlwind scatters autumn leaves.

All this chaotic action makes clouds of gas and dust compress and collapse into star-forming regions. You can see many of them as red and blue blotches scattered along the whirlpool's spiral arms. The red ones are hydrogen clouds (HII regions) that glow red hot from the radiation of young stars spawned within them. These are called emission nebulae. The blue ones are reflection nebulae caused by the light from giant stars reflecting off nearby immense clouds of dust.

The long brownish strings along the spiral arms are dust lanes concentrated by density waves, which, along with gravity, are the architects of galactic structures. The yellow egg yoke-colored centers in both galaxies are filled with mostly older stars with supermassive black holes likely lurking inside.

Look to the left of the whirlpool and you'll see two distant galaxies that are 10 times farther away than M51 is from us. M51 is 23 million light years from Earth and, at that distance, the expansion of space itself is causing the whirlpool to drift away from us.

This image is the result of over three hours of exposure time through six different filters.

The processing time using the astrophotography program called PixInsight took considerably longer (about eight hours of processing work). The wonderful thing about modern digital astro-imaging is one can use powerful processing programs to tease out much more detail compared to old-style emulsion photography.

But you can get carried away by producing an image that exists only in your mind. Rest assured that this image of M51 is the real deal with true colors that reflect the actual starlight emitted by these distant suns.

Gary Zientara is the resident galaxy watcher at Mount Sangre Observatory in Angel Fire.

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