Why do galxies spiral?

[mo wood]

[sorry "galaxies"!]

So there's the density wave and a new theory.

[JulianO]

I think the density wave theory is still the current best bet. Its difficult to picture how it works though.

What is true is that its not the simplistic idea of stars dragging behind in a sort of trail, as these would very quickly wind up and the spiral would be lost.

[spaceboy]

Just a thought and I have a vivid imagination. I'm no physicist so please be gentle if this sounds stupid. Think of an atom. By it's very nature the protons and neutrons are rotating around its nucleus. Everything in the universe comprises of atoms. So as it is said there is a very dense black hole at the center of all galaxies is it possible that in the beginning some atoms due to their incredible density naturally vibrate or spin due to the protons and neutrons encircling the nucleus. Being so dense gravity then lures yet more and more atoms becoming more and more dense and rotating yet more and more. The more dense the core becomes the stronger gravity and larger and larger objects are drawn to the natural rotation ?? Depending on the density of the original atoms would determine size of the galaxies and the rotation would determine the shape of the galaxy. Obviously we are talking atoms that are infinitesimal and cannot be seen and talking about billions of years to become a fully blown galaxy but everything is comprised of atoms. Even the dust clouds of nebula's are atoms and they one day become gigantic suns burning for millions of years. We are obviously talking on massive scales for this to work and hence why we aren't spinning around that is other than each planet rotates on it's own axis that just happens to be spinning around it's own nucleus the sun. Almost a similar way to atoms?

[JulianO]

Just a thought and I have a vivid imagination. I'm no physicist so please be gentle if this sounds stupid. Think of an atom. By it's very nature the protons and neutrons are rotating around its nucleus.

A few comments:

The protons and neutrons are the nucleus, so they don't rotate around it, they are it.

Everything in the universe comprises of atoms. So as it is said there is a very dense black hole at the center of all galaxies is it possible that in the beginning some atoms due to their incredible density naturally vibrate or spin due to the protons and neutrons encircling the nucleus. Being so dense gravity then lures yet more and more atoms becoming more and more dense and rotating yet more and more.

As atoms get bigger they become more unstable. The force that holds together the protons is naturally struggling against the fact that two positive charges repel. So all protons naturally repel each other. Thus it gets more and more difficult to make bigger nuclei, as the strong force holding the neutrons and protons together is finding more and more positive charge to try and hold together. By the time you get up to Uranium and beyond, its a losing battle. All such atoms naturally fall apart sooner or later, in a process we call radioactivity.

The more dense the core becomes the stronger gravity and larger and larger objects are drawn to the natural rotation ??

This more like a black hole, where when you get massive amounts of atoms together, the extremely weak force of gravity finally has some effect and overcomes most other forces.

Depending on the density of the original atoms would determine size of the galaxies and the rotation would determine the shape of the galaxy. Obviously we are talking atoms that are infinitesimal and cannot be seen and talking about billions of years to become a fully blown galaxy but everything is comprised of atoms. Even the dust clouds of nebula's are atoms and they one day become gigantic suns burning for millions of years. We are obviously talking on massive scales for this to work and hence why we aren't spinning around that is other than each planet rotates on it's own axis that just happens to be spinning around it's own nucleus the sun. Almost a similar way to atoms?

It's a nice idea, but doesn't really square with modern physics.

[ollypenrice]

I don't follow the new theory. For the density wave model my mental picture, which may be wrong, is not to think of spiral arms as entities but as those parts of a larger entity, a disk, which happen to be lit up.

Imagine a circular board covered by evenly spaced light bulbs all switched off. A galaxy. There are no spiral arms in there, it is just a disk. Now contrive some switching so that some of the light bulbs switch on, creating a spiral pattern of illumination. By manipulating the switching you can make the spiral of illumination rotate even though the board itself is fixed. Or you could rotate the whole board and use the switching to create a spiral of light which did not rotate but remained fixed.

