What does the sun orbit around?

[MichaelEmouse]

Is it accurate to say that the sun orbits around something? Is it an orbit or some other movement? How fast does the sun move? For that matter, how fast does the earth move around the sun?

If the sun doesn't revolve directly around the center of the galaxy, what does it orbit? Does that thing revolve around the center of the galaxy directly or something else?

Does the galaxy itself revolve around something other than itself? How fast does it outer edge move? It the galaxy moves (other than spinning on itself), how fast does it move?

[DCnDC]

The sun, and the entire solar system, orbits the center of the galaxy. A complete orbit takes about 230 million years.

[iamnotbatman]

And this nasa page fills in a couple gaps in the page linked below.

[DCnDC]

As for the Milky Way, it's part of a "local group" of galaxies that orbit a gravitationally central point, which in our case is somewhere between the two most massive members of the group, the Milky Way and the Andromeda Galaxy.

[XT]

Is it accurate to say that the sun orbits around something? Is it an orbit or some other movement? How fast does the sun move?

As others have said, the sun (and solar system) orbits around the Milky Ways center of gravity. IIRC, it take approximately 200 million years to complete an orbit, and is approximately 26000 light years from the center...which, unless my napkin calculation between flights is completely wrong, is about 500,000 miles per hour. Give or take a mountain of salt.

For that matter, how fast does the earth move around the sun?

I think it's something like 70,000 miles per hour.

If the sun doesn't revolve directly around the center of the galaxy, what does it orbit?

Galactic center of mass.

Does that thing revolve around the center of the galaxy directly or something else?

Not sure what this means. The best visualization I remember from my physics classes years ago is to picture one of those guys doing a hammer throw. Basically, it's a guy swinging a weight on the end of a rope. The guy wobbles a bit due to the weight at the end, while the weight rotates or orbits around the guy as he swings it around his body. This is the same kind of thing...exactly the same thing as the earth rotating around the sun.

-XT

[Anne Neville]

First things first- if you're going to say something is moving at a certain speed, you have to specify "relative to what". There is no absolute standard of rest in the universe. There is nothing in the universe that you could point to and say "that thing there, it's really not moving, and everything moving relative to it is moving." If you want to say the sun is moving, you have to specify relative to what.

When we talk about orbits, we often pretend that the thing in the center (the sun, in the case of the solar system, or actually the center of mass of the solar system) is at rest and everything else is moving.

Originally Posted by MichaelEmouse

Is it accurate to say that the sun orbits around something?

Yes. The sun orbits around the center of the galaxy, as other posters have noted. However, the sun also orbits around the center of mass of the solar system, which is not the same as the center of the sun. We can measure this motion for other stars, and this is a popular way to find extrasolar planets. We think the sun is orbiting this center of mass at about 12 meters per second, plus or minus, mostly due to the pull of Jupiter on the sun.

How fast does the sun move? For that matter, how fast does the earth move around the sun?

To answer "how fast does the sun move", you have to specify something relative to which you are measuring its speed. If you want the measurement relative to the center of the galaxy, we think the sun is orbiting the center of the galaxy at about 220 kilometers per second. The earth is orbiting the sun at about 30 kilometers per second relative to the sun.

The galactic center is defined as the center of mass of the galaxy, where everything in the galaxy orbits around. The sun being in the galaxy, it orbits around the galactic center.

We're pretty sure there is a supermassive black hole near (not necessarily at) the center of our galaxy. It also orbits around the galactic center. I haven't been able to find any information on its orbital velocity.

[John Mace]

Originally Posted by DCnDC

The sun, and the entire solar system, orbits the center of the galaxy. A complete orbit takes about 230 million years.

Center of mass of the galaxy. But those two are probably very close to each other, on a galactic scale.

[Bytegeist]

Originally Posted by MichaelEmouse

How fast does [ the galaxy's ] outer edge move?

I don't think anyone's answered this question yet.

The profile of rotational speed for spiral galaxies like ours is relatively flat, once you get beyond a certain distance from the center. (See the first graph.) So stars at the edge of the Milky Way are moving at about the same linear speed as the Sun.

The angular speed though — in degrees per million years, or what have you — will be much lower. So the outer stars will take almost twice as long to complete one orbit.

