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Jens
2009-Aug-31, 08:14 AM
I think I could probably find an answer to this question somewhere, but haven't been able to locate it. Do the stars in the spiral arms of galaxies go around the galaxy in circular orbits, or are they moving inward, or moving outward? Do we have measurements of that? I suppose a way to measure it would be to look at the redshift of stars in a galaxy that is edgewise toward us.

DrWho
2009-Aug-31, 08:52 AM
I think that bulk motions are circular with some exceptions, particularly close to the center and the spheroidal halo.

StupendousMan
2009-Aug-31, 01:33 PM
Disk stars in spiral galaxies travel in orbits which
are (to a very good approximation) circular, in the
plane of the disk.

The orbits of stars in elliptical galaxies, and in the
bulge and halo populations of spiral galaxies, are
more complicated to describe.

Hornblower
2009-Aug-31, 10:17 PM
Our own Sun is in a somewhat eccentric orbit around the galactic core. That gives us a velocity of about 12 miles per second relative to the average positions of the stars in our immediate area. The overall velocity around the center is somewhere around 200 miles per second, making this eccentric effect relatively slight by comparison.

01101001
2009-Sep-01, 01:32 AM
I think I could probably find an answer to this question somewhere, but haven't been able to locate it. Do the stars in the spiral arms of galaxies go around the galaxy in circular orbits, or are they moving inward, or moving outward? Do we have measurements of that? I suppose a way to measure it would be to look at the redshift of stars in a galaxy that is edgewise toward us.

Not circular. Very roughly circular in bulk.

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)).

If they were orbiting in circles, the Sun and its current cohorts would start much closer together one galactic orbit ago.

Jens
2009-Sep-01, 01:42 AM
So in other words, there is no (I think this is the right term) tangential movement? So stars in the spiral arms don't get closer to the core or further from the core.

The reason I'm asking is that I think that in the spiral arms of tropical storms, the air is spiraling in, and then spiraling out at a different level. I know they are very different physical processes, but the resemblance is interesting so I wondered if the stars might also be going either inward or outward.

Cougar
2009-Sep-01, 02:56 AM
So in other words, there is no (I think this is the right term) tangential movement?

I don't think that's the right term :) , mainly because the tangential velocity of the Sun's orbit around the center of the galaxy is what initiated and maintains its near-circular path.


The reason I'm asking is that I think that in the spiral arms of tropical storms, the air is spiraling in, and then spiraling out at a different level. I know they are very different physical processes...

Right, quite different. Tracking the orbits of test bodies in a storm would be much more complicated than gravity. With gravity, there's a highly accurate algorithm. Works every time (within its broad range of applicability), even if you've got a lot of masses to contend with. With a storm, there will be places where two 'particles' will be very close together one moment, and in the next they may be surprisingly far apart. This signals sensitivity to initial conditions and takes us into the analysis of complex systems (http://en.wikipedia.org/wiki/Complex_system).

Jeff Root
2009-Sep-01, 03:54 AM
Jens,

Rather than 'tangential', I think you mean the opposite, 'radial'. Yes?

Though as Zero One's linked animation shows, the orbits do
have radial components: they do move closer to and farther from the
galactic center.

-- Jeff, in Minneapolis

01101001
2009-Sep-01, 03:54 AM
So stars in the spiral arms don't get closer to the core or further from the core.

I can't tell. Are you aware that being in a galactic spiral arm is just temporary work for a long-lived star? The rotation periods for stars are different than the rotation periods for arms.

For instance, Wikipedia (http://en.wikipedia.org/wiki/Milky_Way) has it:


Sun's galactic rotation period 220 million years (negative rotation)
Spiral pattern rotation period 50 million years

Jens
2009-Sep-01, 05:08 AM
It also occurred to me that this might be very hard to measure in the first place. My reasoning is like this: you can tell that a galaxy is spinning by taking Doppler measurements of an edge-on galaxy and comparing the left and right sides of the disk. Presumably, you could see if the stars are moving outward if there is a blueshift for the stars facing us. But if you actually take the measurement, you can't tell if it is due to the proper motion of the galaxy or due to an outward movement of stars. Maybe there's a way to do it by comparing the measurements for the stars in the front of the disk with those on the edges?

