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Robert Tulip
2009-Feb-10, 01:52 PM
The stars within 50 ly http://www.atlasoftheuniverse.com/50lys.html
the stars within 250 ly http://www.atlasoftheuniverse.com/250lys.html
Against these star maps of 50 (http://www.atlasoftheuniverse.com/50lys.html) and 250 (http://www.atlasoftheuniverse.com/250lys.html) light years from the sun, what path has the sun travelled?
Can an arrow be drawn on these maps showing which direction the sun came from and where it is going, pace of travel and curvature to describe this part of the sun's orbital path around the galaxy?

Jeff Root
2009-Feb-10, 03:17 PM
Somewhere, maybe Before Internet and not online, I saw a diagram
of nearby stars with arrows to show their directions and speeds of
relative motion. I've seen that for the stars of the Big Dipper in
several places. The Sun is probably considered to be at rest for
these diagrams.

-- Jeff, in Minneapolis

NEOWatcher
2009-Feb-10, 03:24 PM
Somewhere, maybe Before Internet and not online, I saw a diagram
of nearby stars with arrows to show their directions and speeds of
relative motion. I've seen that for the stars of the Big Dipper in
several places. The Sun is probably considered to be at rest for
these diagrams.
That would be something that my curiosity would like to see.
Robert's post sounds to me like only the sun has relative motion among the stars, but I do know that they are all moving.

Something similar to the diagrams given with each stars vector of motion relative to the sun would be interesting to me.

I've seen various older forms of what Jeff is saying. I think it was more from the Cosmos series. IIRC Sagan had not only showed illustrations of how the constellations change over time, but some flybys of constellations to show their depth. I particularly remember a big dipper flyby. (I may also be confusing this with another show)

01101001
2009-Feb-10, 07:23 PM
[...] what path has the sun travelled?

Not directly responsive, but interesting, for its sidebar animation of the Sun's much longer travel during the last trip around Galactic Center:

Space.com: The Crazy Cosmos: Stars Near Sun are Wild & Wayward (http://www.space.com/scienceastronomy/milkyway_movement_040406.html)


The survey does not reveal the Sun's birthplace, but it offers some clues to the past. An animation shows the stars' movements in relation to the Sun projected back through the past 250 million years, or about the time it took the Sun to make its most recent trip around the center of the galaxy.

A lot has changed. 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.

And that's nothing compared to the much longer history of the Sun.

"The Sun has made some 20 laps around the galactic center since it was born, and its 'sisters and brothers' have dispersed long ago," Andersen said of any stars that might have been born near the Sun.

I'm not sure it informs us much to specify the Sun's travel vector against a static map of current neighbors, for while the sun was moving so were all the neighbors. It would be a snapshot, wrong an instant later.

aurora
2009-Feb-10, 07:40 PM
Against these star maps of 50 (http://www.atlasoftheuniverse.com/50lys.html) and 250 (http://www.atlasoftheuniverse.com/250lys.html) light years from the sun, what path has the sun travelled?


I'm not sure if it would be possible, since all the stars are moving and they are all moving with slightly different directions and velocities. So to do such a map, something would have to be held constant. Maybe you were thinking of the galactic center?

Robert Tulip
2009-Feb-11, 02:43 AM
Looking at the grid as constant, with the sun moving off the 0,0 point, I expect that all or at least most of the stars are moving together, in a roughly common direction and pace. What I am wondering is the direction and pace of the whole picture, and how far the sun is a proxy for our part of the galactic river. The sun is moving towards one point of the sky in its galactic orbit. I am wondering where on these pictures is that part of the sky, and how long will we take to get to a point near whatever star is near there now.

Tim Thompson
2009-Feb-11, 03:42 AM
See my post #19 (http://www.bautforum.com/space-astronomy-questions-answers/84554-sun-located-nebula-cluster-what.html#post1431969) in the discussion "Is the Sun Located in a Nebula, Cluster, or What? (http://www.bautforum.com/space-astronomy-questions-answers/84554-sun-located-nebula-cluster-what.html). I think it is relevant to the question asked here.

