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?
By the way, the image above is not a photograph, it’s a drawing– there’s no way to get outside the galaxy and take a picture like this looking back. It would be a loooong walk home! Click the picture to embiggen and get more details (which is true for all the pictures in this post).
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.
Comentário postado no artigo:
March 13th, 2008 at 11:46 am
Great post, very user friendly.
I knew all – 8, ok 7 – and a half, of these.
I have to admit, while I understand that the spiral arms are an illusion, I still don’t grasp how they occur. I’m not sure the traffic jam analogy works. I understand that gas and dust can slam into slower moving gas and dust and create stars that illuminate the arms. However, cars speed up again when they exit the traffic jam leaving the slower cars behind and can then run into a second traffic jam. But and I don’t suppose the stars or gas and dust speed up when they leave the arm. So how do we get the second spiral arm (as we move out from the center)? And why are the arms spiraled as opposed to concentric rings?