Why Do Galaxies Spin? Exploring the Science Behind It

[Trying2Learn]

TL;DR Summary

Just that question.

I googled it, and almost every source I find says something like this:

"These stars were then gravitationally attracted to each other to create gigantic clusters of stars enshrouded in clouds of gas. Eventually these groupings of stars come together through the attraction of gravity and together they start to spin around a common centre of mass"

But that does not explain the spinning. It only explains that they are moving "directly" toward the center of mass of the galaxy. UNLESS! The center of mass is changing its position?

Some sources say that at the center, there is a massive black hole that spins and it "drags" the other stars with it. But then that (if, of course it is true in all cases) defers my question to: why is the black hole spinning?

Other sources say that the universe was born spinning. I guess that could make sense.

Could someone explain why the galaxy is spiraling?

 

[Orodruin]

All the matter in the universe is not at rest initially. Even if it were gravitational instabilities would result in it not doing so for very long. When a galaxy forms from matter with motion in random directions, chances are that it will have some angular momentum. As the galaxy contracts, conservation of angular momentum means it must spin faster and faster to maintain the angular momentum with a smaller size. (Compare to a skater making a spin speeding up rotation as they pull in their arms but at a much larger scale.)

There are also galaxies that are not dominated by the rotating motion.

 

[Orodruin]

Also, please give specific sources rather than "I read somewhere that".

 

[Trying2Learn]

Also, please give specific sources rather than "I read somewhere that".

Oh, I just googled it, and it came up in nearly every source
https://www.google.com/search?q=why...69i61j69i60.4093j0j4&sourceid=chrome&ie=UTF-8But, you seem to be saying that there must be an initial angular momentum to start this, and that what they mean by "moving toward the center" is the speed up of the angular velocity as the system contracts. That it is like the ice dancer pulling in her arms and spinning faster -- it seems there must be some sort of initial spin.

OK, that makes sense.

Thank you

 

[Orodruin]

Oh, I just googled it, and it came up in nearly every source

That's not how this game works. Please read the PF rules. For us to be able to help you we generally need to know exactly what you have read and which parts you are taking issue with. The reasons you are taking issue may be anything from misunderstanding the text to the text being wrong.

what they mean

Again, who are "they"? You have not given a specific source.

That it is like the ice dancer pulling in her arms and spinning faster -- it seems there must be some sort of initial spin.

Yes, but as I already explained, this can occur due to the random motion of the original constituents.

 

[Trying2Learn]

That's not how this game works. Please read the PF rules. For us to be able to help you we generally need to know exactly what you have read and which parts you are taking issue with. The reasons you are taking issue may be anything from misunderstanding the text to the text being wrong.

Yes, but I gave you the link. Click that link and the exact quote is at the top of the page. It is the exact same quote I italicized.

I only stated that all other links, make the same statement.

I DID follow the rules.

But that is not an issue anymore.

Again, who are "they"? You have not given a specific source.Yes, but as I already explained, this can occur due to the random motion of the original constituents.

 

[Ibix]

Summary: Just that question.

But that does not explain the spinning. It only explains that they are moving "directly" toward the center of mass of the galaxy. UNLESS! The center of mass is changing its position?

If you pick two particles at random in a gas cloud they are probably not on a collision course. They will pass each other and go on their way, which means they had non-zero angular momentum about their center of gravity. What's true of those two particles is true of a very large collection of particles. If they have a little bit of random motion, it's probably not directly towards the center of mass of the system. So there's angular momentum there, and as collapse happens the velocities increase like the spinning dancer, as you noted.

 

[Borg]

Yes, but I gave you the link. Click that link and the exact quote is at the top of the page. It is the exact same quote I italicized.

Google maintains cookies on your browser that affect the results based on your previous history. Your history isn't going to match someone else's history so the results can be different even when you each search for the same phrase.

 

[Orodruin]

Yes, but I gave you the link. Click that link and the exact quote is at the top of the page. It is the exact same quote I italicized.