Hornblower
2009-Feb-11, 01:34 PM
In an inertial frame of reference in which the core of the galaxy is stationary, the stars are orbiting at roughly 200 miles per second, approximately parallel to the 90 degree axis. That is toward Cygnus as seen from our vantage point. There is some individual variation because the stars are not in identical orbits.

If we choose a frame in which the Sun is stationary, the nearby stars as shown on these maps are streaming by at an average of about 12 miles per second, from a point of origin in Hercules. There is considerable scatter among the apparent motions of individual stars.

If we set our frame of reference in motion so that the motions of these stars average out to zero, the Sun will appear be moving about 12 miles per second toward Hercules. It would appear to move about 1 light-year in 15,000 years.

Over a range of a few light-years, these motions cover such short arcs of galactic orbits that straight lines will be good approximations.

My sources include Norton's Star Atlas, along with various articles in Sky and Telescope over the past few decades.

Centaur
2009-Feb-11, 06:46 PM
Against these star maps of 50 (http://www.atlasoftheuniverse.com/50lys.html) and 250 (http://www.atlasoftheuniverse.com/250lys.html) light years from the sun, what path has the sun travelled?
Can an arrow be drawn on these maps showing which direction the sun came from and where it is going, pace of travel and curvature to describe this part of the sun's orbital path around the galaxy?

All motion is relative. Any point can be chosen as the origin of a coordinate system. In maps of the solar neighborhood, the Sun is typically chosen as that central point.

Robert Tulip
2009-Feb-11, 08:24 PM
In an inertial frame of reference in which the core of the galaxy is stationary, the stars are orbiting at roughly 200 miles per second, approximately parallel to the 90 degree axis. That is toward Cygnus as seen from our vantage point. There is some individual variation because the stars are not in identical orbits. If we choose a frame in which the Sun is stationary, the nearby stars as shown on these maps are streaming by at an average of about 12 miles per second, from a point of origin in Hercules. There is considerable scatter among the apparent motions of individual stars. If we set our frame of reference in motion so that the motions of these stars average out to zero, the Sun will appear be moving about 12 miles per second toward Hercules. It would appear to move about 1 light-year in 15,000 years. Over a range of a few light-years, these motions cover such short arcs of galactic orbits that straight lines will be good approximations. My sources include Norton's Star Atlas, along with various articles in Sky and Telescope over the past few decades.

Thanks Hornblower. So is the attached rough picture (http://www.bautforum.com/attachment.php?attachmentid=9581&stc=1&d=1234383799) accurate?

Hornblower
2009-Feb-11, 09:45 PM
Thanks Hornblower. So is the attached rough picture (http://www.bautforum.com/attachment.php?attachmentid=9581&stc=1&d=1234383799) accurate?
I think this one is more accurate.
http://img102.imageshack.us/img102/5503/sunapexmotionyf3.jpg

The best information I can find has the apex at approximate galactic latitude 23 degrees and longitude 56 degrees. That places it well above the galactic plane as shown. The vertical yellow line drops to a point in the plane with the same longitude, as is done with the stars in this view. The length of the vector is fine, about 50 ly in 750,000 years.

Many of the other stars will show similar amounts of local motion, in random directions. Arcturus will be far off the chart. It is a halo star in an eccentric, steeply inclined orbit that gives it a relative motion many times faster than the disk stars which are predominant here.

Robert Tulip
2009-Feb-11, 11:33 PM
I think this one is more accurate. http://img102.imageshack.us/img102/5503/sunapexmotionyf3.jpg The best information I can find has the apex at approximate galactic latitude 23 degrees and longitude 56 degrees. That places it well above the galactic plane as shown. The vertical yellow line drops to a point in the plane with the same longitude, as is done with the stars in this view. The length of the vector is fine, about 50 ly in 750,000 years. Many of the other stars will show similar amounts of local motion, in random directions. Arcturus will be far off the chart. It is a halo star in an eccentric, steeply inclined orbit that gives it a relative motion many times faster than the disk stars which are predominant here.Many thanks Hornblower, much appreciated. Just to see if I understand this, are the following statements correct?