[Chronos]

And the flat rotation curve is one of the primary pieces of evidence for the existence of dark matter. If most of the mass were concentrated in the center of the Galaxy, or even if it were distributed about the same way as the stuff we see, the outer parts of the Galaxy would be moving much slower than the inner parts. Since they don't, there must be a bunch of extra mass that we don't see.

[Doubticus]

Originally Posted by Anne Neville

First things first- if you're going to say something is moving at a certain speed, you have to specify "relative to what".

I have a bad habit of being a contrarian, so I saw this quote and I felt compelled. I'm not a physicist, so feel free to point out my ignorance. Recently I was exposed to an idea I had never considered before on a science program. The presenter, was talking about the nature of space, not as a void, but as a place where matter could exist. As an example, he talked about spinning objects. When an object spins, the object experiences centripetal force acting on it. Even though relative to itself, it is not moving. In other words, relative to "space" it was rotating, no relative objects required.

[Bosda Di'Chi of Tricor]

What does the sun orbit around?

<blushes modestly>Me.</blushes modestly>

[Chronos]

As an example, he talked about spinning objects. When an object spins, the object experiences centripetal force acting on it. Even though relative to itself, it is not moving. In other words, relative to "space" it was rotating, no relative objects required.

That's (maybe) true about rotation, but not about linear motion.

[leahcim]

Originally Posted by Doubticus

As an example, he talked about spinning objects. When an object spins, the object experiences centripetal force acting on it. Even though relative to itself, it is not moving. In other words, relative to "space" it was rotating, no relative objects required.

First: Obligatory xkcd.

This is not surprising. It is velocity that needs a reference frame to be specified. In Newtonian mechanics acceleration is the same between reference frames, and rotation is a form of acceleration.

[Carl Pham]

Originally Posted by Chronos

That's (maybe) true about rotation, but not about linear motion.

You could measure velocity relative to the cosmic background radiation (via Doppler shift), and that strikes me as a pretty good definition of absolute linear velocity.

[Polycarp]

For any specified two-object system, the actual movement is that both orbit the combined center of mass of the two. However, within our solar system, the difference in masses is such that with the possible exception of Pluto.Charon, a good first approximation of such movement is described by the smaller/less massive orbiting the larger/more massive. Even as large as the Moon is relative to Earth, the center of mass of the Earth-Moon system remains within Earth's core. As the combined Earth-Moon system orbits the Sun, the Earth wobbles slightly, while the Moon describes a series of rosettes somewhat resembling the edge of a doily. (Technically, the Sun orbits the center of mass of the Sun/Earth/Moon system, too, but that point is perhaps a micron from the center of the Sun taken separately, since it is so massive compared to the other two components.)

All the stars in the Galaxy orbit around their combined center of mass, which may or may not be the locus of a supermassive black hole; theories differ on this, and no direct observation is as yet possible.

[Bytegeist]

Originally Posted by Carl Pham

You could measure velocity relative to the cosmic background radiation (via Doppler shift), and that strikes me as a pretty good definition of absolute linear velocity.

Chronos will probably give a better reply here, but here goes my attempt.

I think that although you could determine a "preferred" reference frame that's pegged to the CMB, at least in a flat enough region of space — maybe by zeroing out your Doppler shift, or by minimizing its total incident energy on you — that reference frame is still nothing special as far as the laws of physics are concerned.

The CMB is like a giant spherical billboard that happens to be viewable from almost everywhere in the universe, but your velocity with respect to it doesn't affect how anything else behaves physically, so it doesn't really constitute an "absolute" reference frame.

[ZenBeam]

Originally Posted by Polycarp

As the combined Earth-Moon system orbits the Sun, the Earth wobbles slightly, while the Moon describes a series of rosettes somewhat resembling the edge of a doily.

No rosettes, or even inflections. The Moon's orbit is always concave towards the Sun.

[JBDivmstr]

Originally Posted by Bosda Di'Chi of Tricor

<blushes modestly>Me.</blushes modestly>

Originally Posted by DCnDC

The sun, and the entire solar system, orbits the center of the galaxy. A complete orbit takes about 230 million years.

(bolding mine)

Which would be <clears throat> ahem, me.

