# The Sun's Orbit - Is it What We Think?

I was thinking that the sun must orbit something within our spiral arm in the Milky Way, or be affected by other astronomical bodies - surely not just the supermassive black hole centre.

I have looked at some information and they say that the sun has a sine wave-like movement as it orbits the galactic centre. But what is causing this movement? And what sort of size or density would an object need to be to tug on our solar system uniformly? As I imagine the galaxy is like a soup, with everything churning around itself - still with a common centre (Galactic Centre.)

I'm looking at it in this way: There are black holes between us and the one at the centre which I would assume have enough gravitational energy to affect our star - if not have our star orbit one of them. Is that correct?

• Sgr A*, the SMBH at the centre of our galaxy, has a mass around 4 million solar masses, which is pretty impressive, and it certainly has a big gravitational impact on its neighborhood. But it doesn't gravitationally dominate the whole galaxy, which has a mass around $10^{12}$ solar masses. Also bear in mind that it's over 25,000 light years away. – PM 2Ring Apr 17 '18 at 20:34
• That was a good comment !! and was helpful in putting things in perspective, but what would explain the fact that everything is on its way towards it then (in our Galaxy) - if it isn't gravitationally dominant? – QuantuM Apr 18 '18 at 6:21

The Sun orbits in the gravitational potential of the entire Galaxy, not just the black hole at the centre (which is negligble actually - it has a mass of $4\times 10^{6}M_{\odot}$ compared to the rest of the mass inside the solar orbit, which is of order $10^{11}M_{\odot}$) or the few stars that are closest to us.

There are sufficient separate stars, gas, dust etc. to ensure that the "granularity" in the potential is reasonably small and so the approximation that the Sun moves in a smooth gravitational field is a very good one, unless it comes very close to another body (in which case the $r^{-2}$ influence of gravity comes to the fore).

The reason that the Sun executes a sinusoidal path up and down through the galactic disk, is because the galaxy has... a disc. i.e. The gravitational potential is not spherically symmetric and to a first approximation, the Sun executes a pseudo-simple harmonic motion, with a restoring force set by the gradient of the potential due to a planar disc.

• right so... haha in layman's terms - does that mean that the sun is on its sinusoidal path just like if you were to swing a ball around on a string and knock it up of the centre it would wobble until back on a smooth centrifugal path ... Correct ?? (i hope you understand what I'm getting at here haha) – QuantuM Apr 18 '18 at 8:57
• @QuantuM Crudely, yes. – Rob Jeffries Apr 18 '18 at 8:58
• Arghhh ... well, Thank you very much !! haha yes I thought that might make people in the know cringe :) much appreciated. – QuantuM Apr 18 '18 at 9:05

An object other than the SMBH at the galactic center does not have to be large like a black hole to influence the orbit of the sun around the galactic center. Remember that gravitational forces scale as $1/r^2$ so objects have a larger gravitation attraction as they get more massive but also as they get closer. The sun could be noticeably perturbed (albeit by a very small amount) even by nearby mass density in the form of stars, dust and gas. These interactions typically occur over fairly long timescales and certain effects such as resonances, either in the mean motion or secular behavior, can increase the amplitude of the interactions periodically, leading to the behavior you describe.

• Glad I could provide some interesting comments. The field of perturbations in galactic disks is broad so it is difficult to add anything more specific without making the post ridiculously long. I this subject interests you there are lots of papers that describe how interactions between objects in the disk lead to the structure we observe. – fhorrobin Apr 17 '18 at 12:58
• Also, if this answer helped you, I would appreciate it if you could select it as the "accepted answer". Thanks! – fhorrobin Apr 17 '18 at 12:59
• appologies, done that just now. and yes it does. – QuantuM Apr 17 '18 at 13:00
• although it's not my field of expertise - in any way, I do like to try and gather my own view, if not expand it with information with others. as you say it is so complex I like the lure of the unknown. (should have paid more attention in the school days) haha But yes very interesting answer. i will continue to be interested and update anything else i hear on the subject :) – QuantuM Apr 17 '18 at 13:03