<p>The prior gas cloud will start with some small random motions and density variations left over from its  formation. As a patch starts to contract any small amount of rotation gets amplified as the cloud collapses, because of the conservation of angular momentum.</p>

CC BY-SA 4.0
2020-11-18T06:41:49.633Z
2020-11-18T08:33:32.310Z
2020-11-18T08:33:32.310Z
blanci
<p>The answer is turbulence. Stars are formed from large gas clouds that collapse under their own weight. As they do so, they become unstable to fragmentation and break up into smaller collapsing pieces. Turbulence in the original cloud means that each of these pieces has its own individual angular momentum and rotational energy, even if the total angular momentum of the original cloud was zero. The original turbulence can be injected in many ways, including gravitational interactions and collisions with other clouds, the tidal effects of the galaxy, shocks from supernovae, the winds of massive stars, etc.</p>
<p>As the collapse proceeds, dissipative interactions in the collapsing cloud cause it to radiate away energy, but very little angular momentum. Both quantities must be conserved, so the system tends towards a configuration with minimum energy for a fixed amount of angular momentum - which is a rotating disk.</p>
<p>In order to accrete into the central protostar, the material in the disk has to somehow lose angular momentum in order to fall inwards. It does this by transferring angular momentum outwards through various viscous processes. Ultimately though, some of the disk material has to escape with the angular momentum to allow some of the material to accrete into the star. There are a number of processes that can achieve this including &quot;disk winds&quot; and photoevaporation.</p>

CC BY-SA 4.0
2020-11-23T18:49:46.867Z
I wonder if there is a small Coriolis effect due to the rotation of the cloud around the center of the galaxy.
Jack R. Woods
CC BY-SA 4.0
2020-11-18T07:51:29.810Z
2020-11-18T07:51:29.810Z
ProfRob
<p>Other answers good, but this question left behind:</p>
<blockquote>
<p>Why isn't all the disk material been swallowed by the star?</p>
</blockquote>
<p>Because of the excess of angular momentum carried by some particles. They insist on orbiting the star instead of falling directly to it.</p>
<p>There is a complex process called <a href="https://en.wikipedia.org/wiki/Accretion_(astrophysics)" rel="noreferrer">accretion</a>. The disk constantly transfers angular momentum to the outside, material to the inside and thermal radiation away. In most cases, some material is also blown away in polar jets.</p>
<p>The physics is the same, no matter if it is a galaxy, a protoplanetary disk, a shiny disk around a neutron star or a black hole, or something smaller like a planetary ring. Well, the scales differ.</p>
<p>There are three catches:</p>
<ol>
<li><p>For the accretion to work, one needs some friction between the concentric rings of material. Easy when you have dense enough gas or plasma and stops working when you don't have gas anymore. Dust works somehow, large rocks (planets, moons) - not really.</p>
</li>
<li><p>Some local disturbances in the accretion process may form local mini-accretions in the disk. They form planets/moons that &quot;steal&quot; some material from the star.</p>
</li>
<li><p>Light - once the star is bright enough to reach <a href="https://en.wikipedia.org/wiki/Eddington_luminosity" rel="noreferrer">Eddington limit</a>, the light pressure over the accreting gas overcomes the gravity and the gas gets blown away. The star stops growing and its planets stop growing, too. The dust leftovers get gravitationally stirred until most particles either coalesce over the star and the planets, or get an escape orbit and, well, escape.</p>
</li>
</ol>