[obbn]

So, excuse me if this is a stupid follow up question, but since the sun is revolving around the center of the galaxy, does that mean that the Earth is dual rotating then, revolving around the sun and the galaxy (in a seperare orbit of course)?

[JBDivmstr]

Originally Posted by obbn

So, excuse me if this is a stupid follow up question, but since the sun is revolving around the center of the galaxy, does that mean that the Earth is dual rotating then, revolving around the sun and the galaxy (in a seperare orbit of course)?

Yes, the logical answer as John Mace's and DCnDC's answers very plainly state.

[Carl Pham]

Originally Posted by Bytegeist

Chronos will probably give a better reply here, but here goes my attempt.

I think that although you could determine a "preferred" reference frame that's pegged to the CMB, at least in a flat enough region of space — maybe by zeroing out your Doppler shift, or by minimizing its total incident energy on you — that reference frame is still nothing special as far as the laws of physics are concerned.

The CMB is like a giant spherical billboard that happens to be viewable from almost everywhere in the universe, but your velocity with respect to it doesn't affect how anything else behaves physically, so it doesn't really constitute an "absolute" reference frame.

First, I don't see what the local curvature of space has to do with measuring your velocity relative to the frame in which CMB radiation has zero dipole moment. What difference does it make whether your local space is flat or curved?

Second, I don't see where changes in the laws of physics enter into the question. Absolute frames of reference do not require the laws of physics to be unique in them, and different elsewhere. They only require certain observations to be unique in them, so that all observers, whatever their own reference frame, can agree on whether a certain observer is or is not in the absolute frame. The frame in which the CMB has zero dipole moment certainly qualifies: all observers will agree on whether or not a particular observer is in that frame -- it is therefore unique, and you can define "absolute" velocity as velocity measured in this frame, if you like.

I didn't say it was necessarily useful to do so. I just said it can be done, and that therefore there is a way to define absolute velocity if you want to.

[kferr]

Here's the Obligitary Monty Python link

[Capt Kirk]

Originally Posted by kferr

Here's the Obligitary Monty Python link

Thanks Kferr, needed that

[Crafter_Man]

Originally Posted by DCnDC

The sun, and the entire solar system, orbits the center of the galaxy.

So... is our galaxy orbiting around something?

[Leo Bloom]

Perhaps this will orient ourselves, the Wiki section on the "Galactic context" of the solar system.

Within that section is a pisser of a multi-image graphic/photo reconstructions, with ever larger scales of the placement of the solar system relative to six ever-larger stellar groupings, ending with the "observable Universe." (Note that the image can be enlarged with a click.)


Side question: What is the difference between the "observable" universe and the one witout the adjective? Is it playing hide-and-seek somewhere?

[MichaelEmouse]

Originally Posted by Leo Bloom

Perhaps this will orient ourselves, the Wiki section on the "Galactic context" of the solar system.

Within that section is a pisser of a multi-image graphic/photo reconstructions, with ever larger scales of the placement of the solar system relative to six ever-larger stellar groupings, ending with the "observable Universe." (Note that the image can be enlarged with a click.)


Side question: What is the difference between the "observable" universe and the one witout the adjective? Is it playing hide-and-seek somewhere?

I think it has to do with the amount of light getting to us as well as whether or not the light has had enough time to get to us.

The first is easy enough to understand: The fact that something is bright doesn't mean it's bright enough for us to see it. Far enough away, things might be too faint for us to see.

The second one is that if the universe is about 15 billion years old, we can only see 15 billion light years away and no more because the light that would allow us to see 16 billion light years away hasn't gotten here yet.

[Chronos]

Theoretically, you can use any reference frame you want. In practice, it's often convenient to pick some large object, and say that you're using a reference frame where that large object is at rest. In everyday life, the largest object we interact with directly is the Earth, so we often treat the Earth as being at rest, and measure velocities relative to the Earth. If you're studying the Solar System as a whole, then this becomes inconvenient, and so we instead treat the largest object in the system, the Sun, as being at rest, and measure velocities of other things relative to it. Likewise, if you're working on yet larger scales, you might treat the Galaxy as at rest, or a cluster of galaxies, or whatever. Well, the material emitting the cosmic microwave background radiation is the biggest object we'll ever be able to see, so on extremely large scales, it's most convenient to consider velocities relative to the CMB frame. But it's still just another big object, qualitatively no different than the Earth, Sun, or Galaxy.

