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nickc
2009-Jun-26, 07:54 PM
Greetings,
I have some questions concerning the movement of the Sun through the Galaxy.
Here is what I know so far, please correct if necessary:
-Sun is 25,000 ly from the center of the galaxy, + or - a few thousand ly's
-velocity = 150 mi/sec

Questions:
1. What is the current state of knowledge concerning this motion, is the Sun revolving as a part of the spiral arm?
2. What is the configuration of the Sun's orbit? is it a low or high eccentricity ellipitical orbit?
3. Is there any movement toward or away from the center, ie along the spiral arm?
4. What tools do astronomers use to measure these motions?

thank you,
nick

Celestial Mechanic
2009-Jun-26, 09:12 PM
[Snip!] Questions:
Welcome to BAUTForum! I'll try to answer a few of these for you, but my specialty is celestial mechanics, so I hope someone will correct me if I'm wrong on some of these answers.

1. What is the current state of knowledge concerning this motion, is the Sun revolving as a part of the spiral arm?
We have a reasonable idea of its period, about 250 milliion years to circle the galactic center, and its oscillation period, where it sort of bobs up and down through the galactic plane, about 80 million years. (These are approximate numbers, someone please correct me if they know of better ones.)

Stars (including the Sun) do not stay within spiral arms. A spiral arm is a lot like a stretch of highway where the vehicles have to slow down for construction. Once past the construction zone, all the vehicles speed up again, and something like that happens in spiral arms.

2. What is the configuration of the Sun's orbit? is it a low or high eccentricity elliptical orbit?
I'm pretty sure the eccentricity is low.

3. Is there any movement toward or away from the center, ie along the spiral arm?
Again, the Sun does is not always a part of the spiral arm. There will be some movement towards or away from the center, but not too much as the orbit has low eccentricity.

4. What tools do astronomers use to measure these motions?
I read about it once but I'm sure I don't remember as much of it as I should. For closer stars for which we can determine a distance, we can measure proper motion against the background stars and sometimes we can detect motion towards or away from us by measuring the Doppler shift in the spectrum of the star. This gives the motion of the stars relative to us. I'm not sure of all the steps in inferring the motion of our Sun with respect to the galactic center.

Can anyone else clarify and expand on this?

nickc
2009-Jun-26, 10:09 PM
Hi Celestial Mechanic,
Thanks for your response. It clarified some of the questions and helped me to visualize the Sun's movement through the Milky Way.

We have a reasonable idea of its period, about 250 milliion years to circle the galactic center, and its oscillation period, where it sort of bobs up and down through the galactic plane, about 80 million years.
What would be the cause of such an oscillation?

Thanks in advance, to any replies.

Nick

Amber Robot
2009-Jun-26, 10:14 PM
If the Sun were formed with some velocity out of the plane of the Galaxy, it would continue to oscillate in the gravitational potential well of the Galaxy's disk unless it hit something or drag forces from the interaction with the interstellar medium dampen the oscillation.

Hornblower
2009-Jun-26, 11:36 PM
Hi Celestial Mechanic,
Thanks for your response. It clarified some of the questions and helped me to visualize the Sun's movement through the Milky Way.

What would be the cause of such an oscillation?

Thanks in advance, to any replies.

Nick

If I am not mistaken, that oscillation is caused by the gravitational perturbation of concentrations of mass in the spiral arms.

StupendousMan
2009-Jun-26, 11:45 PM
For closer stars for which we can determine a distance, we can measure proper motion against the background stars and sometimes we can detect motion towards or away from us by measuring the Doppler shift in the spectrum of the star. This gives the motion of the stars relative to us. I'm not sure of all the steps in inferring the motion of our Sun with respect to the galactic center.

Can anyone else clarify and expand on this?

Astronomers can measure the Sun's motion relative to nearby stars to get an idea for the "peculiar motion" of the Sun, relative to the general circular motion it shares with other stars in the disk. We can measure the Sun's motion to distant globular clusters (or other objects in the halo) to get a decent estimate of this general orbital motion in the disk.
These, and other "classical" techniques, have been used for a hundred years or so.

And, for the past fifteen years or so, we can actually measure the motion of the Sun relative to the compact object at the center of the galaxy directly, using radio observations of the accretion disk around the black hole relative to background galaxies.

Jeff Root
2009-Jun-27, 03:34 AM
I'm just amplifying Amber Robot's reply: Drag forces are virtually nonexistant.
If the Solar System started out with a slight motion toward one side of the
galactic disk, the motion would continue, and the Solar System would rise
out of the disk on that side. The motion would be slowed by the disk's
gravity, and the Solar System would be pulled back into the disk. Motion in
the new direction would continue and the Solar System would move through
the disk and rise out of the other side, and so on, for the lifetime of the Sun,
unless it happens to interact with another star, changing the motion.

-- Jeff, in Minneapolis

01101001
2009-Jun-27, 08:17 AM
Here's a rough estimate of the sun's last trip around the Milky Way, along with the trips of its current neighbors: animation of last orbit around the galactic center (http://www.space.com/php/multimedia/imagedisplay/img_display.php?pic=040406_milkyway_anim_02.gif&cap=Animation+shows+the+Sun%27s+last+250-million-year+orbit+around+the+center+of+the+galaxy+%28GC%2 9%2C+and+other+stars+converging+toward+the+Sun%2C+ ending+with+all+of+them+nearer+the+Sun+today.+May+ load+slowly.+Credit%3A+ESO) (from Space.com: The Crazy Cosmos: Stars Near Sun are Wild & Wayward (http://www.space.com/scienceastronomy/milkyway_movement_040406.html)). You might find it instructive, on a first-order level.

Notice that the stars nearby today were spread widely apart 250 million years ago. Stars don't form large long-term persistent portions of galaxies.

Jeff Root
2009-Jun-27, 12:59 PM
Nice animation. It's a GIF so it displays fine even on my computer.
I see that it was computed backward in time, and displays forward in
time, ending at the present, where all the stars are close to the Sun.
I realize that you probably know nothing about this, but...

Why is there a sharp edge on the right side of the mass of stars in
the first 40% or so of the animation?

What is happening at about 7 o'clock, where nearly all the stars appear
to go through a bottleneck? Why would they do that??

Any idea how many stars are shown?

-- Jeff, in Minneapolis

01101001
2009-Jun-27, 02:52 PM
What is happening at about 7 o'clock, where nearly all the stars appear to go through a bottleneck?

The article says 14000 stars were studied. It doesn't seem like that many in the animation.

I see the bottleneck, too. Seems like it would have to be some common effect that affected all the current cohorts. Maybe it's where the group last passed through a dense spiral. I wonder what the difference in rate is between spiral rotation and average individual star rotation.

First article I saw said arms' rotation is approximately that of Sun, so that sort of fits, but it didn't say how rough. Don't have time to delve. Anyone know the ratio?

EnigmaPower
2009-Jun-27, 02:55 PM
With a galactic rotation of 250 million years wouldn't that mean the sun is really about 45 "years" old?

Bearded One
2009-Jun-28, 05:48 AM
With a galactic rotation of 250 million years wouldn't that mean the sun is really about 45 "years" old?I get 18.8 galaxy years old assuming an age of 4.7 billion Earth years for the Sun.

EnigmaPower
2009-Jun-28, 12:57 PM
I get 18.8 galaxy years old assuming an age of 4.7 billion Earth years for the Sun.

oops...I used 14 billion as the age of the universe.