The galaxy is powered in its rotation by the movement of the core, with the arms trailing the central spin.
The sun's orbit around the galaxy aligns with the overall movement of the Orion Arm.
The disk stars in our neighbourhood are moving together, with exceptions like Arcturus.
Most of the stars in the picture will be in roughly the same position relative to the sun in a million years.
The data would be readily available to add short vectors to all the shown stars at 50 or 250 LY indicating direction and speed.

Centaur
2009-Feb-12, 12:16 AM
1. The galaxy is powered in its rotation by the movement of the core, with the arms trailing the central spin.


The galactic rotation is not powered by anything. It simply maintains its original angular momentum. Gravitation, including that from the central core, simply keeps the stars from shooting off along nearly straight lines into intergalactic space.

Hornblower
2009-Feb-12, 12:27 AM
Many thanks Hornblower, much appreciated. Just to see if I understand this, are the following statements correct?

The galaxy is powered in its rotation by the movement of the core, with the arms trailing the central spin.
The sun's orbit around the galaxy aligns with the overall movement of the Orion Arm.
The disk stars in our neighbourhood are moving together, with exceptions like Arcturus.
Most of the stars in the picture will be in roughly the same position relative to the sun in a million years.
The data would be readily available to add short vectors to all the shown stars at 50 or 250 LY indicating direction and speed.


1. No. Just momentum from primordial turbulence.

2. I don't know.

3. Approximately.

4. No. Lots of scatter.

5. Yes, the proper motions and radial velocities are readily available in star catalogues. These give the motion relative to the Sun, and some coordinate transformation and number crunching would be needed to draw the vectors.

Robert Tulip
2009-Feb-12, 12:59 AM
The galactic rotation is not powered by anything. It simply maintains its original angular momentum. Gravitation, including that from the central core, simply keeps the stars from shooting off along nearly straight lines into intergalactic space.Saturn's rings are 'powered' by Saturn in the sense that the angular momentum of Saturn 'enables' the orbiting structure of its rings. Does the angular momentum of the galactic core similarly enable the trailing movement of the galactic arms? Is the rotating core dragging the arms around to create the spiral shape?

ETA: These other pictures show the position of the sun within the galactic arm (http://www.atlasoftheuniverse.com/5000lys.html) and the Milky Way showing central core and spiral arms (http://www.atlasoftheuniverse.com/galaxy.html) .

Centaur
2009-Feb-12, 05:53 AM
Saturn's rings are 'powered' by Saturn in the sense that the angular momentum of Saturn 'enables' the orbiting structure of its rings. Does the angular momentum of the galactic core similarly enable the trailing movement of the galactic arms? Is the rotating core dragging the arms around to create the spiral shape?



Sorry, Robert, but you seem to have missed my point and that of Newton and Galileo. I would have responded to your other statements too, but I had to rush to an appointment. But your first statement was based on such a common misconception that was supposedly put to rest centuries ago, that I had to respond quickly. Hornblower did a fine job responding to almost all of them, although the correct response to number two is essentially yes.

Newton’s first law (or axiom as he put it) of motion was basically borrowed from Galileo and states that, “A body persists its state of rest or of uniform motion unless acted upon by an external unbalanced force.” This essentially says that momentum is conserved unless acted upon by a force; friction is one example of such a force. Combining this with Newton’s second law which simplifies to: force equals mass times acceleration, he was able to derive a similar law for the conservation of angular momentum. This latter law even applies to the scenario I gave earlier of the stars flying off into intergalactic space in the absence of gravity, since they would not be flying directly outward from the galactic center but would maintain some component of lateral motion relative to the center.