[Fear Itself]

Originally Posted by DCnDC

The sun, and the entire solar system, orbits the center of the galaxy. A complete orbit takes about 230 million years.

Seems odd that the galaxy has only rotated 53 times since it was created 13.2 billion years ago.

[XT]

Well, it probably rotated for a bit before it initially formed, but why does that seem odd? It's freaking huge. To paraphrase from HHG, the galaxy...it's big. Really, REALLY big. You just wouldn't BELIEVE how vastly, hugely, mindblowingly vast and fully of hugery it is! You might think it's a long way to get to Poughkeepsie, but that's nuffin compared to the size of the galaxy, let me tell ya!



-XT

[Fear Itself]

Originally Posted by XT

Well, it probably rotated for a bit before it initially formed, but why does that seem odd?

I suppose it underlines the youth of the galaxy. That such structures only exist because of relatively rapid rotation that balances the force of gravity, could have been created so recently as to only rotate 56 times since creation.

[XT]

Well, keep in mind that if you were closer to the core things would be substantially different and you would have made many more rotations on the merry go round. We're pretty far out on the edge here...a backwater more than halfway out of our spiral arm.

Also, and I'm not sure of this point, but I think our galaxy is actually the concatenation (if that's the right word) of several proto-galaxies, which probably had a few more spins in them prior to crashing into ours early on. IIRC, that's our eventual fate in the future as well...another huge galaxy (Andromeda?) is currently en route to smacking us and forming an even bigger galaxy a few billion years in the future. Which might suck for us, depending on how it plays out, and whether there are any of 'us' (meaning any life from Earth) at that time, which seems doubtful.

-XT

[Chronos]

IIRC, that's our eventual fate in the future as well...another huge galaxy (Andromeda?) is currently en route to smacking us and forming an even bigger galaxy a few billion years in the future.

Our Galaxy and the Andromeda galaxy are currently getting closer together, but that's about all we can say. Yeah, maybe they'll collide, but it's far more likely that they'll just swing past each other and continue to orbit. That depends on the other components of the motion, which are much more difficult to measure. And even if they do collide, it probably won't have much effect, if any, on anything on our scale. Space is mostly empty, and colliding galaxies are mostly just a matter of stars passing through the vast gaps between other stars.

[XT]

Well, maybe I'm misremembering then...I thought it was a pretty sure thing. As to space being vast, that's true enough, but there will be some serious gravitational effects with suns being flung in all sorts of crazy ways...not to mention that there is a huge supermassive blackhole presumably at the center of Andromeda as well, which would be slamming through our galaxy, even as ours slams through theirs. Could be...interesting. Though it's only likely to be academic, assuming it happens, since it's billions of years from my IIRC.

-XT

[TonySinclair]

Originally Posted by XT

To paraphrase from HHG

What is HHG? Sounds interesting.

[XT]

Hitch Hikers Guide (I took some liberties with acronyms ).

-XT

[TonySinclair]

Should have known.

[dtilque]

Originally Posted by XT

Well, maybe I'm misremembering then...I thought it was a pretty sure thing. As to space being vast, that's true enough, but there will be some serious gravitational effects with suns being flung in all sorts of crazy ways...not to mention that there is a huge supermassive blackhole presumably at the center of Andromeda as well, which would be slamming through our galaxy, even as ours slams through theirs. Could be...interesting. Though it's only likely to be academic, assuming it happens, since it's billions of years from my IIRC.

My understanding is that a collision with Andromeda is pretty sure. If they don't collide directly, then they'll make a short loop and collide soon after. Of course "soon" here is relative, say half a billion years later. A direct collision will be in a bit under 3 billion years.

In order for them not to collide at all, they would have to be gravitationally unbound, so they'd go sailing past each other at quite a distance. If they get even sort of close, they'll raise tides in each other which will reduce their orbital energy so they collide eventually.

