Questions about Galaxies

Why do galaxies spin? Do all galaxies spin when they are formed? What causes the initial rotation?

 

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I thought the stars were spinning around the center, like planets do around a sun.

 

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Pollux V,

They do. The question is why? What natural phenomena caused the stars to spin around the center of the galaxy in the first place? Is there a natural phenomena that cause all massive objects to rotate (and/or orbit)?

Tom

 

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Okay, let me try to explain this. I don't doubt that one of our many physics weenies could do better, but, alas, they do not appear to be online as of now.

When you toss a rock through the air, presumably with force behind it, it will go forward and curve downward from gravity. I believe this is called a trajectory. The gravity is always trying to pull it down, but the force behind the rock is keeping it aloft for longer than it would have been had it been tossed straight up or with less force. So, stars in space are like the rocks. If they have stable trajectory's around the center of gravity, then they'll simply circle around it. If they're too close then they won't have much luck, that's why the rock falls back to Earth while a positioned satellite will orbit perfectly.

Like I said, I don't doubt that there's more to it than that. Hope I helped, though!

 

Pollux V,

I know that. I wanted to know what caused the stars to start circling the the center of the galaxies in the first place. I don't think that the stars in our galaxy where "caught" by the super black hole in the center, like a satelite is "caught" by the Earth's gravitational field.

Tom

 

Heheh. Sorry.

Okay, well, I guess I would have to say that I'm not sure either. When the first star was born, it eventually died, and gave rise to others. And they died, and gave rise to others. But I think that the stars only began in one place, and if that's so, then how did they spread out into galaxies like they have today? I can understand how a single galaxy would be born, but not a universe full of them.

 

For a start...

http://www.mpa-garching.mpg.de/Galaxien/Prep.html

http://www.ticam.utexas.edu/CCV/projects/DiDi/cosmo/cosmo.html

http://www.damtp.cam.ac.uk/cosmos/Public/galaxy.html

http://hermes.physics.ox.ac.uk/users/Astrophysics/guides/universe/galformation.shtml

 

Q,

Here is a quote from your fourth link:

"As the ordinary matter flowed inwards, it rotated faster and faster around the centre of the halo in the same way that water flowing out of a bathtub rotates faster and faster around the vortex that usually forms in the middle of the plug-hole."

This explanation is clearly wrong. The only reason why water going down a drain rotates is because of the rotation of the Earth. At the equator, water goes straight down the drain without rotating.

Unless the whole universe is rotating, this explanation can't be used to explain why galaxies rotate.

Tom

 

Prosoothus

I agree that's not a very good analogy - but it is just an anology, more or less dumbed-down.

Correct me if I'm wrong but you're not a supporter of relativity therefore, according to your logic regarding an absolute frame, the universe must be rotating, right?

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Most of the Galaxy's rotate because of small imperfections in the `cloud` that they were born from, as they contracted they rotate faster...if a particular `bit` of this cloud could not rotate then it would have flow off into deep intergalactic space ( unlikely) or been swallowed by the black-hole in the core...

Of course the spiral pattern we see is only the product of short-lived stars that outline a rotating shock-wave, (some-times moving in the opposite direction! to the actual star rotation).

 

do elliptical and irregular galaxies spin too? or are they ust kinda... sitting there.

 

Yes, except the shock wave that initiated the star formation is disrupted or missing...so no nice spiral arms.

Just remember that most of the mass in Galaxy's are in the dark portions...the bright bits are where there is less mass!

 

I thought that our galaxy's oldest stars are at the center of our galaxy, therefore the black holes of expired stars are a collective mass in the center, gradually pulling the chain of stars into orbit around it.

 

That's actually a common misconception. At the scale of a drain, the Coriolis effect from the Earth's rotation is much too weak to cause the vortex. Much more prevalent are small imperfections in the geometry of the drain. which causes a slight perference for the water to flow in one direction. The rest is just conservaton of angular momentum

Likewise, a slight imbalance in the random motions of the gases that initially form the galaxy, cause a slight prefernce in angular momentum. As the galaxy contracts,this slight preference is magnified in speed as the radius decreases.

 

@Mikee_bee
hi.
The oldest stars would be found orbiting the galaxy like the normal stars, perhaps in clusters or in orbits of high eccentricity...

The ones near the centre either eat up( collect) too much mass (hydrogen) and blow-up quickly or get eaten by the black-hole.

 

Q,

I would never suggest such a thing.

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blobrana,

Can you explain, in a little more detail, why imperfections in the cloud leads to rotation. I dont understand the mechanics. Wouldn't all the imperfections that cause the galactic cloud to spin clockwise balance all the imperfections that cause the galactic cloud to spin counterclockwise?

Tom

 

Janus58,

As you know, when you flush a toilet in the US the water always goes down in a couterclockwise direction. However, if you ask anyone in Australia, they will tell you that the water in a toilet always goes down clockwise. Are you suggesting that this is the result of all Australian toilets being built differently than all US toilets?

For another example, look at hurricanes. In the northern hemisphere they always spin in the counterclockwise direction. While in the southern hemisphere, hurricanes are called cyclones, and they always spin in the clockwise direction.

Tom

 

I suggest you check out the two links given below:

http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html

http://math.ucr.edu/home/baez/physics/General/bathtub.html

Note that I said, "At the scale of a drain, the Coriolis effect from the Earth's rotation is much too weak to cause the vortex."

I made no mention of hurricanes, which are of large enough scale and long-lived enough for the Coriolis Effect to be a factor.

With drains, other factors come into play which exert more influence.

 

<FONT size=+2>Y</FONT>es the imperfections would cancel out... unless there were some thing that favoured one over the other... for example the <B>magnetic</B> field emanating from the (spinning?) black hole at the center, would certainly have an effect...

<li>20 March 03 the mass of the most distant black hole yet known has been determined
it is the mass of three billion Suns at a distance of 13 billion light years
, at the centre of a quasar(<I>SDSS J1148+5251</I>).<br>

So <B>0.7</B> billion years after the<B> BB</B>; This mass is as big as those black-holes (suspected) to exist today.