Space question for Eric (or someone)
Discussion
I was thinking about something while driving home from work tonight - specifically galaxies.
As I understand it, things which orbit other things bodies move faster the nearer they are to the thing they're orbiting. In the case of our solar system Mercury goes very fast (because it's near the Sun) and Neptune and Pluto trundle along very slowly in comparison (because they're a long way from the Sun). The same goes for satellite orbits round the Earth, the close ones take a few hours while, say, the Moon takes a month.
Anyway, my question: If galaxies rotate round the centre then surely the stars near the middle of the galaxy would be going faster than ones further away from the centre. why is it then that many galaxies (like the milky Way) have spiralling/structures arms? Surely there should be no structure to a galaxy if the stars in the middle are going faster than those further out? Or am I missing something (which clearly I am)!!!
As I understand it, things which orbit other things bodies move faster the nearer they are to the thing they're orbiting. In the case of our solar system Mercury goes very fast (because it's near the Sun) and Neptune and Pluto trundle along very slowly in comparison (because they're a long way from the Sun). The same goes for satellite orbits round the Earth, the close ones take a few hours while, say, the Moon takes a month.
Anyway, my question: If galaxies rotate round the centre then surely the stars near the middle of the galaxy would be going faster than ones further away from the centre. why is it then that many galaxies (like the milky Way) have spiralling/structures arms? Surely there should be no structure to a galaxy if the stars in the middle are going faster than those further out? Or am I missing something (which clearly I am)!!!
Kepler's laws - why things travel faster when they are closer...
Don't forget that there are forces in the arms keeping them together - such as gravity between the various solar systems... Having said that, the arms are constantly changing shape...
Don't forget that there are forces in the arms keeping them together - such as gravity between the various solar systems... Having said that, the arms are constantly changing shape...
Puggit said:
Don't forget that there are forces in the arms keeping them together - such as gravity between the various solar systems... Having said that, the arms are constantly changing shape...
and not everything in the galaxy is the same size or has the same gravitational pull, so stuff clumps together at some points, and spreads out at others.Srebbe64 is right. After a few rotations, the spiral structure ought to disappear. Yet we see lots of spiral galaxies...one explanation is that there is a compression wave also rotating with the matter, and as the gaseous material enters this compression wave it triggers star formation - which if so would explain why we see emission nebulae (the birthplace of stars) and young blue giant stars in the spiral arms. The older stars, red giants, are mostly found in the spherical halo and core regions. So in this picture the arms aren't permanent features, they're continually forming and re-forming as the pressure wave hits different parts of the rotating galaxy's constituent material, or to put it another way, as the material enters the compression regions.
Puggit said:
Here you go: No one knows
Glad I got IBW (in before wiki) 
The compression wave I mentioned in my post is the same as the density wave idea as labelled by wiki. It looks to be the better altternative.
The supernova shockwave model is thought to produce a particular variety of spiral galaxy with characteristic morphology.
grumbledoak said:
Quite the opposite, surely? The stars at the edge must travel far further than those closer in. So, if the 'arms' are at all stable those stars must be travelling far faster?
I think.
Only if they all take the same amount of time to circle the Sun, which they don't...I think.
Puggit said:
Only if they all take the same amount of time to circle the Sun, which they don't...
Yes, but the planets don't form 'arms', they all orbit as Kepler (?) described. Yet there are a lot of spiral galaxies- the arms must be (at least a little) stable, so moving faster at the edges. Or, as mentioned, constantly forming and vanishing.grumbledoak said:
Quite the opposite, surely? The stars at the edge must travel far further than those closer in. So, if the 'arms' are at all stable those stars must be travelling far faster?
I think.
It's possible to get rotational velocities (speeds) from Doppler imaging of galaxies, the arms aren't like stiff spokes poking out from the middle with everything along the spoke (even a pretty curvy spoke) travelling round with the same rotational period. As the pretty pic of NGC 1365 shows:I think.
This mostly shows the overall rotation with bits coming towards us (blue-shifted) and bits moving away (red-shifted) but it's possible pick out differential rotation too.
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