CC BY-SA 4.0
2020-11-18T12:37:19.080Z
2020-11-18T12:37:19.080Z
fraxinus
<p>Answers, in order:</p>
<p><strong>Why did the protoplanetary disk start rotating around the star?</strong></p>
<p>Star and disk form around the same time.<br />
The rotation forms because all other orbits will cross the orbit of the disk and collide. Put another way, the rotation disk is the leftover movement of particles after all symmetrical factors have cancelled each other out in collisions (inelastic ones, elastic ones do not average out anything).</p>
<p><strong>Where did the initial energy to rotate come from?</strong></p>
<p>From the potential energy of the particles that were far away from the center of gravity when the dust/gas could started collapsing.</p>
<p>This also explains why the disk is rotating faster than the original dust cloud: As masses go to the center of a rotation, they rotate faster (conserve angular momentum).</p>
<p><strong>Why isn't all the disk material been swallowed by the star?</strong></p>
<p>I couldn't answer that better than @fraxinus has.</p>

CC BY-SA 4.0
2020-11-22T04:49:05.217Z
Another reason why all the disk material isn’t swallowed by the star is that the star has a “wind” of outward-flowing particles, which does exert a “pressure” on the material outside the star. While this wouldn’t stop large particles or planets from falling inward, it’s enough to keep smaller particles from spiraling inward.
Pierre Paquette
CC BY-SA 4.0
2020-11-22T07:59:19.533Z
@PierrePaquette Not sure what you mean, the solar wind is exactly what fraxinus said.
toolforger
CC BY-SA 4.0
2020-11-22T18:48:49.917Z
Negative; fraxinus mentions light pressure, but the stellar wind is made up of protons and electrons, not only photons, although you are right in saying it has the same effect.
Pierre Paquette
CC BY-SA 4.0
2020-11-23T08:23:30.537Z
Ah right, solar wind is more than just the photons, true. Maybe edit a correction into his answer?
toolforger
CC BY-SA 4.0
2020-11-18T12:47:14.630Z
2020-11-18T12:47:14.630Z
toolforger
<p>Summarizing/adding to previous specific answers:</p>
<p>Why did the protoplanetary disk start rotating around the star?</p>
<p>The disc forms because it was rotating. Without net rotation it wouldn’t form a disc... it will stay a spherical symmetric cloud.</p>
<p>Where did the initial energy to rotate come from?</p>
<p>A small angular momentum NOT energy is required the “angular energy” gets magnified during gravitational infall while conserving angular momentum like the figure skater spins faster pulling arms in with small initial rotation.</p>
<p>Why hasn't all the disk material been swallowed by the star?</p>
<p>As it speeds up it is basically orbiting faster and faster until centrifugal force balances gravity. Equilibrium.</p>

CC BY-SA 4.0
2021-03-06T09:10:25.483Z
2021-03-07T13:57:35.860Z
2021-03-07T13:57:35.860Z
blanci
<p>Two rocks placed in space with no relative motion are going to be attracted by gravity, and hit. 3 rocks, placed in space with no carefully rigged symmetry, will likely miss each other, as the gravitational attraction of the additional rock changes their course. Those near misses are the beginning of rotation. Multiply that effect by trillions, and you have a protoplanetary disk. Of course, a lot of those molecules, dust and rock won't even start out with zero relative velocity, so near misses are inevitable.</p>

