Does the sun rotate?

Question

As implied from the question, does the sun rotate? If so, do other stars not including the sun also rotate? Would there be any consequences if the sun and other stars didn't rotate? Me and my friends have differing views on this, and would like some clarification. Thanks!

What are the odds of angular momentum of the sun being exactly zero? — Bernhard, Mar 26 '13 at 20:32
@JerrySchirmer: infinitesimally close to zero? I don't think so. Classically, ${\vec{0}}$ (or any singleton set) is a null-subset of the set of angular momentum vectors, so one might say the probability is in fact exactly zero. Quantum-mechanically, there are only finitely many states of quantised angular momentum when the energy is limited, so the probability is ridiculously, but not infitesimally small. — leftaroundabout, Mar 26 '13 at 23:38
Aren't you all assuming that the odds of a particular angular momentum are evenly distributed over the set of possible values? If so, why? To put it another way, what are the odds of the number of cats in my pocket being exactly zero? Close to 1! Obviously the situation is different, but do we really have warrant to assume that all possible values for angular momentum are equally likely? — LarsH, Mar 27 '13 at 01:11
@leftaroundabout: Classically, you would interpret the probability of a singleton in the continuum as an infinitesimal number, at least if you're working in a set that allows for infinite and infinitesimal numbers. You know exactly what I mean, though. — Zo the Relativist, Mar 27 '13 at 02:20
@LarsH Well you could roughly bound the rotational energy by the gravitational binding energy of the sun. You could probably do better using the virial theorem. In any case the angular momentum of the sun is bounded. Now you could argue about the distribution over that domain of angular momenta, but at least the domain is finite and well defined. — Michael, Jul 10 '13 at 15:24
Here's a video made by a user of Helioviewer. It doesn't answer the whole question so I'll levae it as a comment. It's a movie comprising 7 days of images from the Atmospheric Imaging Assembly (AIA) aboard NASA's Solar Dynamics Observatory (SDO). From the look of the images, I think this is the 304 Å channel. The satellite is in a geosynchronous orbit, so the motion of the Sun that you see is its surface rotation. — Warrick, Mar 11 '14 at 05:52
And coincidentally, today's Astronomy Picture of the Day shows the Sun's rotation in seven bands. — Warrick, Mar 12 '14 at 13:49

score 41 · Answer 1 · answered Mar 26 '13 at 21:48

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The sun does rotate. We can see the rotation of the sun by the doppler shift of the light we get from the sun.

Doppler shift plot of the sun .

(Image from this page.)

Since we know the characteristic spectrum of light from a hot body of a given temperature, we can use the same effect to determine if other stars rotate as well. Note that this only gives the spread in velocities along the line of sight, so a star may be rotating much more than the amount measured.