No you did not. You do realize that Google shows different results for different people making the exact same search, right?

This is the exact quote from the top of the page for me:

Galaxies, stars, and planets all formed from great clouds of cosmic gas and dust. As gravity caused these clouds to collapse, even the smallest bit of rotation was amplified. So it is natural that they all spin.

from here.

So no, you need to provide the actual context, which you did not.

 

[FactChecker]

It would take a lot of luck for any group of objects to have exactly zero spin. Suppose there is even a slight amount of spin. Now suppose those objects are attracted toward a central point. The spin increases and becomes more obvious.

 

[Drakkith]

No you did not. You do realize that Google shows different results for different people making the exact same search, right?

Huh. I wasn't aware of this.

 

[Drakkith]

Some sources say that at the center, there is a massive black hole that spins and it "drags" the other stars with it. But then that (if, of course it is true in all cases) defers my question to: why is the black hole spinning?

Even supermassive black holes are only a tiny fraction of the total mass of a galaxy, so they are certainly not 'dragging' anything along with them.

It would take a lot of luck for any group of objects to have exactly zero spin. Suppose there is even a slight amount of spin. Now suppose those objects are attracted toward a central point. The spin increases and becomes more obvious.

To expand on this, when talking about a group of objects moving solely under the influence of gravity, even a difference in linear velocity can lead to spin. For example, if a group of objects are all moving in a direction with approximate speed, and close enough to start being drawn together under gravity, an object moving slightly faster than the rest of the group will not 'fall' toward the exact center of mass of the group, but will be slightly ahead. The same as true for an object that is moving slightly slower than the average, except it will be slightly behind the center of mass as it falls in. Further collisions and interactions with other members of the group will tend to spread this around, so that even objects that initially had no relative motion with respect to the center of mass now do so. The net result is spin.

Hope that makes sense.

 

[Borg]

Then there's the whole bit about radiation pressure on intersteller gasses due when a start shines and especially when a supernova occurs. Everything adds to the mixing.

 

[phinds]

For example, if a group of objects are all moving in a direction with approximate speed, and close enough to start being drawn together under gravity, an object moving slightly faster than the rest of the group will not 'fall' toward the exact center of mass of the group, but will be slightly ahead. The same as true for an object that is moving slightly slower than the average, except it will be slightly behind the center of mass as it falls in. Further collisions and interactions with other members of the group will tend to spread this around, so that even objects that initially had no relative motion with respect to the center of mass now do so. The net result is spin.

Nicely stated.

 

[DaveC426913]

It might help to visualize this spinning phenom by thinking about how difficult it is to avoid.

Imagine two stellar-sized objects that are, say, 10,000 light years apart. Were they to fall toward each other - even without the influence of hundred of millions of other nearby stars - they would have to fall 5,000 light years to hit a target that is only 5 light seconds wide in order to not end up in an orbit.

That's one part in 30 billion.

That's akin to firing a bullet from a rifle to hit another bullet 30,000km away (and ignoring the howling wind). Make that shot, and you avoid the spinning scenario for two objects.

 

[snorkack]

It might help to visualize this spinning phenom by thinking about how difficult it is to avoid.

Imagine two stellar-sized objects that are, say, 10,000 light years apart. Were they to fall toward each other - even without the influence of hundred of millions of other nearby stars - they would have to fall 5,000 light years to hit a target that is only 5 light seconds wide in order to not end up in an orbit.

But it is trivially easy to not end up in an orbit except in the trivial sense of hyperbolic one.
Gravity is not a sufficient reason to bring distant objects together! Something must slow them down for them to stay together.

That's one part in 30 billion.

That's akin to firing a bullet from a rifle to hit another bullet 30,000km away (and ignoring the howling wind). Make that shot, and you avoid the spinning scenario for two objects.

But the alternative is the objects not getting stuck to each other.
Do galaxies preferentially catch infalling bodies which pass in forward direction and amplify preexisting rotation?