CC BY-SA 4.0
2020-11-17T16:35:33.727Z
+1 upvote, I like this answer, short and sweet.  The energy comes from gravitational interactions!  Just like big swirling eddies on the Colorado River (also caused by gravity), the universe can be a turbulent place.  Even if a dust cloud is rotating ever so slowly, it speeds up as it coalesces under gravity, like a spinning figure-skater speeds up as she pulls her limbs closer to herself!
Connor Garcia
CC BY-SA 4.0
2020-11-17T21:58:26.353Z
Stated another way, it would take an incredible coincidence for all the random motions of the material drawn into the forming star system to perfectly cancel out and produce a non-rotating star.
Christopher James Huff
CC BY-SA 4.0
2020-11-18T21:23:39.687Z
Although with a random n-body starting state, one would expect an equal tendency for matter to rotate in either direction relative to an axis pointing in a random direction, with the eventual angular momentum being "small". I suppose that the interesting question is what "small" means in this context.
Mark Morgan Lloyd
CC BY-SA 4.0
2020-11-18T21:32:48.150Z
@MarkMorganLloyd I think that assumes that the N bodies all collapse simultaneously. If we instead consider a nucleation site of just 2 particles which happen to be closer and slower than the rest, and a cascade emanating from there, then one small initial bias gets magnified over the whole mass.
Lawnmower Man
CC BY-SA 4.0
2020-11-18T21:41:48.457Z
@LawnmowerMan OK I see what you're getting at, but the direction would still be random.
Mark Morgan Lloyd
CC BY-SA 4.0
2020-11-18T22:09:25.150Z
Although it sounds good, this is not correct. The rotational momentum of the system is _constant_, and doesn't 'come' from gravity or anything else. It exists within the dust cloud, and just becomes more observable when it condenses.
Aganju
CC BY-SA 4.0
2020-11-19T22:38:06.133Z
@Aganju But the rotational momentum of our Solar System's dust cloud wasn't constant, since it was being pulled by gravity in various other directions from other bodies in the Milky Way.  If you plopped a cloud of gas with no angular momentum into orbit around the Milky Way center, it would pick up angular momentum as it collapsed, since the gas closer to the Milky Way Center would be orbiting faster than the gas farther out.
Connor Garcia
CC BY-SA 4.0
2020-11-20T12:57:21.770Z
This explanation doesn't make sense to me. By conservation of angular momentum, if three masses accelerating past each other in a Newtonian sense (i.e. traveling on geodesics past each other in a relativistic sense) are "the beginning of rotation", then you're saying that placing multiple masses anywhere in space with zero initial velocity imbues the whole system with angular momentum.
Luke Hutchison
CC BY-SA 4.0
2020-11-20T15:13:00.530Z
Gravity imbues the objects with large potential energy. Since there's no friction, that results in movement.
Wayfaring Stranger
CC BY-SA 4.0
2020-11-20T15:53:30.653Z
@LukeHutchison Even if the entire system angular momentum of the universe was zero, it wouldn't be zero at a local dust cloud.  Gravity causes the system to be locally turbulent.
Connor Garcia
CC BY-SA 4.0
2020-11-17T15:59:05.157Z
2020-11-17T15:59:05.157Z
Wayfaring Stranger
<p>Planets form from a protoplanetary disk that has been rotating around its star. The initial energy that makes them rotate really matters to me.</p>
<ul>
<li>Why did the protoplanetary disk start rotating around the star?</li>
<li>Where did the initial energy to rotate come from?</li>
<li>Why hasn't all the disk material been swallowed by the star?</li>
</ul>

CC BY-SA 4.0
2020-11-17T16:01:16.453Z
Are you familiar with the conservation of [angular momentum](https://en.wikipedia.org/wiki/Angular_momentum)?
PM 2Ring
CC BY-SA 4.0
2020-11-17T22:23:18.323Z
Technically, the dynamic/kinetic energy comes from the gravitational potential energy of the original dust clouds.
RBarryYoung
<orbit><planet><solar-system><gravity><proto-planetary-disk>
What makes protoplanetary disks start rotating? (Initial energy needed to rotate)
Farid Rjb
# What makes protoplanetary disks start rotating? (Initial energy needed to rotate)

(Asked by: username_0 on 2020-11-17)

Planets form from a protoplanetary disk that has been rotating around its star. The initial energy that makes them rotate really matters to me.

* Why did the protoplanetary disk start rotating around the star?
* Where did the initial energy to rotate come from?
* Why hasn't all the disk material been swallowed by the star?

**Tags:** orbit, planet, solar-system, gravity, proto-planetary-disk
> **Comments:**
> **username_1**: Are you familiar with the conservation of [angular momentum](https://en.wikipedia.org/wiki/Angular_momentum)?  
> **username_2**: Technically, the dynamic/kinetic energy comes from the gravitational potential energy of the original dust clouds.  