answered Mar 26 '13 at 21:48

Dan

5,685

Out of curiosity, do modern telescopes have the resolving power to find the individual doppler shifts of the various parts of faraway stars? – Shivam Sarodia Mar 26 '13 at 22:18
@Draksis: I don't know, but probably not. It's still possible to measure the size of some stars, though, with the Hanbury-Brown and Twiss effect. – Dan Mar 26 '13 at 22:23
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I am curious to learn why someone downvoted this answer? – Bernhard Mar 26 '13 at 22:29
I made it in stages; some of the early revisions weren't very good. – Dan Mar 26 '13 at 22:31
@Dan You say that we can use the spectrum of light from a hot body to determine rotation. It is probably my lack of astronomy knowledge speaking, but how would one do that? – Shivam Sarodia Mar 26 '13 at 23:52
So... how fast does it rotate? – ErikE Mar 27 '13 at 00:22
@Draksis: With a big enough nearby star, like Betelgeuse, I guess it might be just barely possible to obtain separate Doppler spectra of different parts of the surface. My quick Google search didn't find any mentions of anyone actually having done that, though. – Ilmari Karonen Mar 27 '13 at 00:39
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So, like, dudes, how should I tan so I get both sides of my face the same? – Kaz Mar 27 '13 at 01:08
While I don't disagree, doesn't doppler merely prove that it rotates relative to us? – Brad Mace Mar 27 '13 at 02:09
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@BradMace Presumably you're referring to the popular notion that "all motion is relative", which doesn't apply to rotation. The Sun certainly is rotating relative to us, but that just means that it is rotating "relative to itself" as well. Physics in a rotating frame of reference is measurably different from in a non-rotating frame. – Mark Mitchison Mar 27 '13 at 04:15
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@MarkMitchison I realize rotation isn't subjective, but my (limited) understanding of doppler is that it only detects relative motion, and that you'd need to look at something such as the trajectory of ejected material in order to prove that the apparent rotation is not simply the result of the Earth moving around the Sun. – Brad Mace Mar 27 '13 at 15:10
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@BradMace But the period of the Earth's rotation around the Sun (a year) is well-known by other means :) Also, the Doppler shift shows how one side of the sun is moving towards an approximately inertial reference frame (the Earth), while the other side is moving away from it, hence the Sun is rotating around its own axis. Because the Earth is so far away from the Sun, the apparent motion of the Sun's surface due to the Earth's orbit is approximately perpendicular to the line pointing from the Sun to the Earth, which wouldn't have an effect on Doppler measurements. – Mark Mitchison Mar 27 '13 at 16:06
@Mark: Actually, I don't think the latter part of your statement is true. The change in distance between the Earth and various points on the Sun's photosphere -- and thus the observed Doppler effect -- is purely a function of the difference between the relative angular velocities of the Sun's rotation and the Earth's orbital motion. In particular, if the Sun were a rigid body rotating with a period of exactly one year (which it of course is not), we wouldn't observe any net Doppler effect. Still, as you note, the Earth's orbital motion is well known and easy to compensate for. – Ilmari Karonen Mar 28 '13 at 18:59
@IlmariKaronen Yes, my argument implicitly assumes that the angular velocity of the Sun is much larger than that of the Earth's orbital motion, so that the Earth can be considered as moving in a straight line in a direction perpendicular to the Earth-Sun radius for the duration of the measurement. – Mark Mitchison Mar 28 '13 at 20:37
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@Mark: It's not, though: the Sun's rotation speed varies between 10 to 15 degrees per day depending on latitude, compared to just under one degree per day for the Earth's orbit. Larger, yes, but hardly much larger. – Ilmari Karonen Mar 28 '13 at 20:51
@IlmariKaronen well then I stand gratefully corrected :) – Mark Mitchison Mar 28 '13 at 20:55
Ignoring the large slow change from left to right on the image, why do the small features appear to be rotationally symmetric from our perspective? I would expect them to look more like the horizontal stripes on saturn or jupiter. – endolith Mar 28 '13 at 21:49
@endolith: To be honest, I don't know, but I'd guess it must have something to do with the much greater heat flux out from the Sun driving more vigorous convection. – Ilmari Karonen Mar 29 '13 at 18:50
@endolith: It has to do with a lot of things. Helioseismology is an enormous field, and all sorts of magnetohydrodynamic effects play a role. – Dan Mar 29 '13 at 19:39
@IlmariKaronen: See above post about helioseismology. – Dan Mar 29 '13 at 19:40
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This is getting kind of ridiculously long, so I started a discussion in chat so we can all talk about the sun there. – Dan Mar 29 '13 at 19:43
@Draksis: Question answered in chat. – Dan Mar 29 '13 at 19:47

Shivam Sarodia · Accepted Answer · 2013-03-26T21:37:43.533

Yes, the sun and nearly all other stars do rotate.

One can see the rotation of the sun by looking at the motion of sunspots on its surface. Over time, the sunspots will move across the sun's surface - proof of its rotation. Furthermore, the rate of the sun's rotation is not constant throughout the sun; it is higher near the equator and slower near the poles.

Other stars rotate as well. To imagine why this would be requires some thought about the creation of a star. A star begins as an enormous cloud of dust and gas. When these clouds form, they always have some rotation - even if this rotation is incredibly small and imperceptible. Gravity, however, begins pulling the cloud together into a smaller and more compact object (a star). The shrinking of the large cloud into a smaller body hugely decreases its moment of inertia, causing its angular velocity to significantly increase by the conservation of angular momentum. (This is much like how a figure skater increases her rate of spinning by pulling in her arms.) Because of this, even the slightest hint of rotation of the large gas cloud is amplified into a rapid spinning once the compact star forms.

