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Author Topic: Cosmos question
KayTi
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Can someone explain a cosmic thing to me, or point me to the right resource?

I've always heard/understood that celestial bodies seem to occupy or...line up along one plane. I'm having trouble expressing myself, but I hope that makes sense. My question is - why? And what is above/below the planetary/star plane.

Gosh I hope this isn't a totally ignorant embarrassing question but...er, well, some of you are going to answer it anyway so have at it! I have faith in everyone's ability to not mock my lack of Astronomy (or is it Astrophysics? or Ballistics? Neuroscience? Nephrology? The study of underwater basket weaving?) knowledge.


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extrinsic
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There's a lot to answering your question, much more than a casual online discussion can answer. I recommend Wikipedia topics like Stellar Formation, Oort Cloud, and such for places to begin.

In the simplest answer, a cloud of molecular dust, mostly hydrogen, is pressed by intervening cosmic events into dense clouds, like from supernovas and other large body gravitational interactions such as colliding galaxies, forming a natal star. The star's gravity in turn attracts an accretion disc of nearby matter. The disc divides into planets, planetesimals, asteroids, and comets, etc. A cloud of less defined matter encircles the star at greater distances, like Sol's Oort Cloud's football-like shape theoretically extending a quarter of a light year out.

Conversely or in evolutionary sequence, a cataclysmic stellar event, like a supernova, casts of copious quantities of matter. A similar stellar system formation process begins gathering an accretion disc of cast off matter that didn't quite escape the remnant star's gravitational attraction.


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KayTi
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Any idea where to find the answer to the question why the matter collects in a disc formation instead of a sphere?

Thanks for the search term suggestions - often that's all I need. Have an ill-formed concept with a need to research but I can't figure out where to start. I'll start with these. Thanks!


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Cheyne
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I listened to a lecture recently explaining this very thing. The professor explained how all things will try to be sperical as a default and that if you look at galaxies or planetary systems you can see that they indeed tried to form spheres but that these spheres failed due to centrifugal forces of the systems' spins.


I am sorry that I can't give you more details.


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extrinsic
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There are a lot of forces in play that cause a disc to form, gravity being at the center [sic]. Newtonian forces, rotational forces, density gradients all contribute, among a host of other forces and effects.

Again, simply put, gravitational attraction by the star pulls matter inward. The natal star moves along in its orbit about a galaxy accompanied by the bolus of undifferentiated matter. Some of the matter falls into the star, very little is lost to interstellar space. If the matter's angular momentum caused by falling in is equivalent to gravitational attraction and it doesn't fall all the way in, the matter enters an orbit around the star that other contributing forces shape into a disc. The rotational trend of the original star-forming impetus informs the rotation of the star and the emerging prevailing disc. The spinning disc stablizes through gyroscopic effects, and gravitational interactions with the star, etc. The disc evolves its own gravitational attraction further gleaning matter from the stellar sphere. Collisions, absorbtion, and accretion add mass to the ever growing prevailing disc. Planets form, further sweeping matter up from the stellar sphere.

Other good Wikipedia topics are Molecular Clouds, the stellar nurseries of the cosmos, and Stellar Evolution, the sequence of stellar lifecycles.


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AstroStewart
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Basically, the tendency for galaxies and solar systems to settle into disks is due to (for the most part) two components.

The following argument can be scaled down for solar systems, but let's look at galaxies for an example. If you have a big blob of hydrogen and helium gas that is going to collapse via gravitational forces into the center, there is bound to be some angular momentum (the gas isn't just sitting still to begin with, but falling into the center FROM somewhere). Just like if you're spinning in a chair and you pull your legs/arms in (or watch dancers on ice pull their limbs in) as a rotating body settles to the center it has to spin faster to conserve angular momentum (conservation of angular momentum is a basic tenant of Newtonian physics).

As a whole, the galaxy that is collapsing is going to prefer some specific direction of rotation, and will form a disk. Once this disk is in place, it will actually act gravitationally on other matter in the system, to pull other gas particles into that same disk (as opposed to spinning around the center on some odd inclination, in some other disk instead of one uniform disk).
(There are also effects, for example, friction which helps by letting gas loose energy and "cool" down into a disk instead of staying more spheroidal.)

It's essentially inescapable from basic physics of a collapsing blob of gas via gravitational forces. Conservation of angular momentum plus gravity (plus energy dissipation by frictional forces within the gas) equals disk-shape.