---

## 6 answers

### Answer by username_9 on 2020-11-17. Score: 49 | Accepted answer

Two rocks placed in space with no relative motion are going to be attracted by gravity, and hit. 3 rocks, placed in space with no carefully rigged symmetry, will likely miss each other, as the gravitational attraction of the additional rock changes their course. Those near misses are the beginning of rotation. Multiply that effect by trillions, and you have a protoplanetary disk. Of course, a lot of those molecules, dust and rock won't even start out with zero relative velocity, so near misses are inevitable.

> **Comments:**
> **username_10**: +1 upvote, I like this answer, short and sweet.  The energy comes from gravitational interactions!  Just like big swirling eddies on the Colorado River (also caused by gravity), the universe can be a turbulent place.  Even if a dust cloud is rotating ever so slowly, it speeds up as it coalesces under gravity, like a spinning figure-skater speeds up as she pulls her limbs closer to herself!  
> **username_11**: Stated another way, it would take an incredible coincidence for all the random motions of the material drawn into the forming star system to perfectly cancel out and produce a non-rotating star.  
> **username_12**: Although with a random n-body starting state, one would expect an equal tendency for matter to rotate in either direction relative to an axis pointing in a random direction, with the eventual angular momentum being "small". I suppose that the interesting question is what "small" means in this context.  
> **username_13**: @MarkMorganLloyd I think that assumes that the N bodies all collapse simultaneously. If we instead consider a nucleation site of just 2 particles which happen to be closer and slower than the rest, and a cascade emanating from there, then one small initial bias gets magnified over the whole mass.  
> **username_12**: @LawnmowerMan OK I see what you're getting at, but the direction would still be random.  
> **username_14**: Although it sounds good, this is not correct. The rotational momentum of the system is _constant_, and doesn't 'come' from gravity or anything else. It exists within the dust cloud, and just becomes more observable when it condenses.  
> **username_10**: @Aganju But the rotational momentum of our Solar System's dust cloud wasn't constant, since it was being pulled by gravity in various other directions from other bodies in the Milky Way.  If you plopped a cloud of gas with no angular momentum into orbit around the Milky Way center, it would pick up angular momentum as it collapsed, since the gas closer to the Milky Way Center would be orbiting faster than the gas farther out.  
> **username_15**: This explanation doesn't make sense to me. By conservation of angular momentum, if three masses accelerating past each other in a Newtonian sense (i.e. traveling on geodesics past each other in a relativistic sense) are "the beginning of rotation", then you're saying that placing multiple masses anywhere in space with zero initial velocity imbues the whole system with angular momentum.  
> **username_9**: Gravity imbues the objects with large potential energy. Since there's no friction, that results in movement.  
> **username_10**: @LukeHutchison Even if the entire system angular momentum of the universe was zero, it wouldn't be zero at a local dust cloud.  Gravity causes the system to be locally turbulent.  

---

### Answer by username_3 on 2020-11-18. Score: 16

The prior gas cloud will start with some small random motions and density variations left over from its formation. As a patch starts to contract any small amount of rotation gets amplified as the cloud collapses, because of the conservation of angular momentum.


---

### Answer by username_4 on 2020-11-18. Score: 13

The answer is turbulence. Stars are formed from large gas clouds that collapse under their own weight. As they do so, they become unstable to fragmentation and break up into smaller collapsing pieces. Turbulence in the original cloud means that each of these pieces has its own individual angular momentum and rotational energy, even if the total angular momentum of the original cloud was zero. The original turbulence can be injected in many ways, including gravitational interactions and collisions with other clouds, the tidal effects of the galaxy, shocks from supernovae, the winds of massive stars, etc.

As the collapse proceeds, dissipative interactions in the collapsing cloud cause it to radiate away energy, but very little angular momentum. Both quantities must be conserved, so the system tends towards a configuration with minimum energy for a fixed amount of angular momentum - which is a rotating disk.