On Earth our experience with ever present friction gives us an intuitive image of continuously added energy always being required to keep a wheel rotating. But in space there is a virtual absence of friction. Hence the Earth keeps rotating on its axis, the planets keep revolving around the Sun, and the Sun keeps revolving around the galactic center, all without power (added energy). Similarly natural satellites like our Moon keep revolving around their central planets. The rings of Saturn are composed of countless small satellites that keep orbiting due to the conservation of angular momentum, not because of any power. Saturn's gravity simply keeps them from escaping its neighborhood.

Power refers to the amount of energy that must be added to a system to keep it going. In the absence of friction, no power (added energy) is needed in order to maintain rotation. The rotating core of the galaxy is not “dragging” the arms.

Centaur
2009-Feb-12, 07:18 AM
Robert, I should note that your intuitive concept of a “dragging” power is not unlike the vortex theory of planetary motion from the brilliant early 17th century philosopher and mathematician René Descartes. Newton in his writings in the latter part of that century discredited this theory.

If the gravitation from the side of the galactic center rotating away from a distant star had a component pulling that star forward in its orbit, then the gravitation from the side of the galactic center rotating toward that star would have an equal but opposite component pulling the star backward in its orbit. There is no frictional influence connecting the distant star with the core’s rotation.

The arms of the galaxy are not formed in a manner that is intuitively apparent. They are due to density waves. The Sun and its neighbors are in slightly different orbits around the galactic center. At times that group is more compact than at other times. The same is true of adjoining stellar neighborhoods. The tendency to become more compact moves along rather continuously from neighborhood to neighborhood. Hence the appearance of traveling waves over long time frames, despite an absence of net relative movement among the neighborhoods. This is similar to waves in mid-ocean, which are not actually far reaching lateral movements of masses of water, but are due to oscillations that temporarily increase the number of water molecules in one small section of ocean water before creating a similar effect in succeeding adjacent sections of ocean water.

Robert Tulip
2009-Feb-12, 07:42 AM
Thank you Centaur for your patient and informative replies. I do not have your intuitive grasp of the laws of physics. I imagine the galaxy as like a spinning top in a bowl of water, with the arms like spiral ripples in the water. Looking at this picture of the galaxy (http://www.atlasoftheuniverse.com/galaxy.html), it looks like the galactic core is generating the arms by losing material from either end of the core, which is spinning faster and allowing the arms to fall behind as they drift away from the centre. I assume this is wrong.

Centaur
2009-Feb-12, 07:53 AM
Thank you Centaur for your patient and informative replies. I do not have your intuitive grasp of the laws of physics. I imagine the galaxy as like a spinning top in a bowl of water, with the arms like spiral ripples in the water. Looking at this picture of the galaxy (http://www.atlasoftheuniverse.com/galaxy.html), it looks like the galactic core is generating the arms by losing material from either end of the core, which is spinning faster and allowing the arms to fall behind as they drift away from the centre. I assume this is wrong.

You’re welcome, Robert. Indeed, your analogy is a fine illustration of the common intuitive perception of the cause of spiral arms. Despite appearances, your assumption is correct that the notion is wrong.

Jeff Root
2009-Feb-12, 12:55 PM
Centaur,

I tried to compose a reply to Robert's post last night, but wasn't
satisfied with it, and deleted what I wrote. You said almost everything
I said and more. I wouldn't have thought of Descartes vortex theory
or comparing density waves to water waves.

One thing I tried to say that I'll add now. Not only does the core
of a galaxy not drag the arms around, but the curve of the arms is
not caused by drag, either. No advancing drag; no retarding drag.
Instead, there is something called "virialization", which I crudely
understand to be the process of exchanging energy between
independent bodies via a force like gravity, without friction. Stars
in an elliptical galaxy or globular cluster virialize when the more
massive stars group near the center of the galaxy, while less
massive stars go into orbits that take them far from the center.

Also, I started to write:

Spiral galaxies and planetary rings obviously rotate. Sometimes,
what is obvious isn't so. Galaxies don't rotate. Individual stars
each follow their own paths. In the case of gravitationally bound
systems, groups of stars or solar systems of stars, planets, and
moons each follow their own paths. Each particle in a planetary
ring orbits the planet separately. The collection of particles gives
the appearance of a rotating ring.