The analogy is not perfect because the bulbs (representing the gas and stars) should rotate as they do in a galaxy. However, the key point is to uncouple the illumination from the matter. The density wave is represented by the switching mechanism which 'passes through' the disk of light bulbs.

As I understand it there is less difference between the matter density within and without the spiral arms of galaxies than would appear, it is just that the passage of the density wave through the disk is enough to trigger star formation and 'switch on' the spiral of illumination.

If I'm up a gum tree in this analogy do please put me right!

Olly

[JulianO]

I don't quite follow it. The stars don't switch on and off, well certainly not fast enough to make spiral arm patterns. I think I may not be understanding your analogy too well.

I was at a talk the other day explaining how stars migrate from inner to outer parts of the galactic disk, which can be seen from their metalicity and where they should be based on that compared to where they actually are. So I think there is reasonable evidence for star migration in galaxies, so there may be something to this new theory.

[DarkSky71]

Spiral galaxies rotate around their central bulge. Elliptical galaxies dont rotate. Their stars have random motions.

In Spirals the stars rotate around the core due to the same rotational forces that cause moons to go round planets and planets round stars. However the total mass of all visible parts of a spiral does not produce enough gravity to maintain this rotation. This is one piece of evidence for dark matter which makes up the majority of the mass of a galaxy and provides the gravity needed. Even though the stars in the spiral arms are rotating around the centre they do not remain in the spiral arms. Density waves rotate around the centre causing new star formation as they go so new stars appear in the arms and existing ones leave the arms. All be it over long time periods.

[Telrad]

Spiral galaxies, like our own, owe their shapes to stars inside the protogalaxy developing at different intervals. The gas between forming stars continues to be compressed, and the resulting gravitational differences act on the protogalaxy's stars, dust and gas. This motion forces everything into a rotating disc and additional differences in gravity build the spiral arms. Oppositely, when a protogalaxy's stars develop at the same time, you have an elliptical galaxy.

[bamus]

I was always wandering if galaxies spins mostly clockwise or anticlockwise? Is it random or maybe there are some rules making one of the way more frequent in universe?

[JulianO]

I think that question is one galaxy zoo is looking into.

However a clockwise spiral viewed from the other side is an anticlockwise spiral.

[bamus]

I However a clockwise spiral viewed from the other side is an anticlockwise spiral.

good point. question is then where to place referene point from which we could check which way they spin.

[JulianO]

I think we're stuck with one reference point for the forseeable future :-)

[DarkSky71]

Spirals can be seen to rotate in either direction , but the spiral arms trail behind the rotation, not lead. There are some that may be going the opposite way but it is believed something dramatic happened to them to change it.

[spaceboy]

A few comments:

The protons and neutrons are the nucleus, so they don't rotate around it, they are it.

As atoms get bigger they become more unstable. The force that holds together the protons is naturally struggling against the fact that two positive charges repel. So all protons naturally repel each other. Thus it gets more and more difficult to make bigger nuclei, as the strong force holding the neutrons and protons together is finding more and more positive charge to try and hold together. By the time you get up to Uranium and beyond, its a losing battle. All such atoms naturally fall apart sooner or later, in a process we call radioactivity.

This more like a black hole, where when you get massive amounts of atoms together, the extremely weak force of gravity finally has some effect and overcomes most other forces.

It's a nice idea, but doesn't really square with modern physics.

Thanks Julian. My usual day dreaming which I have no excuse for. Should have paid more attention in school me thinks.

One more thing you could put me right on.

A recent thread brought to my attention that space is like a fabric. Very dense objects weigh on the fabric of space IE: Black holes that are so dense not even light can escape. Gravity sucking away at everything around it as the fabric of space is distorted. Now I know you said the whole atom idea put forth wouldn't work but there are a lot of things that are spinning in the universe us for one. Now how that is I don't know as I explained I'm no physicist. But is it possible what ever made us spin is also making the black hole spin. Now think of a piece of fabric on a table and if you try to drill the center the fabric will twist. In a similar way to water going down a plug hole. The drill bit being the rotation of a dense object. Or the plug hole and water being gravity and rotation. I understand the rotation at the plug hole is due to the earth and the drill is under external power but only meant as a poor examples from a not too cleaver person

I really don't know why I bother with these threads as most of it goes over my head but I do pick the occasional thing up which is better than nothing I guess

[JulianO]

A recent thread brought to my attention that space is like a fabric. Very dense objects weigh on the fabric of space IE: Black holes that are so dense not even light can escape. Gravity sucking away at everything around it as the fabric of space is distorted.

