Milky Way Trivia

The Bad Astronomer has a post up detailing ten things that you probably don't know about the Milky Way. I'm pretty up on my science knowledge, but there were three that were somewhat new to me. (Forgive me -- it's been awhile since I'm spent any significant time reading about astrophysics and astronomy on the galactic level.)

1) It’s a barred spiral.

You might know that the Milky Way is a spiral galaxy, perhaps the most beautiful galaxy type. You’ve seen ‘em: majestic arms sweeping out from a central hub or bulge of glowing stars. That’s us. But a lot of spirals have a weird feature: a rectangular block of stars at the center instead of a sphere, and the arms radiate away from the ends of the block. Astronomers call this block a bar, and, you guessed it: we have one.

Is fact, ours is pretty big. At 27,000 light years end-to-end, it’s beefier than most bars. Of course, space is a rough neighborhood. Who wouldn’t want a huge bar located right downtown?

Yeah, everybody knows that the Milky Way is a spiral, but a barred spiral? That's pretty awesome.

8) Spiral arms are an illusion.

Well, they’re not an illusion per se, but the number of stars in the spiral arms of our galaxy isn’t really very different than the number between the arms! The arms are like cosmic traffic jams, regions where the local density is enhanced. Like a traffic jam on a highway, cars enter and leave the jam, but the jam itself stays. The arms have stars entering and leaving, but the arms themselves persist (that’s why they don’t wind up like twine on a spindle).

Just like on highways, too, there are fender benders. Giant gas clouds can collide in the arms, which makes them collapse and form stars. The vast majority of these stars are faint, low mass, and very long-lived, so they eventually wander out of the arms. But some rare stars are very massive, hot, and bright, and they illuminate the surrounding gas. These stars don’t live very long, and they die (bang!) before they can move out of the arms. Since the gas clouds in the arms light up this way, it makes the spiral arms more obvious.

We see the arms because the light is better there, not because that’s where all the stars are.

Again, interesting. I always thought the spiral arms were in some sense actual real entities held together by gravitational forces and angular momentum, but that's just one of those things that I just thought I knew.

9) It’s seriously warped.

The Milky Way is a flat disk roughly 100,000 light years across and a few thousand light years thick (depending on how you measure it). It has the same proportion as a stack of four DVDs, if that helps.

Have you ever left a DVD out in the Sun? It can warp as it heats up, getting twisted (old vinyl LPs used to be very prone to this). The Milky Way has a similar warp!

The disk is bent, warped, probably due to the gravitational influence of a pair of orbiting satellite galaxies. One side of the disk is bent up, if you will, and the other down. In a sense, it’s like a ripple in the plane of the Milky Way. It’s not hard to spot in other galaxies; grab an image of the Andromeda galaxy and take a look. At first it’s hard to see, but if you cover the inner part you’ll suddenly notice the disk is flared up on the left and down on the right. Andromeda has satellite galaxies too, and they warp its disk just like our satellite galaxies warp ours.

As far as I can tell, the warp doesn’t really affect us at all. It’s just a cool thing you may not know about the Milky Way. Hey, that would make a good blog entry!

Again, I tend to think of the galaxy as a simple rotating platter, so this is another thing that I simply didn't know. Then again, since most of my knowledge of large-scale astronomy comes from books I read when I was a kid in the early nineties, and most of those books were at least five or six years old even at the time, it's possible that this is stuff that wasn't even known to astronomers at the time I was reading about it.

Guess this is just a sign I need to spend more time reading about large-scale astronomy!