Jeff Root
2009-Sep-01, 06:15 AM
I doubt that redshifts of individual stars in other galaxies can be measured.
But redshifts of individual stars in the Milky Way must have been used in
making the animation Zero One linked to. We can get a good sample
of nearby stars. That's enough to answer your question.

An introductory astronomy textbook I have shows how elliptical orbits of
stars and nebulae around a galactic center may combine to produce density
waves that are spiral arms.

-- Jeff, in Minneapolis

matt.o
2009-Sep-01, 07:29 AM
I doubt that redshifts of individual stars in other galaxies can be measured.


I wouldn't put money on that if I were you (unless you want to lose some money to me by making a bet?).

http://adsabs.harvard.edu/abs/2005ApJ...634..287I

chornedsnorkack
2009-Sep-01, 08:29 AM
Our own Sun is in a somewhat eccentric orbit around the galactic core. That gives us a velocity of about 12 miles per second relative to the average positions of the stars in our immediate area. The overall velocity around the center is somewhere around 200 miles per second, making this eccentric effect relatively slight by comparison.

And what is the speed of Sun relative to circular orbit?

There is an obvious reason why the average positions of stars in a centrally concentrated disk should systematically move backward. Consider that a star on an elliptical orbit is either nearer to its periapse and moving faster than circular orbit or else nearer to its apoapse and moving slower than circular orbit. If there are more stars in the inner parts of the disk than in its outer parts then a region in between should on the average contain more stars that are near apoapse and slow moving than those that are near periapse and fast moving. Thus, the average speed of stars must be slower than the circular orbit speed.

The Sun is not only faster than the average star of neighbourhood, but also is among the minority of stars in solar neighbourhood that is close to periapse.

Would someone give the specific numbers for Sunīs speed relative to circular orbit and relative to neighbourhood average?

antoniseb
2009-Sep-01, 04:28 PM
And what is the speed of Sun relative to circular orbit?...

Papers I've read have shown that the Sun is currently on the inbound leg of its roughly elliptical path, meaning that it's motion is not in the same direction as most other stars around us.

ngc3314
2009-Sep-01, 07:18 PM
There is an obvious reason why the average positions of stars in a centrally concentrated disk should systematically move backward. Consider that a star on an elliptical orbit is either nearer to its periapse and moving faster than circular orbit or else nearer to its apoapse and moving slower than circular orbit. If there are more stars in the inner parts of the disk than in its outer parts then a region in between should on the average contain more stars that are near apoapse and slow moving than those that are near periapse and fast moving. Thus, the average speed of stars must be slower than the circular orbit speed.


This effect is known in "classical" stellar kinematics as the asymmetric drift (http://www.daviddarling.info/encyclopedia/A/asymmetric_drift.html), which gets some interesting Google hits.

forrest noble
2009-Sep-18, 04:41 AM
Hi Jens,


Do the stars in the spiral arms of galaxies go around the galaxy in circular orbits, or are they moving inward, or moving outward? Do we have measurements of that? I suppose a way to measure it would be to look at the redshift of stars in a galaxy that is edgewise toward us.

Spiral arm stars are called disc stars because they are part of the disk as opposed to the bulge of a spiral galaxy like the Milky Way. Since we now know that disc stars do not follow our present gravity models without the inclusion of dark matter little predictions can be made outside actual measurement. Theory asserts that spiral galaxies must rotate as a whole to maintain their distinct spiral form over many rotations of the galaxy.

If there is relative motion within the disc it would seem to be a slow process and very difficult to measure. In our own galaxy this motion has been measured to be flat, meaning the inner disc and outer disc stars are moving at about the same rate of orbital motion. This would indicate some relative motion between successive orbital radii. Theory asserts that in these encounters between stars, some will move both inward and outward from the galactic center for each revolution of the galaxy. Exact predictions for any single star including the sun is not possible unless a close encounter is observed. Stars are so far apart that few close encounters or collisions are thought to occur from this very slow process. Without interference most disc stars' orbit around the center of the galaxy are thought to be generally circular but our sun and some other disc star orbits are thought to be elliptical. The orbit of core stars of our galaxy are known to be much more complicated.

TonyE
2009-Sep-18, 12:04 PM
And what is the speed of Sun relative to circular orbit?


The "standard" orbit of the "Mean Sun" is circular around the galactic centre (GC) with a radius of 8.5kpc and velocity of 220km/s. (This standard is set by the IAU, the real values are probably a bit lower)

The position of the Mean Sun in this orbit is the origin for a set of coordinates called the Local Standard of Rest. The LSR has a period of about 240 million years.