You can think of space as a fabric, a huge flat rubber mat is the usual analogy. If you roll a marble across the mat, it rolls nice and true. Now you put a weight on it, the mat sags under the weight of that making a dip. Now roll a marble, and if it gets near the weight, it tends to bend around the mass a bit so no longer rolls straight. Thats basically general relativity, mass bends space, so things that go in straight lines, even light, bend.

A black hole is just such a heavy weight, that it actually makes a hole in the mat. If the marble rolls by the black hole it bends. If it goes directly towards the black hole - it drops out of sight, never to be seen again.

Now I know you said the whole atom idea put forth wouldn't work but there are a lot of things that are spinning in the universe us for one. Now how that is I don't know as I explained I'm not physicist. But is it possible what ever made us spin is also making the black hole spin. Now think of a piece of fabric on a table and if you try to drill the center the fabric will twist. In a similar way to water going down a plug hole. The drill bit being the rotation of a dense object. Or the plug hole and water being gravity and rotation. I understand the rotation at the plug hole is due to the earth and the drill is under external power but only meant as a poor examples from a not too cleaver person

I think its clear most black holes rotate, because they are formed from rotating suns, and rotating things keep on rotating unless you can put a break on them.

There was a recent satellite experiment that showed that the earth actually drags space around a bit with it. It's very subtle, but has been shown to work.

[spaceboy]

You can think of space as a fabric, a huge flat rubber mat is the usual analogy. If you roll a marble across the mat, it rolls nice and true. Now you put a weight on it, the mat sags under the weight of that making a dip. Now roll a marble, and if it gets near the weight, it tends to bend around the mass a bit so no longer rolls straight. Thats basically general relativity, mass bends space, so things that go in straight lines, even light, bend.

A black hole is just such a heavy weight, that it actually makes a hole in the mat. If the marble rolls by the black hole it bends. If it goes directly towards the black hole - it drops out of sight, never to be seen again.

I think its clear most black holes rotate, because they are formed from rotating suns, and rotating things keep on rotating unless you can put a break on them.

There was a recent satellite experiment that showed that the earth actually drags space around a bit with it. It's very subtle, but has been shown to work.

So I've loosely got it right that black holes and most things like suns rotate and that dense gravitational black holes bends the fabric fabric/rubber of space and anything near to it would also follow the same bend in the fabric and would travel towards the sag. So if the dense gravitational pull that sags space is also rotating is this not how spiral galaxies form ? And depending on the dense gravitational pull and where it sits in space would determine why some galaxies are ecliptic or spiral ?? Sorry for being such a dumb a** turning the thread in to a primary school lesson. I promise this is the last question

[JulianO]

So I've loosely got it right that black holes and most things like suns rotate and that dense gravitational black holes bends the fabric fabric/rubber of space and anything near to it would also follow the same bend in the fabric and would travel towards the sag. So if the dense gravitational pull that sags space is also rotating is this not how spiral galaxies form ? And depending on the dense gravitational pull and where it sits in space would determine why some galaxies are ecliptic or spiral ??

Well I think the bending of space is pretty localised, so although theoretically the black hole at the centre of our galaxy bends space a little bit where we are, in practice its completely negligable. Its offset by other star masses and things like dark matter. Within a light year or two I think the effect of an individual thing is so diminished it doesn't do too much in terms of the overall structure of the galaxy. Nor does the dragging effect. The shapes are probably much more related to how they formed, or what they ran into recently. Also most of the mass is actually in the dark matter halo, not in the stars or black holes or things we can see.