 

[malawi_glenn]

Gravity is not a sufficient reason to bring distant objects together! Something must slow them down for them to stay together.

Like what?

 

[Orodruin]

Like what?

Getting rid of energy by emitting radiation.

 

[malawi_glenn]

Getting rid of energy by emitting radiation.

Like in an accretion disk? The gravitational potential energy is converted to kinetic energy and heat. I can see how heat is generated if there is "friction" amongst the gas particles. What are the other mechanisms for heat generation? If the particles are charged there is radiation emitted due to acceleration, but what if the particles are "big" and electrially neutral? Would each charged constituent (electron, proton) emit such radiation too?

Edit: previously I wrote "syncrotron radiation" but that is just due to acceleration due to rotation. Hence it not the most generic acceleration.

 

[Orodruin]

Like in an accretion disk? The gravitational potential energy is converted to kinetic energy and heat. I can see how heat is generated if there is "friction" amongst the gas particles. What are the other mechanisms for heat generation? If the particles are charged there is radiation emitted due to acceleration, but what if the particles are "big" and electrially neutral? Would each charged constituent (electron, proton) emit such radiation too?

Edit: previously I wrote "syncrotron radiation" but that is just due to acceleration due to rotation. Hence it not the most generic acceleration.

On a microscopic level, any type of collision where radiation can be emitted will do. On a macroscopic level it is just thermodynamics and the assumption that there is a coupling to photons (which will make the system radiate).

On the opposite end, dark matter does not have such an interaction and therefore does not clump in the same fashion as matter does.

 

[Drakkith]

Gravity is not a sufficient reason to bring distant objects together! Something must slow them down for them to stay together.

Remember that galaxies don't exist in a vacuum. Okay they do, but not in the sense that you can just ignore the past history of all the material making up the galaxy. Galaxies are created from enormous clouds of gas (and maybe dust for younger galaxies), and this gas and dust is subject to heating and radiative transfer of energy from collisions between molecules. This transfers energy and momentum from the gas, letting it slow down and clump together. In addition, much of this gas and dust didn't originally exist in a hyperbolic orbit anyways. That is, much of it was already located near to the center of mass of the collapsing system of gas, so even a straight 'fall through' without a collision doesn't let it escape.

 

[snorkack]

There are both spiral (and lenticular) galaxies and elliptical galaxies in the world.
Do elliptical galaxies rotate? If no, why are they elliptical and not exactly globular? If yes, how are they different from lenticular galaxies?
Which are the more common galaxies - spiral or elliptical? And why do both form, rather than just one or the other type?

 

[Drakkith]

Do elliptical galaxies rotate? If no, why are they elliptical and not exactly globular?

According to my very limited knowledge and brief look at wikipedia, it appears that the stars in elliptical galaxies move more in the radial direction, that is, towards or away from the center, than they do around the center like our Sun does here in the Milky Way. So there may be little large scale rotation, but each star still has significant orbital angular momentum around the galaxy.

If yes, how are they different from lenticular galaxies?

Lenticular galaxies still have lots of dust, giving them significant visible detail that can be seen in images. Elliptical galaxies are dust deficient and are smooth and devoid of almost any visible details.

Which are the more common galaxies - spiral or elliptical?

According to table 10 of this paper that surveyed about 14,000 galaxies, 56% were spiral, 18% were lenticular, and 12.2% were elliptical, with the remaining percentage being split between irregular, unsure, or unknown because their images were merged with other galaxies or stars.

And why do both form, rather than just one or the other type?

I don't know and don't have the time to look into that today.

