Saturday, June 17, 2006

A Dearth Of Stars

There are certain times of the year when it seems that the sky is just uninteresting. There's a lack of bright stars, easy to find deep sky objects and the constellations themselves just seem faint. Here in the northern hemisphere, this is spring and autumn, obviously reversed for the southern hemisphere.
Yet if we live in a galaxy that has between 100 and 400 billion stars, then why such a dearth? Well, it has to do with the fact that our Solar System is tilted.
The picture below shows the entire night sky, all 360° of it. You'll notice that most of the brighter stars seem to follow a serpentine pattern. That is the actual galactic plane, what we know as the Milky Way itself.

(image produced with Star Atlas v.06b1 by Youhei Morita)

Our Milky Way galaxy is a barred spiral, essentially a flat, spinning disk with a slight bulge near the center and over 100,000 light years across. Our little Solar System is located some 30,000 light years from the center, as indicated by the circle near the top center of the Milky Way map (which, I admit, is based on the best information we have available and is probably still far from accurate. Incidentally, our Solar System would be near the center of the circle, microscopic in this scale. Very microscopic...).

(image by Robert Little)

If our Solar System was aligned so that our Sun's axis lined up with the galactic axis, we'd never want for Milky Way filled nights, and only the view towards our poles would show fewer stars than around the ecliptic (the main plane of the Solar System, the imaginary belt in which most of the planets lie). Instead, our Solar System is actually tilted 62° from the galactic plane itself, almost lying on its side.
(image by Robert Little)

As a result, there are times when our night sky actually faces out through our galactic plane, through less densely populated sections of our local interstellar neighborhood. This is why the stars in the spring and autumn seem to be lacking. That's not to say that it isn't worth still doing; stargazing should be enjoyed all year round. Still, if your wondering why the night sky is so humdrum, just remember we're tilted. It's all in how we're aligned.

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