M

#### Milamber

I was wondering about stars, and if anyone can answer part or one of these questoins (though answer more if you really want to) i would really be grateful!

Does the Sun move? if it does, how and what are the physics and forces involved in that.

When looking at the sun, ive only found one accepted theory as to why it evolved, and never found one as to how it moves. Are there anyother possible theorys out there[?]

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#### Ambitwistor

Well, in any matter of whether something "moves", the question arises, "moves with respect to what?" That's relativity for you. After all, if you consider the Earth to be at rest, the Sun moves around the Earth.

But ignoring such relativistic subtleties, we can ask the question of whether any interactions influence the motion of the Sun. The answer is gravity: just like the Earth orbits around the Sun, the Sun orbits around the center of the galaxy, at a speed of somewhere between 200 and 250 kilometers per second. (By comparison, the Earth moves at about 30 kilometers per second around the Sun.)

(Depending on what you mean by "move", you might also say that the Sun moves because it spins on its axis, just like the Earth does. But this is "moving in place", not "moving in space". This rotational motion is leftover angular momentum from the collapse of the primordial cloud from which the solar system formed.)

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#### Milamber

Thanks

Thanks for that, and i did mean movement in space. However gravity alone does not make something orbit, the two would siumply move together untill tehy collide. Does anyone have any ideas as to where the sun got its original speed to make it orbit?

#### Janus

Staff Emeritus
Gold Member
Re: Thanks

Originally posted by Milamber
Thanks for that, and i did mean movement in space. However gravity alone does not make something orbit, the two would siumply move together untill tehy collide. Does anyone have any ideas as to where the sun got its original speed to make it orbit?
The moves due to left over angular momentum from when the galaxy formed.

#### Ambitwistor

Re: Thanks

Originally posted by Milamber
Thanks for that, and i did mean movement in space. However gravity alone does not make something orbit, the two would siumply move together untill tehy collide. Does anyone have any ideas as to where the sun got its original speed to make it orbit?
Gravity alone does make things orbit, unless you postulate that the two pieces of matter start out exactly at rest with respect to each other (or maybe, moving directly towards or away from each other). But it would be highly unnatural to assume that the universe started out with every body exactly at rest with respect to every other -- and even if it did, you would have lots of objects all pulling each other in many different directions, so most objects will never be at rest with respect to each other, though they might be momentarily at rest with respect to a few of the other objects.

In more detail, the Sun got its velocity about the center of the galaxy in a way similar to how it got its axial rotation, or how the planets in the solar system got their orbital motion: leftover angular momentum.

Suppose you take a cloud of randomly moving particles. On average, there will be no preferred axis of rotation, if you assume the cloud is perfectly spherically symmetric. But in reality, the average angular momentum may be very close to zero, but since the distribution of particles is random, it's statistically unlikely for the average to be exactly zero.

Over time, the spherical cloud will begin to collapse under its own gravity, but because of the effective "centrifugal" effect of the small net rotation of the cloud, it won't collapse as much in the directions perpendciular to the axis of rotation. The cloud will start to flatten into a disc. And as the disc collapses, it spins faster and faster, like an ice skater twirling and pulling in his or her arms. You are left with a fairly flat, rotating disc of matter.

Locally dense regions of the disc collapse further, and form little rotating discs of their own: solar systems. Then dense regions within those discs collapse further and produce stars, planets, etc. By by this point, the rotation is slow enough that it doesn't flatten the smallest collapsing regions into discs; they end up as spheres.

#### HallsofIvy

Thanks for that, and i did mean movement in space.
Then you still do not understand the first response. All motion has to be relative to some object. There is no such thing as "movement in space" without being relative to something. The other posters assumed you meant "relative to the center of mass of the galaxy".

#### Phobos

Staff Emeritus
Gold Member
Welcome to Physics Forums, Milamber!

#### meteor

Originally posted by Milamber
I was wondering about stars, and if anyone can answer part or one of these questoins (though answer more if you really want to) i would really be grateful!

Does the Sun move? if it does, how and what are the physics and forces involved in that.

When looking at the sun, ive only found one accepted theory as to why it evolved, and never found one as to how it moves. Are there anyother possible theorys out there[?]
The sun is moving around the galaxy towards a point called apex, that in this moment is situated in the constellation of Hercules, very near to the frontier with the constellation of Lyra

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I was always under the impression that the sun orbits the center of our galaxy where there was a super-massive black hole.

The foces involved in this movement are the forces present when an object as dense as a super-massive black hole is created.

#### Ambitwistor

All stars in the galaxy travel in orbits, but this has little to do with the presence of a supermassive black hole; while "supermassive" for a black hole, it is totally insignificant compared to the mass of the galaxy itself. If it weren't there, the orbits of the stars would be hardly changed at all, except for some stars very close to the center.

#### Nommos Prime (Dogon)

I would put forward that the sun (and all stars) do not move at all (barring slight perturbations caused by the feeble force of gravity). What actually makes stars move is the expansion rate of the Universe (ie. a constant process of the unfolding of space/time).

Originally posted by Nommos Prime (Dogon)
I would put forward that the sun (and all stars) do not move at all (barring slight perturbations caused by the feeble force of gravity). What actually makes stars move is the expansion rate of the Universe (ie. a constant process of the unfolding of space/time).
How does your theory explain the paths stars take around the galactic centre then?

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