|So spiral arms are a rotating
density wave, where
the self-gravity of the disk help
"amplify" the spiral features.
How big of an amplification can we get? In some cases, the amplification can be very strong and we can get barred galaxies.
Approximately 1/3 of spiral galaxies are barred -- bars are common, but not universal.
Numerical simulations of galaxies have shown that, by themselves, rotating disks are wildly unstable, and will spontaneously form bars. Why?
Take a disk where all the stars are rotating on perfectly circular orbits - relative to one another, the stars aren't moving. Tweak it just a little, so that there is a little bit of excess density somewhere in the disk. Nearby stars will begin to fall towards this density excess, making it even more excessive. So stars further away will begin to feel the pull, and they will fall in. And we keep going until everyone is falling towards the excess, and we have a bar.Like this: MPEG movie showing a bare disk going bar unstable.At this point, stars are no longer moving on circular orbits, they are moving on highly elongated orbits along the bar -- we say they are "trapped" in the bar.
Here's an MPEG movie of that same galaxy, this time with a massive dark halo around it.
But for barred galaxies, once stars are trapped in the bar, it is very hard to get them out of the bar. So are bars permanent features of galaxies? Can bars be disrupted and destroyed?
To destroy the bar, we want to move the stars off
orbits. How would we do this? Hint:
think about what happens when a star moves through the center of the
and what it might meet.
Here's an example of bar
in action (from Norman, Sellwood,
So put these three pieces of information together: