Spiral Structure 101: (APOD 2009 April 14)

[neufer]

http://apod.nasa.gov/apod/ap090414.html
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Origin of the spiral structure

<<The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realised that the idea of stars arranged permanently in a spiral shape was untenable due to the "winding dilemma". Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy, a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem. Or, the stars on the outermost edge of the galaxy would have to move faster than those near the center, as the galaxy rotates. Neither behaviour is observed.

There are two leading hypotheses or models for the spiral structures of galaxies:

* Star formation caused by density waves in the galactic disk of the galaxy.
* The SSPSF model - Star formation caused by shock waves in the interstellar medium.

These different hypotheses do not have to be mutually-exclusive, as they may explain different types of spiral arms.
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. Density waves model

Bertil Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy’s stars and gas. As gas enters a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms.
...........................................................
. Historical theory of Lin and Shu

The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964.

* They suggested that the spiral arms were manifestations of spiral density waves.

* They assumed that the stars travel in slightly elliptical orbits and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic centre. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits.
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. Star formation caused by density waves

The following hypotheses exist for star formation caused by density waves:

* As gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse (the Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars.

* As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.

* As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars.
...........................................................
. More young stars in spiral arms

The arms appear brighter because there are more young stars (hence more massive, bright stars). These massive, bright stars also die out quickly, which would leave just the (darker) background stellar distribution behind the waves, hence making the waves visible.

While stars, therefore, do not remain forever in the position that we now see them in, they also do not follow the arms. The arms simply appear to pass through the stars as the stars travel in their orbits.
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. Alignment of spin axis with cosmic voids

Recent results suggest that the orientation of the spin axis of spiral galaxies is not a chance result, but instead they are preferentially aligned along the surface of cosmic voids. That is, spiral galaxies tend to be oriented at a high angle of inclination relative to the large-scale structure of the surroundings. They have been described as lining up like "beads on a string," with their axis of rotation following the filaments around the edges of the voids.>>

[aristarchusinexile]

Another fine example of Time altered by the rotation of the spiral (Galaxies trailing their spirals,etc.) with the arms being nearer us in time, the centre being farther ahead in time. In fact, it suddenly occurs to me that the Spiral Galaxy is natures model of one of the Time Machines proposed by reputable PHDs, one of those machines being a huge spiral column: the current hindrance to construction of the column being material development and power source. The Spiral Galaxy is not similarly hindered.

[neufer]

Another fine example of Time altered by the rotation of the spiral (Galaxies trailing their spirals,etc.) with the arms being nearer us in time, the centre being farther ahead in time. In fact, it suddenly occurs to me that the Spiral Galaxy is natures model of one of the Time Machines proposed by reputable PHDs, one of those machines being a huge spiral column: the current hindrance to construction of the column being material development and power source.

And it requires concentration or even additional assistance from other artists.

<<Spirograph is a geometric drawing toy that produces mathematical curves of the variety technically known as hypotrochoids and epitrochoids. The Spirograph was invented by British engineer Denys Fisher who exhibited it in 1965 at the Nuremberg International Toy Fair. It was subsequently produced by his company. Distribution rights were acquired by Kenner, Inc., which introduced it to the United States' market in 1966. The name is a registered trademark of Hasbro, Inc.

A Spirograph consists of a set of plastic gears and other shapes such as rings, triangles, or straight bars. There are several sizes of gears and shapes, and all edges have teeth to engage any other piece. For instance, smaller gears fit inside the larger rings, but also can engage the outside of the rings in such a fashion that they rotate around the inside or along the outside edge of the rings.

To use it, a sheet of paper is placed on a heavy cardboard backing, and one of the plastic pieces is pinned to the paper and cardboard. Another plastic piece is placed so that its teeth engage with those of the pinned piece. For example, a ring may be pinned to the paper and a small gear placed inside the ring – the actual number of arrangements possible by combining different gears is very large. The point of a pen is placed in one of the holes in the moving piece. As the moving part is moved the pen traces out a curve. The pen is used both to draw and provide locomotive force; some practice is required before Spirograph can be operated without disengaging the fixed and moving pieces. More intricate and unusual-shaped patterns may be made through the use of both hands, one to draw and one to guide the pieces. It is possible to move several pieces in relation to each other (say, the triangle around the ring, with a circle "climbing" from the ring onto the triangle), but this requires concentration or even additional assistance from other artists.>>

[Peter B Mockridge]

As a totally uninformed person when it comes to astronomy (and most other topics, if the truth is to be known), I wonder what causes the brightness of the center of M101. Thanks

[apodman]

I wonder what causes the brightness of the center of M101.

Lots of stars, with more Population II (old) stars with a red hue than Population I (young) stars with a blue hue.

[Chris Peterson]

I wonder what causes the brightness of the center of M101.

Lots of stars, with more Population II (old) stars with a red hue than Population I (young) stars with a blue hue.

Also, while most of the galaxy is thin and disc-like, the center bulges and is more sphere-like. So not only is the star density higher, but we are looking through a greater depth of stars.

[BMAONE23]

It is actually similar to a likely explanation for the problem created by Olbers' Paradox.. If you look at any given star, you only see the source of the photons that are coming directly toward your point of view, the point source light. Stars appear as point sources because of their apparent angular size WRT the rest of the sky given their distance from us.
Take Sirius for example, Its apparent magnitude of -1.4 makes it the brigtest star in the night sky but when you look at it, its apparent brightness doesn't extend beyond the star into space even though the photons it gives off are still there. We can't see a photon directly. We can see its source (star, lightbulb, fire, etc.) and its effect as it bounces off (lights up) normal matter. Since we can't directly view a Photon, we can only see its source and effect.
The night sky might be brighter if there were enough widely/evenly scattered normal matter to reflect the photonic energy back to our eyes (which there isn't, space is relatively empty) or if the stellar density of open space were similar to that nearing galactic centers.
We see brght galactic central regions because the stellar density (stars per sq LY) is far greater. In our solar neighborhood, the stellar density is about one star per cube that is 3.26 light-years on each side (cubic parsec). At the Galactic core, around 100 parsecs from the Galactic center, the stellar density has risen to 100 per cubic parsec, crowded together because of gravity. This massing of stars is what increases the glow effect.

[TMark.Bosley]

Something I have noticed in many images of spirals.
It seems that the lanes of dark dust are on the trailing edge of the arm, while HII regions and/or OB associations are on the leading edge.
I would expect the following
leading edge-- dust lanes
then HII regions
trailing edge-- OB associations
But I see the opposite (e.g. M51)
http://apod.nasa.gov/apod/ap080614.html
Is my problem in not grasping how a pressure wave moves?

[neufer]

Something I have noticed in many images of spirals.
It seems that the lanes of dark dust are on the trailing edge of the arm, while HII regions and/or OB associations are on the leading edge.
I would expect the following
leading edge-- dust lanes
then HII regions
trailing edge-- OB associations
But I see the opposite (e.g. M51)
http://apod.nasa.gov/apod/ap080614.html
Is my problem in not grasping how a pressure wave moves?

Think of the spiral arms as similar to stationary mountain lee wave clouds:
http://antwrp.gsfc.nasa.gov/apod/ap090121.html

Spiral arm pressure waves are basically stationary concentrations
with the orbiting stars & gas passing through them
(in a clockwise fashion as seen below for the Milky Way):
http://en.wikipedia.org/wiki/File:Milky ... al_Arm.svg

The "leading edge" as experienced from the moving perspective of orbiting stars & gas
IS, in fact, these dust lanes you refer to. Our Sun (moving clockwise) has recently
entered into the dusty "leading edge" of the Orion/Local Spur:
http://antwrp.gsfc.nasa.gov/apod/ap080606.html
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. Star formation caused by density waves

The following hypotheses exist for star formation caused by density waves:

* As gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse (the Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars.

* As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.

* As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars.
...........................................................
. More young stars in spiral arms

The arms appear brighter because there are more young stars (hence more massive, bright stars). These massive, bright stars also die out quickly, which would leave just the (darker) background stellar distribution behind the waves, hence making the waves visible.

While stars, therefore, do not remain forever in the position that we now see them in, they also do not follow the arms. The arms simply appear to pass through the stars as the stars travel in their orbits.

[aristarchusinexile]

Another fine example of Time altered by the rotation of the spiral (Galaxies trailing their spirals,etc.) with the arms being nearer us in time, the centre being farther ahead in time. In fact, it suddenly occurs to me that the Spiral Galaxy is natures model of one of the Time Machines proposed by reputable PHDs, one of those machines being a huge spiral column: the current hindrance to construction of the column being material development and power source.

And it requires concentration or even additional assistance from other artists.

I'm not sure what you mean, Neufer .. but all I need to see the time alteration is to look at the photo. To me it is easily seen, but it took considerable thought to realize why the arms appear closer than the nucleus. Spirographs are plain two D.

[Jyrki]

Art! Thank you so much for the explanation of the spirals. To me they always seemed to be in violation of Kepler's law My misunderstanding was, of course, that I thought the spiral arms to have originally looked like spokes on a wheel that became spiral due to varying rotational speeds. Silly of me.

[apodman]

Photos are flat.

Seeing depth correctly in a photo is a science. It involves a structured interpretation of the data contained in the photo itself, using the photo to supplement other knowledge that is not itself represented in the photo, and using the other knowledge to enhance the interpretation that can be reasonably made from photographic data.

Imagining depth in a photo is an art. It can reveal many possibilities. It can also lead to self-deception and misinterpretation.

Between science and art lie educated guesses. It can be worthwhile to see how far the science can stretch (reach exceeding grasp and all that stuff) and how far conjecture can go while remaining useful.

The art I'd like to see is a simulation of all these stars in their elliptical orbits about the galactic center resulting in the arm structure as we see it. The visualization of the result is plain to see from the photo, the mathematics results in no visualization for me at all, and I'd like to be lazy and sit back and watch a nice video to put the two ends together. Thinking about a Spirograph gives me hope that this is possible. We tried to contact the artist responsible for spiral galaxies to begin with, but the only response has been that mysteries are good for our intellectual curiosity.

And let's not forget about all the invisible dark matter adding to the gravity of the situation. These waves that collect the stars into arms are fueled by more than what we can see in the photo. I think maybe someone is just trying to avoid controversy by not mentioning it.

And I don't get the time machine references. Can someone tell me in plain language what the theory of that is?

[Doum]

I have somem more link that are also interesting:

http://burro.cwru.edu/Academics/Astr222 ... piral.html
http://ircamera.as.arizona.edu/NatSci10 ... alarms.htm

But there are so many so i stop here.

[apodman]

Thanks for those links, Doum. The kind of simple level of explanation I like. Grannys on the freeway.

[aristarchusinexile]

Photos are flat.

Seeing depth correctly in a photo is a science. It involves a structured interpretation of the data contained in the photo itself, using the photo to supplement other knowledge that is not itself represented in the photo, and using the other knowledge to enhance the interpretation that can be reasonably made from photographic data.

The human brain and senses are far beyond our understanding of their capabilities. The simplicity of our 'instincts', even outside the paranormal, are only now beginning to be revealed as simple mechanisms. Take the example of a man or woman walking on a sidewalk and looking with sexual interest at a man or woman walking ahead of them on the sidewalk in the same direction, with the observed person turning around and saying something like, 'so you're enjoying the view, are you?' Those examples are totally commonplace, if not so clearly evident. It turns out simple brainwave function alerts us to each other's interest, but of course that kind of 'instinct' is educated out of us, except for those who don't give in to the enforced diminishment of our most basic physical stimualtions like sex, hunger, appreciation of beauty, curiousity .. all those 'primitive' things which enable us to survive and flourish in hostile environments. So too, is looking at a photo .. our instincts are simply mostly unrecognized abilities of the brain and spirit .. connecting directly with the formally unknown .. in part our brain's use of the invisible electromagnetic waves which are as real and vital part of our senses as optics and sound. If we can see a photo of a galaxy, its electromagnetic signals are all around us, and our brain is totally capable of distinguishing, perhaps especially without a higher formal education which discredits 'ESP', the nature of those signals. I have read that the human mind's brain waves can be detected 200 miles away from their source .. I suggest not only detected, but a mind that far away can easily 'hear' what someone is thinking those 200 miles away .. but of course, our mind's reading of those waves is classified as 'dangerous' imaginings. An ant can jump very high because his society needs the talents of the ants, and those talents are encouraged, whereas our society's rulers need to suppress our talents .. human parents need to supress their children's abilities to make their own abilities seem superior to keep children 'in line' .. our educational systems work the same way. We are supressed on every level. Probably even in ant society ants showing tremendous potential for leadership are cast out by the ruling class. Such a great sorrow for the intellect.

Imagining depth in a photo is an art. It can reveal many possibilities. It can also lead to self-deception and misinterpretation. Between science and art lie educated guesses. It can be worthwhile to see how far the science can stretch (reach exceeding grasp and all that stuff) and how far conjecture can go while remaining useful.

I think all conjecture is useful.

And I don't get the time machine references. Can someone tell me in plain language what the theory of that is?

I can't .. but I read it in one of the several, recently written, PHD authored books on cosmology I read over the winter.

[rigelan]

I like the density wave example of the cars in a traffic jam. The cars from behind slow down because of the traffic, and the cars in front speed up to escape the jam. If this is the case, and these density wave spirals occur in galaxies because of the elliptical orbit of the stars around the center . . .

Then would our own asteroid belt or Kuiper Belt or Oort cloud show these density wave - spiral phenomenon too?

[apodman]

Then would our own asteroid belt or Kuiper Belt or Oort cloud show these density wave - spiral phenomenon too?

I'm taking a wild stab at your question without a lot of consideration, but ...

I'd say yes to density waves but no to spiral formations.

Rather than a spiral phenomenon as in galactic structure, I think maybe loose density waves occur in the Oort cloud and Kuiper belt as part of the mechanism by which the more massive planets perturb objects into different orbits, and I think maybe density waves could be a way to describe the collection of asteroids at trojan points or orbital resonance of planets and moons with smaller objects.

---

Quick and dirty research reveals that some believe there was more of this density wave stuff going on in the solar nebula during the formation of the solar system, and that spiral structure was involved.

---

With regard to spiral phenomena in galactic structure as well as developing solar systems, try looking at:

- Lindblad resonance
- Lindblad torque
- Differential Lindblad torque

[astrolabe]

Hello aristarchusinexile,

The human brain and senses are far beyond our understanding of their capabilities. The simplicity of our 'instincts', even outside the paranormal, are only now beginning to be revealed as simple mechanisms. Take the example of a man or woman walking on a sidewalk and looking with sexual interest at a man or woman walking ahead of them on the sidewalk in the same direction, with the observed person turning around and saying something like, 'so you're enjoying the view, are you?' Those examples are totally commonplace, if not so clearly evident. It turns out simple brainwave function alerts us to each other's interest, but of course that kind of 'instinct' is educated out of us, except for those who don't give in to the enforced diminishment of our most basic physical stimualtions like sex, hunger, appreciation of beauty, curiousity .. all those 'primitive' things which enable us to survive and flourish in hostile environments. So too, is looking at a photo .. our instincts are simply mostly unrecognized abilities of the brain and spirit .. connecting directly with the formally unknown .. in part our brain's use of the invisible electromagnetic waves which are as real and vital part of our senses as optics and sound. If we can see a photo of a galaxy, its electromagnetic signals are all around us, and our brain is totally capable of distinguishing, perhaps especially without a higher formal education which discredits 'ESP', the nature of those signals. I have read that the human mind's brain waves can be detected 200 miles away from their source .. I suggest not only detected, but a mind that far away can easily 'hear' what someone is thinking those 200 miles away .. but of course, our mind's reading of those waves is classified as 'dangerous' imaginings. An ant can jump very high because his society needs the talents of the ants, and those talents are encouraged, whereas our society's rulers need to suppress our talents .. human parents need to supress their children's abilities to make their own abilities seem superior to keep children 'in line' .. our educational systems work the same way. We are supressed on every level. Probably even in ant society ants showing tremendous potential for leadership are cast out by the ruling class. Such a great sorrow for the intellect.

What do you propose to fix things? You know, get everyone to think like you do: that all is tainted and that everyone has been blinded and stripped of anything meaningful or worse. But thank the stars above that we have you to straighten us out or else we would have continued to be just lost drones without one single original thought to call our own. By the way your particular viewpoint on the way of the world (or is it just the U.S.) is IMOPO about as narrow as they come. It is a bit surprising coming from a professed openminded individual, Ari. And even after coming back from my absence and complimenting you on your moped. Tsk, tsk.

[apodman]

With all the velocity changes in a star's orbit around the galactic center as it lingers in spiral arms and then moves on, how smoothly elliptical is an elliptical orbit really? Is it almost perfectly elliptical with a subtle gradient between fast and slow segments, is it a grossly distorted wobbly path that only resembles an ellipse in the large view, or is it something in between? Does a star linger in one arm per orbit or in more than one? How do the answers differ for stars with orbits close to the galactic plane versus stars whose orbits are greatly inclined to the galactic plane? Since stars' orbital velocity does not decrease with distance from the galactic center as we would expect from Newton and Kepler (an effect attributed to dark matter in a galaxy's halo), how well does a star follow the expected attributes of an elliptical orbit anyway?

---

Is the propagation delay for gravity over 10,000s of light years between a galaxy's arms a factor in the creation, sustaining, and/or spacing of the density waves and spiral arms?

[Chris Peterson]

With all the velocity changes in a star's orbit around the galactic center as it lingers in spiral arms and then moves on, how smoothly elliptical is an elliptical orbit really?

I don't think that's quite the right way of seeing it. In this interpretation, each star is in its own elliptical orbit. Its orbital velocity at any time is just what you'd expect from classical orbital dynamics. Stars don't slow down and linger, and then move on (so the comparison with cars in a traffic jam is poor). If you have stars in elliptical orbits with some moderate degree of eccentricity, and the principal axis of those orbits rotate, you can have regions of higher density (the arms) and of lower density (the voids between arms).

