PDA

View Full Version : A Galaxy in Time-Lapse



tofu
2004-Mar-30, 08:07 PM
What would a time lapse sequence of a spiral galaxy with, say a million years between each frame, look like?

Here's why I ask. Spiral galaxies have arms. The stars in those arms are in orbit about the common center of mass of the galaxy. I can't think of any configuration of stars and orbits that maintains the structure of the arms.

Imagine you had a computer program that let you position a lot of little dots and define an elliptical path for those dots to travel in. You could arrange the dots so that they form spiral arms and you could play with the elliptical orbits, but my intuition is that once you started the simulation running, the structure of the arms would be lost and you'd end up with just a big jumble of dots.

For that reason, I'd like to know what a time-lapse sequence of a spiral galaxy would look like. I'd like to see how the order is maintained.

There are after all a lot of spiral galaxies around. There must be some way to make it work. Maybe it has something to do with interactions between nearby stars, like the way shepard moons maintain the appearance of Saturn's rings. Or maybe it has something to do with the birth and death of stars. That's probably not it though. Our Sun takes around 250,000 million years to complete one orbit so it's certainly gone around the galactic core several times.

Brady Yoon
2004-Mar-30, 09:37 PM
It has something to do with density waves within the spiral arms, but I don't really know what that means. Help someone?

Spaceman Spiff
2004-Mar-30, 09:53 PM
What would a time lapse sequence of a spiral galaxy with, say a million years between each frame, look like?

Here's why I ask. Spiral galaxies have arms. The stars in those arms are in orbit about the common center of mass of the galaxy. I can't think of any configuration of stars and orbits that maintains the structure of the arms.

Imagine you had a computer program that let you position a lot of little dots and define an elliptical path for those dots to travel in. You could arrange the dots so that they form spiral arms and you could play with the elliptical orbits, but my intuition is that once you started the simulation running, the structure of the arms would be lost and you'd end up with just a big jumble of dots.

For that reason, I'd like to know what a time-lapse sequence of a spiral galaxy would look like. I'd like to see how the order is maintained.

There are after all a lot of spiral galaxies around. There must be some way to make it work. Maybe it has something to do with interactions between nearby stars, like the way shepard moons maintain the appearance of Saturn's rings. Or maybe it has something to do with the birth and death of stars. That's probably not it though. Our Sun takes around 250,000 million years to complete one orbit so it's certainly gone around the galactic core several times.

A little correction:
The orbital period of the Sun is 250 million years, give or take.

A little explanation:
The spiral arms of spiral galaxies are a pattern in motion, a density wave. The stars and to some extent the gas clouds pass through the arms. The orbital period of stars as a function of galactocentric distance is different from the period of the pattern (yes, the pattern turns too). The arms represent a localized enhancement in the density of stars and gas clouds. They are not "physical arms" that can become wound-up upon rotation.

Whether the density wave pattern is driven by "Self -Sustaining Star Formation" or a non-axisymetric gravitational field (like that generated by an aspherical bulge or bar, the so-called "density wave theory"), spontaneously from swing amplification of "particle noise" (http://www.ifa.hawaii.edu/faculty/barnes/ast626_95/spirals.html), or some other/additional mechanism is still debated. There are good examples supporting each of the 3 ideas.

Do a google search on "spiral density wave". Here is an interesting site (http://www.kingsu.ab.ca/~brian/astro/course/lectures/winter/chp7.htm)with an animation.

JohnOwens
2004-Mar-31, 11:11 AM
Something I've been wondering about lately that's related enough I'm not going to start my own thread for it: How do we know our orbit within the galaxy? I would assume we could determine a radial velocity by doppler of the stars near/at the core.
But how do we know our tangential or angular speed? It seems that it isn't done by using the orbital period formula with a best estimate of the galaxy's mass, because the best estimates for mass that I've seen don't agree with the best estimates of the period that I've seen (1.4*10^41 kg and 226,000,000 years; if anyone knows fresher, more up-to-date estimates, I'd love a link! :D ). This could be accounted for by the radial velocity, if we're in a rather eccentric orbit, but I doubt that's the case. It could also be accounted for by the dark matter, as I understand, but I thought that's what we were led to believe might exist because the mass didn't match up, so it can't be the explanation.
Or do we measure the apparent motion of the galactic core against the background, to give us an angular speed? Or something else I'm not considering right now? (I think I had a couple of other possible ideas that aren't coming to mind right now.)
What exactly do we measure, how do we measure it, and how accurately is it known?

tofu
2004-Mar-31, 03:43 PM
The orbital period of the Sun is 250 million years, give or take.
Oops!

Thanks for the explanation and the link. It was very interesting reading. I learn a lot from this BB