The real Sun moves in an epicycle around the LSR. This takes it about 300pc nearer to/further from the GC with a period of 170 million years.

Currently the Sun is moving relative to the LSR at 13.4 km/s towards position RA18h Dec 30Deg in the sky. This velocity is made up of:

10km/s towards the GC.
7.2km/s away from the plane of the disk towards the north galactic pole.
5.2km/s faster than its mean motion in a circular orbit.

chornedsnorkack
2009-Sep-18, 12:10 PM
Currently the Sun is moving relative to the LSR at 13.4 km/s towards position RA18h Dec 30Deg in the sky. This velocity is made up of:

10km/s towards the GC.
7.2km/s away from the plane of the disk towards the north galactic pole.
5.2km/s faster than its mean motion in a circular orbit.

Then how far are Sunīs periapse and apoapse compared to Sunīs current distance from galactic centre?

JohnD
2009-Sep-18, 12:49 PM
That animation, and the accompanying article say that 14,000 stars, the Sun's closest neighbours now, were all concentrated close to the Sun one galactic year ago. On the face of it, an extraordinary and unlikely heliocentric conclusion.

What is the rational conclusion? All the Milky Way's stars cannot have been so congested, or else they would have been in some dense cluster, and that is unlikely too. So at the same time others must have been dispersing from a concentration.
Does this obesrvation represent the wavelike effect that forms the spiral arms? The arms are not 'real', in that the concentration of stars that makes them moves through the galactic disc at a different speed to the orbital. A star in an arm will not remain so as the 'concentration wave' passes it and other stars move away. All stars have different orbits, but are somehow 'organised'.

I hope this isn't an 'alternate' theory?

John

Ken G
2009-Sep-18, 01:12 PM
One other point-- the Sun bobs up and down through the disk plane at a rate that is faster than its orbit. This is due to the gravity of the disk itself, i.e., not the same gravity that is responsible for the nearly-circular orbit (which presumably is mostly from dark matter).

01101001
2009-Sep-18, 02:24 PM
That animation, and the accompanying article say that 14,000 stars, the Sun's closest neighbours now, were all concentrated close to the Sun one galactic year ago.

No it doesn't. It (http://www.space.com/scienceastronomy/milkyway_movement_040406.html) says:


A lot has changed [in a galactic year about 250 millions years -- 01101001]. The stars begin the animation spread broadly across about a quarter of the galaxy, then gather to a small, seemingly insignificant huddle at the end.

Now: close neighbors. Then: Spread widely.

They have just come together. They will soon drift apart.

Many did appear to go through a realtively huge bottleneck maybe 2/3 of a galactic year ago -- at 8 o'clock in the animation if it's 3 o'clock now. Is that what you are concerned about?

Ken G
2009-Sep-18, 03:43 PM
I wondered about that bottleneck too, the stars appear to be in a sheet that flips over at that point. It looked very odd.

Romanus
2009-Sep-19, 03:10 PM
IIRC, a galaxy's spiral arms have little or no bearing on stellar orbits. In fact, stellar orbits are in fact not even bound to the direction of the spiral pattern, as exhibited by galaxies like the one in this link:

http://www.newscientist.com/article/dn13161-galaxys-spiral-arms-point-in-opposite-directions.html

TonyE
2009-Sep-20, 01:03 PM
Then how far are Sunīs periapse and apoapse compared to Sunīs current distance from galactic centre?

I have not ignored your question - but failed to find any detailed estimates that would provide a good answer.

The Sun describes a roughly elliptical path in a retrograde direction round the position of the 'mean Sun'.

So we can say, from its current motion, that it is in that part of the ellipse where it is nearer than average to periapsis and still moving towards periapsis.

Zachary
2009-Sep-21, 01:14 PM
Spiral arms in galaxies aren't actually a coherent structure made up of stars. Well, they are, but not the same stars. The arm is actually a density wave that propogates around the galaxy. As it passes through an area of space, the stars in that area bunch up, giving the perception of a spiral arm.

But it is the density wave travelling around the galaxy; not the stars. It doesn't have a long term effect on the orbit of the stars that it affects.