-- Jeff, in Minneapolis

Robert Tulip
2009-Feb-13, 07:04 AM
Not only does the core of a galaxy not drag the arms around, but the curve of the arms is not caused by drag, either. No advancing drag; no retarding drag. Instead, there is something called "virialization", which I crudely understand to be the process of exchanging energy between independent bodies via a force like gravity, without friction. Stars in an elliptical galaxy or globular cluster virialize when the more massive stars group near the center of the galaxy, while less
massive stars go into orbits that take them far from the center. Also, I started to write: Spiral galaxies and planetary rings obviously rotate. Sometimes, what is obvious isn't so. Galaxies don't rotate. Individual stars
each follow their own paths. In the case of gravitationally bound systems, groups of stars or solar systems of stars, planets, and moons each follow their own paths. Each particle in a planetary ring orbits the planet separately. The collection of particles gives the appearance of a rotating ring. -- Jeff, in Minneapolis

Jeff, thanks for this (thanks also for your reply a couple of weeks ago on why the solar system is a flat disk).

Where I am still having trouble is in considering whether the galactic arms are stable structures. Will they still be there with most of their current stars when we next orbit the galaxy in 250 million years? Or does this 'virialisation' (a term I have never seen before - http://en.wikipedia.org/wiki/Virial_theorem ) mean there are waves emanating from the galactic core which cause the wave structure of the arms to change more quickly? Could the Orion Arm dissolve into the Sagittarius Arm, for example?

I still can't help visualising the Orion Arm as like a river, with the disk stars flowing along to create an appearance of a current, and with firm banks between our arm and the neighbours. Sorry if my use of words like current and power creates a false analogy. It would be interesting to see a dynamic model of the galaxy over a billion years, indicating current understanding of the physical evolution of the spiral arms.

I hope I get your point about Saturn's rings, but it still remains the case that if Saturn wasn't there neither would the rings, so Saturn enables the orbit of the rings. A similar situation obtains for the galaxy, in that if the core was not there we would not be here, so the core is a main factor causing the structure of the arms. I am wondering how far the power analogy holds up. For example if we travel in a car the car powers our movement, and if celestial objects travel in a group does the preponderant mass somehow power the entire group, in the sense that Earth's momentum along the solar system Z axis is primarily powered by the sun?

Robert

Centaur
2009-Feb-13, 08:07 AM
Argh!! Sorry, Robert, but I appear to have totally failed with my explanations. You are still not getting it, and instead are reverting to your original assumptions. The waves are not emanating from the core. The waves are more or less orbiting the core. Over the long term the arms are not stable structures. In other epochs the less dense regions become the more dense regions and vice versa. You say that you can’t help visualizing in a certain way. You’ll just have to stop that and try to visualize in the manner I have described. And despite my earlier description of how friction affects wheels on Earth thus requiring power (added energy) to keep turning in a manner that does not apply to rotating objects in frictionless space, you still want to bring up the analogy of an Earth based car. Perhaps coasting is a term that will help you to understand. Objects in space continue moving or rotating as though they were constantly coasting in the absence of friction. The Earth’s momentum is not powered by the Sun. Its angular momentum remains constant. The Sun’s gravity merely prevents that angular momentum from becoming pure linear momentum which would send the Earth moving on a straight line at constant velocity into interstellar space. The same can be said of Saturn’s gravity in relation to the small satellites that form its rings, and of the entire mass of the galaxy (including the core) on its constituent stars.

If you truly desire to begin developing a deeper understanding, I highly recommend registering in a Physics 101 course. Either that or read an English translation of Isaac Newton’s Philosophiĉ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), often abbreviated to simply Principia.