 

[256bits]

Galaxies are created from enormous clouds of gas (and maybe dust for younger galaxies), and this gas and dust is subject to heating and radiative transfer of energy from collisions between molecules. This transfers energy and momentum from the gas, letting it slow down and clump together

how big of a cloud? Larger than the galaxy with stars? same size as the the present size of a galaxy?
One more question.
If it is gas and dust subject to heating from collisions, should not the gas within a galaxy be quite warm, or has it all had a chance to emit the radiation from the heating during the billions of years of life of a galaxy. I had thought that star formation had more to do with pressure waves with a gas rather than the slowing down of tangential velocity around a galactic centre.
I remember some simulations from years ( quite a few years ) ago on galaxy formation from one of the science magazines of the time. should have paid more attention back then,

 

[Drakkith]

how big of a cloud? Larger than the galaxy with stars? same size as the the present size of a galaxy?

Remember that the universe was originally very homogenous, and has subsequently evolved into high-density regions surrounded by large voids. So the original gas clouds must have been much, much larger than the galaxies they collapsed into.

If it is gas and dust subject to heating from collisions, should not the gas within a galaxy be quite warm, or has it all had a chance to emit the radiation from the heating during the billions of years of life of a galaxy.

I don't know enough to answer this, sorry.

I had thought that star formation had more to do with pressure waves with a gas rather than the slowing down of tangential velocity around a galactic centre.

I've heard something similar. But I think the collapse of the original gas clouds into proto-galaxies is the same thing as the collapse of smaller gas clouds into stars, just on a much larger scale of distance and at lower density.

 

[Dragrath]

There are both spiral (and lenticular) galaxies and elliptical galaxies in the world.
Do elliptical galaxies rotate? If no, why are they elliptical and not exactly globular? If yes, how are they different from lenticular galaxies?
Which are the more common galaxies - spiral or elliptical? And why do both form, rather than just one or the other type?

While there are unresolved questions that need answering in general galaxy shape appears to have a lot to do with the overall history of that galaxy i.e. its past interactions. In the very least the large elliptical galaxies appear to form from the merger of two or more off aligned spiral galaxies of comparable sizes while spiral galaxy merger history typically involves the larger galaxy being much more massive than the smaller galaxy allowing the bigger galaxy to tidally strip it apart over billions of years. In the case of our primary examples which we have fairly detailed merger histories for the Milky Way and to a lesser extent Andromeda there appears to be a close linkage between galaxy mergers the spiral arm structure and surprising degrees of nonlinear star formation even if the mechanisms behind it are poorly understood.

The discoveries enabled by ESA's GAIA missions precise astrometry have really opened a whole new world of galactic archeology including identifying that the collision between the Milky Way and the Sagittarius (Sgr) dwarf spheroidal (dSph) galaxy 5.8 Ga seems to have set off one of our galaxies largest starburst episodes the upper trailing end of which coincides with the formation of our solar system.
https://www.esa.int/Science_Explora...ash_may_have_triggered_Solar_System_formation.

Lenticular galaxies are somewhat trickier to classify in that they are more of a transitional category on a spectrum between spiral and elliptical.

 

[sophiecentaur]

It would take a lot of luck for any group of objects to have exactly zero spin. Suppose there is even a slight amount of spin. Now suppose those objects are attracted toward a central point. The spin increases and becomes more obvious.

This true. Take a massive nebula, which can be identified by the fact that, locally, its density is high, relative to the surrounding 'more empty' space. 'Locally', all its content will be pulled towards its CM and other stuff will be pulled elsewhere. Now consider that Angular Momentum depends on the square of the distance from the origin. The distances across large nebulae are massive - much greater than a solar system, formed from some of this mass. So even when the nebula or part of it, is revolving about its CM once in a hundred million years, by the time the majority of the nebula has collapsed to, say, a few light weeks, the mean ω of the central parts will be very significant - significant enough to prevent a planetary disc from ever getting into the centre.
The angular momentum of the Sun is only around 4% of the total angular momentum of the rest of the solar system. That statistic doesn't rely on any guesswork or extrapolation but it comes from some pretty ancient measurements. The same thing must apply to the [Edit: L] of anybody at the centre of a galaxy - it's the distance that counts.

 

[Contrarian]

Summary: Just that question.