To summarise: the dip in space makes a nice bowl shaped playground for marbles to roll around in, but exactly how they roll around in it depends how you flicked them into the bowl in the first place.

Sorry for being such a dumb a** turning the thread in to a primary school lesson. I promise this is the last question

There are no dumb questions - except the unasked ones.

[spaceboy]

Thank you for your time answering my questions Julian and sorry for hijacking the thread Mo wood.

[mo wood]

Spiral galaxies, like our own, owe their shapes to stars inside the protogalaxy developing at different intervals. The gas between forming stars continues to be compressed, and the resulting gravitational differences act on the protogalaxy's stars, dust and gas. This motion forces everything into a rotating disc and additional differences in gravity build the spiral arms. Oppositely, when a protogalaxy's stars develop at the same time, you have an elliptical galaxy.

But then:

"Spiral galaxy NGC 6384 taken by Hubble Space Telescope.The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized 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 (via a standard solar system type of gravitational model), 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. Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain the stability of the spiral structure."

Wikipedia

So the winding problem is about how galaxies retain their spiral arms over time.

And thus how do we know that your account is a sufficient explanation of how spiral galaxies formed in the first instance?

[George Jones]

And thus how do we know that your account is a sufficient explanation of how spiral galaxies formed in the first instance?

How do we know that the work at the link in the original post is sufficient? The link that you gave does not have a reference to a peer-reviewed journal article, nor can I find an arXiv paper, either in preliminary or accepted for publication form.

I do think that the findings are very interesting, but they seem to be at a very preliminary stage in the scientific process, and I would like to know how the results will be/ are being taken by the scientific community.

[meteoriot]

I don't follow the new theory. For the density wave model my mental picture, which may be wrong, is not to think of spiral arms as entities but as those parts of a larger entity, a disk, which happen to be lit up.

Imagine a circular board covered by evenly spaced light bulbs all switched off. A galaxy. There are no spiral arms in there, it is just a disk. Now contrive some switching so that some of the light bulbs switch on, creating a spiral pattern of illumination. By manipulating the switching you can make the spiral of illumination rotate even though the board itself is fixed. Or you could rotate the whole board and use the switching to create a spiral of light which did not rotate but remained fixed.

The analogy is not perfect because the bulbs (representing the gas and stars) should rotate as they do in a galaxy. However, the key point is to uncouple the illumination from the matter. The density wave is represented by the switching mechanism which 'passes through' the disk of light bulbs.

As I understand it there is less difference between the matter density within and without the spiral arms of galaxies than would appear, it is just that the passage of the density wave through the disk is enough to trigger star formation and 'switch on' the spiral of illumination.

If I'm up a gum tree in this analogy do please put me right!

Olly

How about water ripples in a pond. The water itself isn't travelling with the ripple but the ripple still moves across the pond. So the stars aren't moving with the spiral arm etc etc

[JulianO]

How about water ripples in a pond. The water itself isn't travelling with the ripple but the ripple still moves across the pond. So the stars aren't moving with the spiral arm etc etc

Well sort of, the animation on the wiki page gives a good visual idea. If you focus on an individual star you can see it moves between spirals. Jumping from one density to another. To me its not intuitive, but then not all physics is.

[toml42]

I think olly has the right idea, spiral density waves lead to increased rates of star-formation in the 'arms', and it's the short lived giant stars that are the most luminous, and make up the majority of the luminosity in the arms.

The young, luminous stars blow themselves up in supernova on a timescale much shorter than the rotation of the density wave, meaning that we see what appears to be a structure, but is in fact only a region of slightly enhanced density, and the most recently formed stars.

[mo wood]

But then does any theory of spiral galaxies really explain the origin of the spiral structure?

So why do S0 or lenticular galaxies look prety much like spirals in that they have a quite thin disk but have no spiral arms?

And why do SBc galaxies have such a long central bar with only two whisps of spiral arms at each end?

[mo wood]

http://alfalfasurvey.wordpress.com/2009/03/26/a-galaxy-primer/