Rotation is notable in pulsars, which are rapidly rotating neutron stars. Rapidly spinning neutron stars produce magnetic fields, causing electromagnetic radiation (often in the form of X-rays). Beams of the radiation can strike Earth, allowing observatories to observe the rapidly pulsating stars.

Note: if there was no rotation at all of the cloud of dust, then the planets wouldn't have cross-radial velocity neither, collapsing into the sun or not forming at all. The fact that you can ask the question is proof that the Sun rotates. About other stars, read answer above, it's pretty good. — Francisco Presencia, Mar 27 '13 at 05:30
It would be an incredibly unlikely occurrence for the protostellar cloud to have so little net angular momentum that, after collapsing (think of the figure skater) it would not have a noticeable rotation. Possible, but ridiculously improbable. — Larry Gritz, Mar 27 '13 at 19:52

score 6 · Answer 3 · edited Apr 13 '17 at 12:40

Draksis' answer is more than enough...

As implied from the question, does the sun rotate?

Yes, it does rotate. There's an evidence similar to the sunspots. It's not much historical. Yet, we can observe it -The 2012 Venus transit. I noticed this in three of my images (1, 2, 3) which I got during the transit. These are the images from NASA's SDO, captured in the visible spectrum. On viewing these images consecutively, you can clearly see sun's rotation. Or you can also download their amazing video based on the same time-lapse. If you observe it closely, you can see the sun's rotation in the background quite relative to Venus.

Do other stars not including the sun also rotate?

Though Dan's answer provide an excellent evidence, Draksis' answer gives an amazing explanation. As far as we've observed, almost every celestial body in the observable universe rotates about its axis, due to the conservation of angular momentum $mr^2\omega$. As the distance (radius of orbital motion) decreases, $\omega$ should increase. Inertia does this automatically. Interstellar clouds (a few or few thousand LYs long) are massive enough to experience a gravitational collapse. Once this collapse leads to proto-star phase, the center is dense enough, so that the surrounding matter can swirl and finally spiral into it. As the lengthy cloud slowly spiral inwards it spins faster. Hence, almost all stellar objects spin.

Would there be any consequences if the sun and other stars didn't rotate?

Let's assume that the sun and other stars along with the planets suddenly stopped rotating themselves. There won't be a consequence or any effects now. (This necessarily doesn't happen due to conservation of angular momentum). But as Frank says (which according to the currently accepted hypothesis), if the clouds had stopped rotating (no initial angular momentum), there would be no proto-planetary disks surrounding the nebula-forming region, leading to a central non-rotating star with no planets. These proto-stellar disks are the accretion disks (dense arms of the clouds) which sometimes acquire enough density to form a star-like object (unable to fuse further) ending up as a planet.

Enos Oye · Answer 4 · 2014-03-25T14:14:08.983

The photosphere of the Sun rotates with a 25 day period at equator and more than a 36 day period at the poles. Below the photosphere we have the convection zone and below the convection zone everything rotate as it was one solid spherical body. This spherical body is 70 % of the volume of the Sun and even more of the mass and rotates with a period of 26,3 days. The surface of this sphere is called the tachocline and its extremely thin, less than 3% of the solar diameter.

Often magnetic confinement are used as a explanation of how the interior of the Sun can behave like one solid rotating spherical body, and the magnetic field are again hypothesized to come from motion in the convection zone, but this theory is not yet developed to level where it can fully explain what we observe.

Alternatively the interior of the sun is a fluid or solid body, but this conflicts with current fusion theory where the core of the sun is a hot, high pressure, nuclear fusion furnace.

Yes the rotation of stars have some implications, like neutron stars that rotate faster than a dentists drill. If they where made of ordinary matter the centrifugal forces would rip them apart, so we postulated that they where made of neutrons with huge gravity forces holding them together. Our Sun is the most spherical object we know of and we would excpect it to be like a compressed oval ball due to centrifugal forces, but current theory states that it is the magnetic field which confines the hot plasma ball into the perfect sphere we observe.

Does the sun rotate?

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