For a cool looking movie that shows this pretty well:
http://www.astro.washington.edu/fabio/movies/Merger.mpg


Pretty much all of this can be applied to a smaller gas cloud forming a star and subsequently a solar system. The biggest difference is that in a solar system, matter collects into planets, which sweep out most of the little debris within their orbits.


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TaleSpinner
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Hmm, sounds complicated. Let me say it back in terms I understand and see if I have it right.

When a galaxy or a solar system comes into existence, it does so around something heavy -- a black hole or a star (sun).

If everything were perfectly symmetrical and the thing in the middle was perfectly stationary, the system would take a spherical form.

But pure symmetry and a state of perfect rest rarely exist in nature. The heavy thing at the middle can't be at perfect rest so it spins, and anything it throws out will be slung into a disc formation.

Objects above or below the disc will be attracted (by gravity) towards the greater collective mass of the bodies in the disc and over a long period of time assume an orbit in the plane of the disc.

To put it another way: the heavy mass at the centre of a solar system or galaxy cannot be perfectly still, so it spins. A sphere is inherently unstable because it needs absolute symmetry to maintain itself. Over time, the planets or galaxies with more mass will attract their lesser companions into a disc.

Is that anywhere near right?
Pat

[This message has been edited by TaleSpinner (edited December 30, 2008).]

[This message has been edited by TaleSpinner (edited December 30, 2008).]


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AstroStewart
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I would say that is a very good simplified summation (sorry if I get too technical, I study galaxy formation for a living, and the details of why things are disks really is complicated and messy and not fully understood).

The one caveat I have to say regarding your re-summation, however, is that most of the times things like galaxies or solar systems form, there isn't a star or black hole or something compact and massive at the center. Generally, the center of the big cloud is not much more dense than the rest of the cloud.

That is, I get the impression (and correct me if I'm wrong) that you're thinking along the lines of "there's a heavy thing at the center, and all the gas around it is attracted to this heavy thing by gravity, so it falls to the center, and the heavy thing is spinning." But remember, gravity works for any/every object in the universe (if it has mass). So it's not that all the gas is attracted to one heavy thing at the center. Instead, all the gas is attracted to ALL the OTHER gas.

Think of a cloud, kinda like clouds you see in the sky but way way bigger, like the size of our solar system or something. Also think of this cloud as being basically alone, with no stars, planets, black holes, or anything close enough to really DO anything to the cloud. Now picture a single particle of gas on the very OUTside of the cloud. This particle will be attracted, by gravity, to every single particle in the rest of the cloud. Since this outlying particle is on the outer edge of the cloud, the TOTAL net force due to all these thousands and millions of minute gravitational attractions will add up to attract this outlying particle roughly towards the center of the cloud. So it's not that all the gas is attracted to some OBJECT at the center of the cloud, it's just that if you do all the math for a big puffy, uniform cloud like that, they ALL tend to get pushed towards the center.

And in this case, it's not that the CENTER of the cloud is spinning, necessarily. But that the infalling gas doesn't go DIRECTLY towards the center, perfectly. (Think of playing tether-ball. If you hit the ball *exactly* towards the center, it hits the pole and stops, but if you miss the pole, it goes beyond the center and circles around. So, too, the gas won't go *perfectly* towards the center, but will go around the center into orbit, which leads to a disk and angular momentum conservation).

But other than this point, I would say you have rephrased the situation into your own words pretty well. Sorry if I've just confused you more with this description, haha.


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TaleSpinner
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Cool, thanks AstroStewart, I learned something.

Taking it further: As the particles get attracted to the centre of the cloud they get compacted by the increasing gravity (due to their increasing density) until matter starts to form.

But there are other things going on too.

Particles collide and get broken up into simpler particles, which later recombine in fresh combinations -- different elements.

The collisions also create energy -- heat. When the gas is sufficiently dense large numbers of collisions are inevitable, producing the blistering heat of a star.

On a larger scale, the lumps of matter and the primitive stars attract more gases and matter towards themselves -- and still not perfectly so more complex collisions and orbits occur, and you get this roily dance of colliding, orbiting, heating and cooling gasses and rocks that eventually coalesce into planets, stars and galaxies.

(And to understand this better we built the LHC. And broke it.)

Learning, hopefully,
Pat

[This message has been edited by TaleSpinner (edited December 31, 2008).]


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