Quote:
Originally Posted by multiweb
I don't get that one. Are you saying:
1_ the solar system orbit around the galaxy core is approx 30-35million years. (I didn't realise the galaxy was spinning that fast)
2_ we are passing through spiral arms? I thought all the stars in a spiral arm travel uniformely in the same direction? I mean the arm is a bunch of stars right, so we are a part of the arm?
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No...we pass through the spiral arms once every 30-35 million years. As the Sun orbits about the Galaxy (every 250Ma), it bobs up and down with respect to the plane of the Galaxy....roughly +/- 250ly above and below. Right at present, we're about 1-2 million years out and 25ly above the last encounter with a spiral arm...which was in the direction of Centaurus. We're not out of the woods completely, yet. At present we're heading towards what's known as the Aquila-Cygnus Rift, which is a dark, nebula filled region loaded with star formation regions and all sorts of nasty stuff. We won't get there for about 1-1.5 million years. Fortunately, we'll miss the core of the region...just.
The arms themselves travel more slowly than the stars which form in them. If the stars live long enough, they leave the arms to pursue their own orbits about the Galaxy. Most of the stars within a galaxy orbit about it on their own orbits and whilst most orbit roughly in the same direction, their orbits are not all nicely spaced or heading in specifically the same direction. Some orbit the galaxy at high angles, others have almost flat orbits. Some have highly elliptical orbits and others have more or less circular orbits.