[Doum]

Thanks for those links, Doum. The kind of simple level of explanation I like. Grannys on the freeway.

Yea but it is understandable (Wich i also like). I know that our sun is moving above and below a spiral its in while orbiting the galactic center. So it is obviouly more complex then Grannys on the freeway.

P.S. May be we should add that some grannys got lost on the freeway and get stuck in it (spiral). Uhh thats not helping.

[neufer]

Thanks for those links, Doum.

http://burro.cwru.edu/Academics/Astr222 ... piral.html
http://ircamera.as.arizona.edu/NatSci10 ... alarms.htm

The kind of simple level of explanation I like. Grannys on the freeway.

Yea but it is understandable (Which i also like). I know that our sun is moving above and below a spiral its in while orbiting the galactic center. So it is obviously more complex then Grannys on the freeway.

P.S. May be we should add that some grannys got lost on the freeway and get stuck in it (spiral). Uhh thats not helping.

They can't be flocculent grannys, however.
------------------------------------------

Some words in constant use he rarely explained.
I remember once sticking up my hand and asking him in the full flow of description,
"Please, sir, what is flocculent?"

"The precipitate is."

"Yes, sir, but what does it mean?"

"Oh! flocculent! " said my father, "flocculent! Why--"
he extended his hand and arm and twiddled his fingers for a second in the air.
"Like that," he said.

I thought the explanation sufficient, but he paused for a moment after giving it.
"As in a flock bed, you know," he added and resumed his discourse.

------------------------------------------

[aristarchusinexile]

What do you propose to fix things? You know, get everyone to think like you do: that all is tainted and that everyone has been blinded and stripped of anything meaningful or worse. But thank the stars above that we have you to straighten us out or else we would have continued to be just lost drones without one single original thought to call our own. By the way your particular viewpoint on the way of the world (or is it just the U.S.) is IMOPO about as narrow as they come. It is a bit surprising coming from a professed openminded individual, Ari. And even after coming back from my absence and complimenting you on your moped. Tsk, tsk.

I'm sorry you're taking my observations personally and to the extreme, Astro, and that you think I want everyone to think as I do. My observations are merely observations, not dictates, and of course I have no fix even though I would like to see the human intellect and spirit advance, because the human condition is far from totally devoid of merit, but it is terminal .. and not centred on the U.S. My comments on the U.S. educational system were a simple response to a highly overinflated evaluation of U.S. educational merits. If the U.S. and all of humanity were as advanced as they promote themselves we would be living in a far different world: peaceful, prosperous in spirit and health, joyous in song, rivers, skies and air would be clean, crime would be minimal, war would not exist, children would not be lowest caste citizens. Simple observations.

[Peter B Mockridge]

I wonder what causes the brightness of the center of M101.

Lots of stars, with more Population II (old) stars with a red hue than Population I (young) stars with a blue hue.

Also, while most of the galaxy is thin and disc-like, the center bulges and is more sphere-like. So not only is the star density higher, but we are looking through a greater depth of stars.

Thanks - I actually understood that. It begs the next naive question though - what is the diameter of the bright center of 101? APOD is, hands down, my favorite internet site. It reminds me just exactly how anthropocentric our thinking is and just how inconsequential, in the overall scheme of things, we humans (and the earth) are . . . Peter

[apodman]

what is the diameter of the bright center of 101?

The overall diameter of M101 is estimated to be
170,000 light years (1,000,000,000,000,000,000 miles)

Depending on how much of the hub you count as the bright center, its diameter is anywhere from
24,000 light years (140,000,000,000,000,000 miles) to
40,000 light years (240,000,000,000,000,000 miles)

For comparison, our own Milky Way galaxy's estimated diameter is
100,000 light years (600,000,000,000,000,000 miles)

Some might consider my size estimates of the hub a little large, but for the larger number I'm using a proportion from the discoverer's original estimate of 6 to 7 minutes of an arc for what he could see in his 18th century telescope versus 28 minutes overall (almost the size of a full moon), guessing that what can be seen in a small or crude scope is what can be considered bright - I had to draw the line (or circle) somewhere. You can name your own proportion from the picture.

The fact that M101 is almost the size of a full moon but you need a telescope to see it (and a large telescope or time exposure photograph to see brightness and detail) tells you how bright it isn't for the naked eye to see. For comparison, the Andromeda galaxy M31 (which appears several times larger and 3.9 magnitudes - 36 times - brighter than M101) is barely visible to the naked eye as a fuzzball in medium dark skies.