In order to accrete into the central protostar, the material in the disk has to somehow lose angular momentum in order to fall inwards. It does this by transferring angular momentum outwards through various viscous processes. Ultimately though, some of the disk material has to escape with the angular momentum to allow some of the material to accrete into the star. There are a number of processes that can achieve this including "disk winds" and photoevaporation.

> **Comments:**
> **username_5**: I wonder if there is a small Coriolis effect due to the rotation of the cloud around the center of the galaxy.  

---

### Answer by username_6 on 2020-11-18. Score: 8

Other answers good, but this question left behind:

> Why isn't all the disk material been swallowed by the star?

Because of the excess of angular momentum carried by some particles. They insist on orbiting the star instead of falling directly to it.

There is a complex process called [accretion](https://en.wikipedia.org/wiki/Accretion_(astrophysics)). The disk constantly transfers angular momentum to the outside, material to the inside and thermal radiation away. In most cases, some material is also blown away in polar jets.

The physics is the same, no matter if it is a galaxy, a protoplanetary disk, a shiny disk around a neutron star or a black hole, or something smaller like a planetary ring. Well, the scales differ.

There are three catches:

1. For the accretion to work, one needs some friction between the concentric rings of material. Easy when you have dense enough gas or plasma and stops working when you don't have gas anymore. Dust works somehow, large rocks (planets, moons) - not really.
2. Some local disturbances in the accretion process may form local mini-accretions in the disk. They form planets/moons that "steal" some material from the star.
3. Light - once the star is bright enough to reach [Eddington limit](https://en.wikipedia.org/wiki/Eddington_luminosity), the light pressure over the accreting gas overcomes the gravity and the gas gets blown away. The star stops growing and its planets stop growing, too. The dust leftovers get gravitationally stirred until most particles either coalesce over the star and the planets, or get an escape orbit and, well, escape.


---

### Answer by username_7 on 2020-11-18. Score: 3

Answers, in order:

**Why did the protoplanetary disk start rotating around the star?**

Star and disk form around the same time.  
The rotation forms because all other orbits will cross the orbit of the disk and collide. Put another way, the rotation disk is the leftover movement of particles after all symmetrical factors have cancelled each other out in collisions (inelastic ones, elastic ones do not average out anything).

**Where did the initial energy to rotate come from?**

From the potential energy of the particles that were far away from the center of gravity when the dust/gas could started collapsing.

This also explains why the disk is rotating faster than the original dust cloud: As masses go to the center of a rotation, they rotate faster (conserve angular momentum).

**Why isn't all the disk material been swallowed by the star?**

I couldn't answer that better than @fraxinus has.

> **Comments:**
> **username_8**: Another reason why all the disk material isn’t swallowed by the star is that the star has a “wind” of outward-flowing particles, which does exert a “pressure” on the material outside the star. While this wouldn’t stop large particles or planets from falling inward, it’s enough to keep smaller particles from spiraling inward.  
> **username_7**: @PierrePaquette Not sure what you mean, the solar wind is exactly what fraxinus said.  
> **username_8**: Negative; fraxinus mentions light pressure, but the stellar wind is made up of protons and electrons, not only photons, although you are right in saying it has the same effect.  
> **username_7**: Ah right, solar wind is more than just the photons, true. Maybe edit a correction into his answer?  

---

### Answer by username_3 on 2021-03-06. Score: -1

Summarizing/adding to previous specific answers:

Why did the protoplanetary disk start rotating around the star?

The disc forms because it was rotating. Without net rotation it wouldn’t form a disc... it will stay a spherical symmetric cloud.

Where did the initial energy to rotate come from?

A small angular momentum NOT energy is required the “angular energy” gets magnified during gravitational infall while conserving angular momentum like the figure skater spins faster pulling arms in with small initial rotation.

Why hasn't all the disk material been swallowed by the star?

As it speeds up it is basically orbiting faster and faster until centrifugal force balances gravity. Equilibrium.


---

