Summer Milky Way
22 Jul 2012
This week we cover another basic concept in astronomy, the Milky Way. And by the time we are finished you will either want to go outside and observe for yourself, or fire off an email. Or both. Here we go!
Since humans have walked the Earth they have observed a distinct band of light on the starry canopy overhead, one that can be seen all year long.
For the longest time no one knew what it was but it did make its way into countless myths. How could something that majestic and awe-inspiring not be included in the great stories?
We of course call it the Milky Way, derived from the Latin "via lactea," given us by the Romans. But what is it really?
Well, believe it or not, we didn't have a full grasp of the true nature of the band of light until these last centuries.
The great telescopes of the last couple hundred years first helped us here by seeing that the Milky Way consisted of literally countless stars which were just like the thousands of individual stars we see every night.
The fact that it was a band of light all around us in every direction implied at least that it was a huge disk of stars, not a random scattering. But were we in the middle of this disk or off to a side somewhere?
It turns out that some very clever men at the beginning of the 20th century discovered that we are actually not in the center; we are off to one side. The center of the starry disk is some 27,000 light years away, over near the constellation Sagittarius. But even though the center of the galaxy is far more populated with stars, it doesn't look that much brighter over there. Why? Because there is so much interstellar dust from eons of supernova explosions that it blocks much the light of those stars.
It was also discovered through some special laws of physics and the way light behaves that the disk was swirling around this center like water around a drain, and that the swirling disk had arms.
The glorious band of light we call the Milky Way is actually a spiral galaxy, seen edge-on, teeming with hundreds of billions of stars. We see it best during summer because our night skies are aiming towards the star-rich inner sanctum. During winter our night skies are directed towards the scanty outskirts.
Go out and take a look tonight after the Moon sets. Let your eyes adapt for 10-15 minutes then, starting in the southern skies where it is brightest, follow the great white way overhead into the northern skies. It is a stunning, awesome sight.
What's that? Your skies are too light polluted to see it? Try and get out to the desert this summer. In the meantime, you can email International Dark-Sky Association (darksky.org) and ask what you can do to reduce light pollution in your area before we tragically lose the Milky Way for this next generation.