Webbo
2009-Sep-23, 04:57 PM
Spiral arms in galaxies aren't actually a coherent structure made up of stars. Well, they are, but not the same stars. The arm is actually a density wave that propogates around the galaxy. As it passes through an area of space, the stars in that area bunch up, giving the perception of a spiral arm.

But it is the density wave travelling around the galaxy; not the stars. It doesn't have a long term effect on the orbit of the stars that it affects.

That means, to maintain the structure, either the individual stars would need to speed up and slow down as they orbited or, stars orbiting at the same distance from the centre of the galaxy orbit at different speeds. These descriptions both sound ATM.

Surely theory predicts that they should all orbit at the same speed depending on their distance from the centre, therefore the arm struture and the specific stars that form it must have always been the same in the past as they will be in the future.

TonyE
2009-Sep-23, 05:20 PM
The number density of ordinary middle-aged stars is not much different inside and outside spiral arms. Individual stars go about their orbits passing into and out of the arms.

However, the arms contain a lot of gas (molecular clouds) and this is where new stars are formed. The arms have the star-making regions where young high-mass stars produce most of the light.

So the spiral density wave shows up bright because it has the bright high-mass stars. By the time the density wave moves on, the high-mass stars will burn out. The low mass stars live much longer and will drift out of the arm in which they were created.

Webbo
2009-Sep-23, 05:33 PM
The number density of ordinary middle-aged stars is not much different inside and outside spiral arms. Individual stars go about their orbits passing into and out of the arms.

However, the arms contain a lot of gas (molecular clouds) and this is where new stars are formed. The arms have the star-making regions where young high-mass stars produce most of the light.

So the spiral density wave shows up bright because it has the bright high-mass stars. By the time the density wave moves on, the high-mass stars will burn out. The low mass stars live much longer and will drift out of the arm in which they were created.

Shouldn't the gas, and all other forms of mass no matter how large or small orbit at the same velocity? What is holding the gas stationary while the stars move through it?

ngc3314
2009-Sep-23, 06:23 PM
Shouldn't the gas, and all other forms of mass no matter how large or small orbit at the same velocity? What is holding the gas stationary while the stars move through it?

It's not that individual gas atoms are stationary with respect to the spiral pattern - it is the location of the density maximum which defines the arm peak. Like stars, individual atoms go in and out (spending longer within the arm than the mean velocity would imply due to the density enhancement). If we measure the whole Doppler-shift field of a suitably inclined spiral galaxy, we can see the effect as "ripples" in velocity on each side of the arms. Here is an example from Westerbork data (by H.C.D. Visser) on neutral hydrogen for M81:

http://www.astr.ua.edu/keel/galaxies/m81velhi.gif

The perturbation on gas motions caused by the spiral arms is especially clear at lower left, as the isovelocity contours kink on both sides of the arm.

chornedsnorkack
2009-Sep-23, 07:48 PM
That means, to maintain the structure, either the individual stars would need to speed up and slow down as they orbited
Which they do because of Kepler laws.

or, stars orbiting at the same distance from the centre of the galaxy orbit at different speeds. These descriptions both sound ATM.
Kepler laws also require that


Surely theory predicts that they should all orbit at the same speed depending on their distance from the centre, therefore the arm struture and the specific stars that form it must have always been the same in the past as they will be in the future.

No. This would only apply if all stars were on exactly circular orbits, and stationary with respect to nearby stars.

However, the Milky Wat contains stars on eccentric orbits, with different eccentricities, mean distances and positions on orbit. Which is why stars near each other have peculiar motions past each other.

A star in a perfect, armless disc of a lenticular galaxy will necessarily spend just as much time moving inwards towards periapse as moving outwards towards apoapse. And each local region in an armless disc would at any time contain equal number of stars moving inwards and stars moving outwards, because the stars in different parts of orbit are randomly mixed. Note, however, that there would be asymmetric drift - each star spends more time near apoapse than periapse (because it is slower near apoapse), and in each neighbourhood, there would be more stars near apoapse than stars near their periapse (because density grows inwards).

But arms of a spiral galaxy mean that the orbits of stars are not random, but somehow aligned. In which direction does a spiral arm move?

Webbo
2009-Sep-24, 01:07 AM
It's not that individual gas atoms are stationary with respect to the spiral pattern - it is the location of the density maximum which defines the arm peak.
So what you are saying is the gas molecules also orbit in the same way the stars do; but that is not the cause of the stars maintaining arm structure. Something else would need to be influencing both of them.