As for what happens when they collide, several things. One is that they could cause long streams of gas, dust and stars to extend from each other. See the Antenna Galaxy for an example of this (you'll have to find your own link -- I'm too tired tonight to provide one). Also, while the stars won't collide, the gas and dust clouds will. Which means they'll collapse into stars. See star-burst galaxies for examples. Since all the gas and dust gets used up in this, the result after everything settles down is an elliptical galaxy. The black holes will eventually merge with each other.

[Anne Neville]

Originally Posted by Crafter_Man

So... is our galaxy orbiting around something?

Yes, the center of mass of the Local Group.

[Waenara]

Originally Posted by MichaelEmouse

I think it has to do with the amount of light getting to us as well as whether or not the light has had enough time to get to us.

The first is easy enough to understand: The fact that something is bright doesn't mean it's bright enough for us to see it. Far enough away, things might be too faint for us to see.

The second one is that if the universe is about 15 billion years old, we can only see 15 billion light years away and no more because the light that would allow us to see 16 billion light years away hasn't gotten here yet.

FWIW, according to Wikipedia the size of the observable universe is what can be observed in principle because its' light has had time to reach us, regardless of whether there are technological limitations on how well we can see it.

I just learned from that article that there is a distinction between the visible universe (dating from a time when the universe was no longer filled with a plasma opaque to photons, approximately 300,000 years after the Big Bang) and the observable universe (dating from the end of the inflationary epoch, approximately 10^-33 seconds after the Big Bang). Also, due to the expansion of space, we can see things that are now further away than the age of the universe.

Detailed quote (my emphasis added):

The comoving distance (current proper distance) to the particles which emitted the CMBR, representing the radius of the visible universe, is calculated to be about 14.0 billion parsecs (about 45.7 billion light years), while the comoving distance to the edge of the observable universe is calculated to be 14.3 billion parsecs (about 46.6 billion light years), about 2% larger.

The age of the universe is about 13.75 billion years, but due to the expansion of space humans are observing objects that were originally much closer but are now considerably farther away (as defined in terms of cosmological proper distance, which is equal to the comoving distance at the present time) than a static 13.75 billion light-years distance. The diameter of the observable universe is estimated to be about 28 billion parsecs (93 billion light-years), putting the edge of the observable universe at about 46–47 billion light-years away.

[JBDivmstr]

Originally Posted by dtilque

My understanding is that a collision with Andromeda is pretty sure. If they don't collide directly, then they'll make a short loop and collide soon after. Of course "soon" here is relative, say half a billion years later. A direct collision will be in a bit under 3 billion years.

In order for them not to collide at all, they would have to be gravitationally unbound, so they'd go sailing past each other at quite a distance. If they get even sort of close, they'll raise tides in each other which will reduce their orbital energy so they collide eventually.

As for what happens when they collide, several things. One is that they could cause long streams of gas, dust and stars to extend from each other. See the Antenna Galaxy for an example of this (you'll have to find your own link -- I'm too tired tonight to provide one). Also, while the stars won't collide, the gas and dust clouds will. Which means they'll collapse into stars. See star-burst galaxies for examples. Since all the gas and dust gets used up in this, the result after everything settles down is an elliptical galaxy. The black holes will eventually merge with each other.

(bolding mine)

They won't?
Although I am in no way qualified to dispute your statement, (my) simple logic says that they would.
I mean, in the event of a galactic collision it stands to reason that there are going to be at least a few stars that will meet head on, if for no other reason than their immense gravitational pull.
Or am I missing something?

And on a different note... Whose insurance is going to have to pay for damages?

[Anne Neville]

Originally Posted by JBDivmstr

(bolding mine)

They won't?
Although I am in no way qualified to dispute your statement, (my) simple logic says that they would.
I mean, in the event of a galactic collision it stands to reason that there are going to be at least a few stars that will meet head on, if for no other reason than their immense gravitational pull.
Or am I missing something?

Yes. You're missing the scale of the distances between stars in most of the galaxy. The Sagan Planet Walk in Ithaca, NY, is a scale model of the solar system, with distances and sizes of each object represented on the same 1:5,000,000,000 scale. The Sun in this model is 27.8 cm in diameter, a little less than 11 inches. If they were to show the nearest star, Alpha Centauri, on the same scale, it would be somewhere in Hawaii. No kidding.