Jeff Root
2009-Feb-13, 08:27 AM
Where I am still having trouble is in considering whether the galactic
arms are stable structures. Will they still be there with most of their
current stars when we next orbit the galaxy in 250 million years?
The arms are not structures, in the sense I would use the word.
Just as a group of people waiting at a curb for the light to change
is not a structure, a group of stars comprising a galactic arm is not
a structure. Nothing holds them together. They are not together
because some force binds them; they are together because the
way they speed up and slow down as they move causes them to
bunch up at regular intervals. Switching to Centaur's analogy, the
water of the ocean holds together, but the waves do not. They
are just groups of water molecules that bunch together periodically
because of how they speed up and slow down as they move. Stars
are gravitationally bound to the galaxy, but the arms are just places
where stars are temporarily bunched together.

An arm is somewhat stable, but it is dynamically stable. It depends
on the stars constantly speeding up and slowing down so that they
are always bunching together in new ways. Stars are constantly
moving into a galactic arm while other stars leave. The number of
stars in an arm may be fairly constant, but the stars that make up
that arm are constantly changing. Same as molecules of water in
a water wave.

The speeding up and slowing down of the stars is due to their
orbits around the galaxy. Although not strict Keplerian ellipses,
they have a significant degree of umm... Keplarian character.
They move away from the center of the galaxy when they move
too fast to be in circular orbit, and fall back toward the center
when they move too slowly. Then they lose speed as they rise
away from the center, or gain speed as they fall toward the center.
So they are in a continual harmonic oscillation which makes them
bunch at regular intervals, creating density waves.



Or does this 'virialisation' (a term I have never seen before -
http://en.wikipedia.org/wiki/Virial_theorem ) mean there are waves
emanating from the galactic core which cause the wave structure
of the arms to change more quickly? Could the Orion Arm dissolve
into the Sagittarius Arm, for example?
The arms only have as much identity as water waves have. If you
watch a wave moving across a lake or river, you may be able to
follow its progress for a long distance. But at some point you are
likely to get confused and not be sure which of two wave crests
is the one you were following. Not so much because you weren't
paying close enough attention, but because waves sometimes pass
through one another and momentarily lose their identities as they
do so. You then have to sort out which wave is which according
to your own definition of the wave you were attempting to follow.

You might follow the Orion and Sagittarius arms for a few million
years, and then suddenly realize that you can't identify the arms
anymore, but you can identify an arm over there, which you didn't
notice before.

The stars in a galaxy are all gravitationally bound to each other,
and to the galaxy's core, just as the molecules of water in the
ocean are bound to each other, and to the Earth. But just as the
waves of water are not created by the Earth's gravity or by the
forces that bind the water molecules together, the density waves
of galactic arms are not created by the gravitational force of the
stars at the galactic center. In both cases the binding forces are
required for the waves to exist, but they do not cause the waves,
and they do not provide energy to generate them or keep them
moving.

-- Jeff, in Minneapolis

NEOWatcher
2009-Feb-13, 12:47 PM
I don't want to take any steam away from Jeff's answer, but the way I first heard it and continue to think of is the following:

The analogy is a highway with cars/trucks of varying speed.

There's nothing external to the traffic causing slowdowns or stops or anything other than the road defining the path. (could equate to orbit)

The traffic tends to bunch up around the slower objects. (could equate to the gravitational influences of each other)

Viewed from above, you will see this in waves of dense traffic. (could equate to the waves of the dense arms)

Jeff Root
2009-Feb-13, 03:29 PM
There's nothing external to the traffic causing slowdowns or stops
Good point. That makes for a much better analogy. The traffic
itself causes the slowdowns. That analogy still shouldn't be taken
literally, of course: Stars don't get in each other's way, causing
them to slow down! :D

Here is another analogy that gets around that problem: A juggler
tossing a large number of balls in the air in rapid succession. At a
given instant, he may have one ball in his hand and six in the air.
One of those six may be just a few inches above his empty hand,
falling toward it. But most of the balls will be high over his head,
bunched together at the top of their arced trajectories because
that is where they are moving the slowest.