I googled it, and almost every source I find says something like this:

"These stars were then gravitationally attracted to each other to create gigantic clusters of stars enshrouded in clouds of gas. Eventually these groupings of stars come together through the attraction of gravity and together they start to spin around a common centre of mass"

But that does not explain the spinning. It only explains that they are moving "directly" toward the center of mass of the galaxy. UNLESS! The center of mass is changing its position?

Some sources say that at the center, there is a massive black hole that spins and it "drags" the other stars with it. But then that (if, of course it is true in all cases) defers my question to: why is the black hole spinning?

Other sources say that the universe was born spinning. I guess that could make sense.

Could someone explain why the galaxy is spiraling?

If 2 bodies move toward each other, but not directly, as in center to center, they have angular momentum. Perhaps the ones moving directly toward each other formed the Black holes.

 

[phinds]

Perhaps the ones moving directly toward each other formed the Black holes.

Sigh. You really think black holes never spin?

You really should learn some basic cosmology if you are going to post about it. You have three posts ** now and all three contain your own personal theories, divorced from actual science.

** EDIT: Make that 2 posts. Your thread with personal theory was removed.

EDIT AGAIN: Make that 1 post. I see one of your other personal theory posts was also deleted.

@Contrarian you probably didn't read the forum rules before posting. This is not one of those causual social media forums where you can post pretty much anything you want. This is a serious science forum with the specific charter of discussing mainstream physics. Personal theories are explicitly prohibited.

 

[Drakkith]

Perhaps the ones moving directly toward each other formed the Black holes.

Unlikely. The black holes in the centers of galaxies are likely formed by complex gravitational interactions that causes more and more matter to fall into them over time, especially in the early stages of galaxy formation where there is an abundance of matter in the center of proto-galaxies in the form of gas.

 

[sophiecentaur]

If 2 bodies move toward each other,

What two bodies will do is no basis for predicting how a galaxy can form. If you take just two bodies, in the absence of anything else, then they will be in an orbit with each other (unchanging and lasting for ever) or else they may collide / have collided (if their trajectories coincide exactly). But that is an unreal and theoretical situation. In the presence of many other objects, there will be interactions between all the objects in the region and the result will be that they gather together in a local region.

More collisions / very near misses will occur causing Kinetic Energy to be lost (but not Angular Momentum). This produces clumping of material with higher angular velocity around the same position of the local centre of mass (which will also be moving linearly).

 

[FactChecker]

What two bodies will do is no basis for predicting how a galaxy can form. If you take just two bodies, in the absence of anything else, then they will be in an orbit with each other (unchanging and lasting for ever) or else they may collide / have collided (if their trajectories coincide exactly). But that is an unreal and theoretical situation. In the presence of many other objects, there will be interactions between all the objects in the region and the result will be that they gather together in a local region.

I took his statement as more of a simple statement of why some angular momentum is much more likely than none. I did not take it as trying to explain the entire theory of how galaxies formed.

 

[sophiecentaur]

I did not take it as trying to explain the entire theory of how galaxies formed.

Once you get me started . . . .
I was just repeating the message about angular momentum, basically. And, of course, the likelihood that a massive cloud of 'stuff' could ever have zero angular momentum is pretty damn low. Plus, there's a maximum value of net angular momentum because the outermost parts would have escaped if they were going too fast past the CM (hyperbolic orbit).

 

[snorkack]

To think of it, if an elliptical galaxy possesses a central black hole then the individual stars in the galaxy must possesses angular momentum - otherwise they would fall in on the first orbit.
How precisely are the elliptical galaxies devoid of combined angular momentum?

 

[sophiecentaur]

How precisely are the elliptical galaxies devoid of combined angular momentum?

Why should they be devoid? These so-called spiral arms in our galaxy are not like the spirals you see in water going down a plughole. This link discusses them quite fully. The arms are density waves and stars move in and out of them, not 'orbiting' at the same rate that the arms rotate.

Needless to say, it's a non-intuitive business.