Like stars, individual atoms go in and out (spending longer within the arm than the mean velocity would imply due to the density enhancement). If we measure the whole Doppler-shift field of a suitably inclined spiral galaxy, we can see the effect as "ripples" in velocity on each side of the arms. Here is an example from Westerbork data (by H.C.D. Visser) on neutral hydrogen for M81:

The perturbation on gas motions caused by the spiral arms is especially clear at lower left, as the isovelocity contours kink on both sides of the arm.
Again this just describes the motion not why that particular structure is created and maintained.

Webbo
2009-Sep-24, 01:39 AM
Which they do because of Kepler laws.

Kepler laws also require that


No. This would only apply if all stars were on exactly circular orbits, and stationary with respect to nearby stars.

However, the Milky Wat contains stars on eccentric orbits, with different eccentricities, mean distances and positions on orbit. Which is why stars near each other have peculiar motions past each other.

A star in a perfect, armless disc of a lenticular galaxy will necessarily spend just as much time moving inwards towards periapse as moving outwards towards apoapse. And each local region in an armless disc would at any time contain equal number of stars moving inwards and stars moving outwards, because the stars in different parts of orbit are randomly mixed. Note, however, that there would be asymmetric drift - each star spends more time near apoapse than periapse (because it is slower near apoapse), and in each neighbourhood, there would be more stars near apoapse than stars near their periapse (because density grows inwards).

But arms of a spiral galaxy mean that the orbits of stars are not random, but somehow aligned. In which direction does a spiral arm move?

Yes but for Keplers Laws to create these structures (which are supposedly not always made from the same stars) the stars would need to be in highly elliptical orbits for the velocity differentials to create a definitive structure. I thought that these kind of orbits were rare (possibly Barnard's Star for example) not the norm. Also there would almost certainly need to be galaxy wide communication between all the participating stars otherwise it would just be random noise. Like you stated, for this to work something needs to be causing alignment. What exactly could be doing that? It's not the gas because that just appears to be orbiting in the same manner.

In addition, I thought that it was accepted that galaxies do not rotate as per Keplerian dynamics, as highlighted by the rotation curve problem and the subsequent necessity for a dark matter ring.

StupendousMan
2009-Sep-26, 02:49 PM
Webbo,

You're asking questions which can only be answered by lengthy explanations involving the physics of density waves. It would take a long time for any one of us to write a full explanation, especially if we had to start with a description of waves in general.

I suggest you go find a copy of the book "The Physics of Astrophysics", by Frank Shu. It's an introductory astronomy textbook, but at a more advanced mathematical level than most. Shu was one of the first astronomers to work through the mechanism of spiral density waves, and he has a very good ability to communicate with casual readers.

Please read the section(s) of that book which deal with spiral galaxies, and then come back to us with your questions.

Webbo
2009-Sep-27, 03:04 AM
Well yes but a density wave requires the object within the denser portion to slow down and then speed up again when it leaves (like cars in a traffic jam). Is there an explanation how orbiting stars manage this without having their orbits seriously perturbed? And the expected change in speed due to normal ellipsical orbit could not account for this unlesss there is galaxy wide communication to ensure the arm structure is created and maintained. Also, closer to the center the arms can be very thin which means they are only slowing for a relatively short period of time, which wouldn't make sense for normal orbital change in speeds due to the elipse.

In addition the density wave theory is only necessary because of the winding problem, but there is no such problem as all stars orbit at a constant speed relative to their radius. Something which is explained with the addition of dark matter. Therefore is Shu's theory even relevant any more? Will the calculations work with the different (and unexpected) orbiting chracteristics? Is/was dark matter factored in?

TonyE
2009-Sep-27, 11:31 AM
In which direction does a spiral arm move?

If we take the density wave theory as being our best guess as to the mechanism that creates/maintains spiral arms then the entire pattern rotates in the same direction as the stars.

However, stars in the inner region of the galaxy have a much shorter orbital period than stars in the outer regions. This means that the pattern is rotating slower than the inner stars and faster than the outer stars. Only at one distance from the centre are the stars and pattern rotating with the same period. This is the 'co-rotation speed'.

From what I can see in various papers there is not much agreements as to what the co-rotation speed is in the Milky Way. It is very difficult to measure being so slow and having a view 'from the inside'.