-- Jeff, in Minneapolis

Robert Tulip
2009-Feb-13, 07:14 PM
Argh!! Sorry, Robert, but I appear to have totally failed with my explanations. You are still not getting it, and instead are reverting to your original assumptions. The waves are not emanating from the core. The waves are more or less orbiting the core. Over the long term the arms are not stable structures. In other epochs the less dense regions become the more dense regions and vice versa. You say that you can’t help visualizing in a certain way. You’ll just have to stop that and try to visualize in the manner I have described. And despite my earlier description of how friction affects wheels on Earth thus requiring power (added energy) to keep turning in a manner that does not apply to rotating objects in frictionless space, you still want to bring up the analogy of an Earth based car. Perhaps coasting is a term that will help you to understand. Objects in space continue moving or rotating as though they were constantly coasting in the absence of friction. The Earth’s momentum is not powered by the Sun. Its angular momentum remains constant. The Sun’s gravity merely prevents that angular momentum from becoming pure linear momentum which would send the Earth moving on a straight line at constant velocity into interstellar space. The same can be said of Saturn’s gravity in relation to the small satellites that form its rings, and of the entire mass of the galaxy (including the core) on its constituent stars.

If you truly desire to begin developing a deeper understanding, I highly recommend registering in a Physics 101 course. Either that or read an English translation of Isaac Newton’s Philosophiĉ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), often abbreviated to simply Principia.

Thanks again Centaur and Jeff. I appreciate that you can bear with me as I try to match the language of physics with my naive observations. Obviously my use of the term 'power' was wrong because power has a precise technical meaning of 'adding energy'. That wasn't what I was trying to say, rather I was using 'power' in the more general sense of 'enabling' or 'structuring'. Rather like in law, where the 'powers' defined in the Constitution 'enable' what people can do now, in quite a different use of the term from physics. My sense is still that the sun is carrying us along for the ride around the galaxy, structuring the path of the earth. I accept the sun is not adding energy to the earth's momentum, but there is still this broader sense in which earth's movement around the galaxy is a result of the angular momentum of the sun, which contains most of our local mass.

I was quite surprised by Jeff's earlier comment that galaxies don't rotate. This looks like saying rivers don't flow. For example, http://www.physorg.com/news141045997.html uses the term 'the Milky Way's spin'.

http://wiki.answers.com/Q/Does_the_milky_way_spin says "The bar in the center of the spiral rotates every 15 to 18 million years, while the spiral arm pattern (these are a pressure wave effect) rotates every 50 million years. Our sun which is 26,000 light-years form the center, rotates about the galactic core once every 220 million years. Viewed from above, the direction of rotation is anti clockwise."

This last point has me really confused as the physorg.com diagram above seems to show the galaxy moving clockwise. Which is right?

Also, http://en.wikipedia.org/wiki/Milky_Way#Spiral_arms states "astronomers propose that the spiral arms form as a result of a matter-density wave emanating from the galactic center."

Centaur
2009-Feb-13, 09:45 PM
"...Viewed from above, the direction of rotation is anti clockwise."

This last point has me really confused as the physorg.com diagram above seems to show the galaxy moving clockwise. Which is right?



You’re welcome, Robert. The phrase “viewed from above” is meaningless in this context. There is no top or bottom side to the galaxy. What must have been meant is what is seen when viewed from a point on what has been defined as the north side of the galaxy. And that is defined by convention as the side from which viewing from outside makes the rotation appear to be counterclockwise (or anticlockwise as some say). It’s the same with the Earth and the solar system.

“Seems to show” got you again. The appearance of the galaxy rotating clockwise when viewed from the north side is again due to the mistaken analogy of a whirlpool on Earth. Once again, the apparent structure of the arms is not due to a whirling effect, but rather to density waves.

Waves are studied in a physics course. If you and your friend stretch a long slightly loose rope between you, a vertical snap at one end will result in a wave traveling from one side to the other. The rope material from one side did not move to the other. But the wave did.

