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One for the (astro) physicists.

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I've been admiring the wonderful galactic images posted here on SGL  (I just do visual myself, though dabbled with FujiColor 1600 many moons ago). 

The face on spirals are wonderful but pondering upon these images has got me thinking.  How do stars that may be up to 100, 000 light years apart  "know" they are part of such a vast system.  Given that no information, including gravitational (?) can travel faster than light, how are stars on opposite extremes informed of the distribution of mass, and thereby form these beautiful spirals.  Just how dependent is the spiral form on the constant 'c'.  

 

 

 

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I believe the current theory, is that it is due to the dark matter. It hasn't been discovered, but scientists suspect its existence, because there's no other way to explain some things, like the movement of galaxies, like you mentioned.

What is interesting, is that scientists believe we (the ordinary matter) are just a few % of the total mass of the universe, the rest being dark matter. 

Quote

Dark matter is a hypothetical form of matter that is thought to account for approximately 85% of the matter in the universe, and about a quarter of its total energy density. The majority of dark matter is thought to be non-baryonic in nature, possibly being composed of some as-yet undiscovered subatomic particles.[note 1] Its presence is implied in a variety of astrophysical observations, including gravitational effects that cannot be explained unless more matter is present than can be seen. For this reason, most experts think dark matter to be ubiquitous in the universe and to have had a strong influence on its structure and evolution. Dark matter is called dark because it does not appear to interact with observable electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum, making it extremely difficult to detect using usual astronomical equipment.[1]

The primary evidence for dark matter is that calculations show that many galaxies would fly apart instead of rotating, or would not have formed or move as they do, if they did not contain a large amount of unseen matter.

Source: https://en.wikipedia.org/wiki/Dark_matter

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Because gravity bends space-time, and another factor is that you can't suddenly drop star straight into galaxy and you can't take one out - make it vanish.

If we were to suddenly "create" single star at some distance to our sun, it would "not know" about it until gravity catches up with speed of light, but we can't just pop matter out of nothing, neither can we make it go away to make abrupt changes in space-time curvature. It is gradual process of changing space time curvature as stars redistribute them selves (think of smooth curves rather than sudden jumps in graph).

You are right that at any given time star at one side of galaxy "sees" sort of gravity snapshot as it was some time ago - and "acts accordingly". Star velocities are much smaller than the speed of light, and there is no sudden changes in gravity that any one star sees. On the other hand, this motion of stars within a galaxy and limit to how fast disturbance in space time can propagate - speed of light, has an effect - gravitational waves. Each galaxy is "pulsating" gravitational waves as dynamic of star motion unfolds.

Actual mechanics of spiral arms is very complex. Spiral arms are not made of same stars all the time, but rather they represent density waves that move at different rate than individual stars. Star can be located in spiral arm at one point, but at some time in future (a long time by human standards) - same star will no longer be in spiral arm, but somewhere in between.

Look at this video for very nice simulation of spiral arms formation and time evolution:

https://www.youtube.com/watch?v=C8BHLgfFx7I

Btw, spiral arms are not "imprint" of gravity waves - these propagate at light speed. They are phenomena more related to "springiness" of chain of stars. Imagine that you have a chain of stars and you "disturb" one star from its position. It will "pull" on neighboring stars that in turn pull on their neighbors - much like bunch of beads connected by elastic spring. It is due to inertia of individual stars that wave is propagating slowly.  

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As I recall, It is still Enigma...
I was always sure, gravity of the central Galaxy region, which includes super massive Balk Hole, plus that "dark something",  - formed them...
But spiral arms, - do no fit into my head :)
Ok, maybe, I can imagine formation of the spiral galaxy due to rotation of the central region of the galaxy, - but the Central Galaxy Cone do not fit into it completely : )

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I am no astrophysicist, but have sat through a series of lectures at Oxford Astrophysics department on the subject of galactic dynamics. I say 'sat through' as the vast majority of what was said entered my brain, but drifted on through and out the other side, largely not understood...!

But, as far as I do understand it, the matter on one side of a galaxy doesn't know that much about the stuff on the other side - it doesn't need to. However, all stuff is interacting with everything at a gravitational level, obeying the inverse square law. As stuff is acreted so its gravitational pull increases, thereby pulling in more stuff. Space is clumpy at all levels, not least the galactic level, so as the spiral arms start to form, so the formation increases. Everything is travelling around the centre of gravitational force in the galaxy, thought to be the black hole at the centre of almost all galaxies, creating the beautiful spiral arms as the matter obeys the conservation of momentum - most things in space are round(ish). As for how the different speeds observed at different distances from the galactic centre are maintained to create the shapes observed, I refer to my learned colleagues responses above and invoke the need for Dark Matter and Dark Energy... If anyone can find these two Dark things, there is a Nobel prize awaiting them!!

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9 minutes ago, PhotoGav said:

If anyone can find these two Dark things, there is a Nobel prize awaiting them!!

I don't think it's the matter (I almost intended that pun there :D ) of finding dark matter and dark energy - we already found them - plenty of evidence for them. It's understanding what they are.

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Guest

Hi folks, thanks for all the info, will take me a while to digest.  

Have you ever noticed a spiral galaxy in your coffee ?  My other half rolls her eyes at me when I'm 'cooing' at some of the spirals, mostly barred spirals ;)

 

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2 minutes ago, AstroCiaran123 said:

Hi folks, thanks for all the info, will take me a while to digest.  

Have you ever noticed a spiral galaxy in your coffee ?  My other half rolls her eyes at me when I'm 'cooing' at some of the spirals, mostly barred spirals ;)

 

I don't take sugar in my coffee so I usually don't get spiral arms in it :D (those are due to stirring of coffee)

It is same phenomena as whirlpools - density distribution due to circular motion.

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7 minutes ago, vlaiv said:

I don't think it's the matter (I almost intended that pun there :D ) of finding dark matter and dark energy - we already found them - plenty of evidence for them. It's understanding what they are.

The Nobel prize is available for detecting Dark Matter and / or Dark Energy... Yes, plenty of evidence to suggest that they should exist, but so far both go undetected. I would be delighted to work with people to try and find them.

In parallel and certainly once found, we can apply ourselves to understanding them... and collect a second Nobel prize for the mantlepiece!!!

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I think it could be something like that. Whether or not matter is finite, it can and is destroyed to some extent. By forces acting against each other. I guess it's just a matter of time. 

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