Even if the stars are exerting a gravitational force on each other, they're just so far apart as compared to their size that they are not likely to collide.

[ZenBeam]

I remembered in an earlier thread, someone had calculated a probability of two stars colliding for an Andromeda/milky Way collision. Turns out, it was me. Who knew?

(The calculation is in Post 22, with some discussion in later posts. I got a 0.16 percent chance, but that's a very rough number.)

[Robot Arm]

Originally Posted by XT

IIRC, it take approximately 200 million years to complete an orbit, and is approximately 26000 light years from the center...

Originally Posted by kferr

Here's the Obligitary Monty Python link

We're thirty-thousand light years from galactic central point.
We go 'round every two-hundred-million years.

The face that it's roughly accurate puts that song squarely into the realm of genius.

[Dr. Strangelove]

Originally Posted by Carl Pham

The frame in which the CMB has zero dipole moment certainly qualifies: all observers will agree on whether or not a particular observer is in that frame -- it is therefore unique, and you can define "absolute" velocity as velocity measured in this frame, if you like.

Is this true? I thought that two widely separated observers, each measuring a zero dipole moment in the CMB, will have a velocity relative to each other (due to Hubble expansion). Sure, you could define a rest frame that works for a single galaxy, but that's still not "absolute".

[ZenBeam]

What you want is the Comoving coordinate frame..

[JBDivmstr]

Originally Posted by Anne Neville

Yes. You're missing the scale of the distances between stars in most of the galaxy. The Sagan Planet Walk in Ithaca, NY, is a scale model of the solar system, with distances and sizes of each object represented on the same 1:5,000,000,000 scale. The Sun in this model is 27.8 cm in diameter, a little less than 11 inches. If they were to show the nearest star, Alpha Centauri, on the same scale, it would be somewhere in Hawaii. No kidding.

Even if the stars are exerting a gravitational force on each other, they're just so far apart as compared to their size that they are not likely to collide.

(bolding mine)

I concur with you that it's very unlikely that two stars will directly collide.

Thanks ZenBeam, for posting the link to the earlier discussion on the subject.


The reason I originally posted was because I parsed dtilque's statement to mean that there was a reason (other than the vast distances involved) that no collision(s) would occur.

[Jragon]

How big is the "orbiting hierarchy" between the sun and the galaxy's CM?

Somebody also said the Sun is orbiting the center of mass of the solar system (which in turn is orbiting the center of mass of the galaxy).

So we have

Luna->Earth->Sol->Solar System's CM->?...->Galaxy CM->Local Group CM->...

How many non-trivial "?" are in there? Is there any formal hierarchy like "group of solar systems"->"cluster of solar system groups"->"group of clusters" etc in between? The terms themselves I made up, but I hope they kind of illustrate what I'm getting at.

[Anne Neville]

Originally Posted by JBDivmstr

The reason I originally posted was because I parsed dtilque's statement to mean that there was a reason (other than the vast distances involved) that no collision(s) would occur.

dtilque didn't say it can't happen, only that it won't. An asteroid could fall on my house today. There's nothing in the laws of physics that says that can't happen. But I can pretty confidently predict that it won't happen.

Originally Posted by Jragon

Luna->Earth->Sol->Solar System's CM->?...->Galaxy CM->Local Group CM->...

Earth is not orbiting the Sun, Earth is orbiting the solar system's center of mass, as is the Sun. The solar system's center of mass is close enough to the Sun that saying "the Earth orbits the Sun" isn't too far off.

[Malden Capell]

In the Solar System, Venus orbits in a different direction from the other planets, and Uranus 'rolls' on its side as it orbits. Do any stars copy such a thing on a galactic scale? As in, orbiting against the trend or 'rolling' around Galactic Central Point?

[Anne Neville]

Originally Posted by Malden Capell

In the Solar System, Venus orbits in a different direction from the other planets

It rotates on its axis in a different direction from the other planets. It orbits the Sun in the same direction as everything else.

Do any stars copy such a thing on a galactic scale? As in, orbiting against the trend or 'rolling' around Galactic Central Point?

The plane in which the planets orbit, called the ecliptic, is tilted at a 60 degree angle to the plane of the galaxy.