Robert Tulip
2009-Feb-14, 12:01 AM
Okay, so looking at the 250 light year picture (http://www.atlasoftheuniverse.com/250lys.html), we are headed for Vega, and on the Orion Arm picture (http://www.atlasoftheuniverse.com/5000lys.html) we are headed for Cygnus, apparently moving along the arm in a clockwise direction, from this reference point of galactic north. Why then do people say the galaxy is spinning anti-clockwise?

Hornblower
2009-Feb-14, 01:50 AM
It appears that there are some differences of opinion on how to define "above" or "north" in galactic polar coordinates.

In the views linked in the opening post, the galactic plane is horizontal, and the pole above it clearly is in the direction of Coma Berenices, north of the celestial equator and the ecliptic. To me it is second nature to call it the North Galactic Pole, and Wil Tirion concurred in his Sky Atlas 2000. From this viewpoint the orbital motion is clockwise.

As seen from afar in the direction of Coma, our planets' orbits around the Sun are retrograde with respect to the Sun's galactic orbit. Apparently there was enough turbulence in the primordial nebula to override the sort of rotation I intuitively would have expected.

Jeff Root
2009-Feb-14, 02:11 AM
I was quite surprised by Jeff's earlier comment that galaxies
don't rotate. This looks like saying rivers don't flow.
:)


For example, http://www.physorg.com/news141045997.html
uses the term 'the Milky Way's spin'.
I will point out, in case someone didn't notice, that the illustration
on that page shows a small rectangle that doesn't match what is
shown in the main image. The small rectangle is on the left side of
the galaxy, while the main image is on the right side of the galaxy.
Having pointed that out, it really doesn't matter. The rotation is
clockwise in both the small image and the main image.

The direction of rotation is arms trailing, just like you'd expect.

Larger versions of the image are at:
http://www.eso.org/public/outreach/press-rel/pr-2008/phot-30-08.html



http://wiki.answers.com/Q/Does_the_milky_way_spin
says "The bar in the center of the spiral rotates every 15 to 18 million
years, while the spiral arm pattern (these are a pressure wave effect)
rotates every 50 million years. Our sun which is 26,000 light-years
form the center, rotates about the galactic core once every 220 million
years. Viewed from above, the direction of rotation is anti clockwise."

This last point has me really confused as the physorg.com diagram
above seems to show the galaxy moving clockwise. Which is right?
As Centaur said, the expression "viewed from above" is meaningless,
unless they define what they mean by "above", which they do not.

They also call the spiral arm pattern "a pressure wave effect",
which I think is nonsense. As Centaur and I said before, and as
you quote the Wikipedia article, it is a density wave. There is
no pressure between stars.

-- Jeff, in Minneapolis

Jeff Root
2009-Feb-14, 02:27 AM
As seen from afar in the direction of Coma, our planets' orbits
around the Sun are retrograde with respect to the Sun's galactic
orbit. Apparently there was enough turbulence in the primordial
nebula to override the sort of rotation I intuitively would have
expected.
??? Are you saying that you would have expected the plane of
the orbits of planets in the Solar System to be in some way
related to the plane of the Galaxy??? If so-- NO, NO, NO, NO,
NO, NO, NO! NO connection. :)

Take a look at Hubble images of any giant molecular cloud, and
tell me if you see any hint of it aligning with the plane of the
Galaxy. The orientations are completely random.

-- Jeff, in Minneapolis

Hornblower
2009-Feb-14, 10:46 AM
??? Are you saying that you would have expected the plane of
the orbits of planets in the Solar System to be in some way
related to the plane of the Galaxy??? If so-- NO, NO, NO, NO,
NO, NO, NO! NO connection. :)

Take a look at Hubble images of any giant molecular cloud, and
tell me if you see any hint of it aligning with the plane of the
Galaxy. The orientations are completely random.

-- Jeff, in Minneapolis
I never argued otherwise. I merely acknowledged that the real-galaxy dynamics are much more complex than what I might have naively expected from conservation of angular momentum.