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Author Topic: why are galaxies flat instead of round?  (Read 12583 times)

Offline Boo

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why are galaxies flat instead of round?
« on: 16/08/2006 17:52:04 »
im sure this has something to do with the theory of relativity..... or something haha. but why are galaxies formed on a plane, instead of as a sphere like the planets and stars within it? or are there galaxies that are sphyrical? why doesnt all of the matter form into one larger ball from gravity, like planets and stars do?8)

becca :)


 

Offline neilep

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Re: why are galaxies flat instead of round?
« Reply #1 on: 16/08/2006 17:55:18 »
Hi Becca....I don't mean to hijack your lovely thread but thought this thread may help to answer your question

http://www.thenakedscientists.com/forum/topic.asp?TOPIC_ID=3829

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Offline Boo

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Re: why are galaxies flat instead of round?
« Reply #2 on: 16/08/2006 17:57:30 »
yup that helps... but it brings me to another question then... what determines which shape a galaxy will become? why are some spirals, some ellipical and many other shapes?

becca :)
 

Offline neilep

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Re: why are galaxies flat instead of round?
« Reply #3 on: 16/08/2006 18:04:34 »
Well !!..it's like this y'see !!.................I haven't got a clue :)

This is a question for the academic bods...of whom Soulsurfer is the expert !

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Offline Hadrian

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Re: why are galaxies flat instead of round?
« Reply #4 on: 16/08/2006 19:20:34 »
not all galaxy’s if any are totally flat

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Offline Soul Surfer

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Re: why are galaxies flat instead of round?
« Reply #5 on: 19/08/2006 10:30:50 »
Let us start with an evenly distributed vast area of hydrogen and helium gas   That is expanding rapidly and cooling down  like the conditions in our universe just after the cosmic microwave backkground was created.  If the gas is evenly distributed it can be shown that gravitational attraction has no effect because every bit attracts every other bit in all directions and the forces all balance out.  But the moment there is any unevenness in the distribution gravitational attraction starts to have an effect. Initially all the atoms were moving too fast for much to happen but eventually it got cool enough for some lumps to start to collapse in a vaguely speherical way.  As these lumps slowly contracted they got denser and the effect of any residual angular momentum (tendancy to rotate) in the cloud was amplified this made it more difficult for the cloud to contract towards the axis of rotation than perpendicular to the plane of rotation because the centrifugal force associated with the rotation acted againset gravity in that direction so the blobs became oval and finally tended to flatten out into thick discs because the material got so dense that the atoms tended to bump into each other more frequently and settle down there.  This process applies to all gravitational collapses however large.

Gravitational collapse tends to happen from the inside out bcause clouds tend to get denser and in the middle and the particles have lesss distance to fall but eventually the angular momentum collected would stop the collapse,  but something different usually happens, in that the core becomes  hot because of the energy of the collapse  this tends again to reduce the tendancy to collapse but things can get so hot they emit light and form stars.  (nuclear reactions help stars to last longer but are not an essential part of this process)

These were the first stars  this collapse process usually takes a few million years.  The first stars were often inside very much larger contracting and rotating shells that tended to form discs  (there was also dark matter but that just diluted the atoms and played very little part in this process of slow gravitational collapse).  however the density and turbulence of the discs allowed many seperate stars to form  in the early days of the universe it eas also possible to form black holes directly if the conditions are just right and there is not much angular momentum in the cloud.

The first generations of stars went through ther lives. A big star will only last a few million years before the nuclear reactions inside the star go unstable and cause it to blow up as a supernova.    Small stars last for many billions of years and just hang around to show us what the galaxy is doing. Once a star has formed it is very small with respect to the space between the stars so it very unlikely to collide with another star so the galxies develop.

Exploding supernovae create great shock waves that compress the gas for thousands of light years around the exploding star so supernovae trigger star forrmation and the whole process is regenerative.  It is these shock waves of newly formed bright stars that we see as spiral arms in the spiral galaxies.  The shapes and nature if these arms depend on the history density speed of rotation etc of the galaxy so form a wide range of possible shapes with a generally spiral pattern.

Now this tendancy to collapse from the inside means that the structure of the universe tends to develop into a sort of stringy blobby shape with the biggest galaxies at the centre of blobs.

Now galaxies are quite big compared with the seperations between them so it is quite probable that over many hundreds of millions of years galaxies will collide with each other and merge together  this is particularly at the denser areas near the cores of big galaxy clusters  when this happens large elliptical or spherical galaxies are formed because when the stars go into orbit round the very massive centre of the large galaxy they keep the direction that they came in from and do not bump into each other forming strems of small long lived stars as ghosts of the original galaxy that last for thousands of millions of years.

Our galaxy has absorbed smaller galaxies and can be seen to be absorbing some at the present time the streams of stars and globular cluster residues of them can be seen.

Initially the interactions enhance the spiral structure of the galaxy but later it all becomes indestinct and blobby evolving into an elliptical.

The Barred spiral structure is probably formed when a normal spiral galaxy partly merges with another spiral galaxy and we have in effect a double spiral in two planes with the bar being the inner spiral seen on edge and the other spiral seen face on.



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« Last Edit: 19/08/2006 10:39:21 by Soul Surfer »
 

Offline thebrain13

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Re: why are galaxies flat instead of round?
« Reply #6 on: 23/08/2006 19:07:12 »
lets put it this way, if a galaxy wasnt spinning, all the matter in it would simply fall to the center due to gravity. If a galaxy was a sphere then the area near the two poles would be spinning very slowly, and would get pulled down, flattening out the galaxy. So if a galaxy was a sphere it wouldnt remain that way very long.
 

Offline thebrain13

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Re: why are galaxies flat instead of round?
« Reply #7 on: 23/08/2006 19:07:12 »
lets put it this way, if a galaxy wasnt spinning, all the matter in it would simply fall to the center due to gravity. If a galaxy was a sphere then the area near the two poles would be spinning very slowly, and would get pulled down, flattening out the galaxy. So if a galaxy was a sphere it wouldnt remain that way very long.
 

Offline thebrain13

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Re: why are galaxies flat instead of round?
« Reply #8 on: 23/08/2006 19:07:12 »
lets put it this way, if a galaxy wasnt spinning, all the matter in it would simply fall to the center due to gravity. If a galaxy was a sphere then the area near the two poles would be spinning very slowly, and would get pulled down, flattening out the galaxy. So if a galaxy was a sphere it wouldnt remain that way very long.
 

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Re: why are galaxies flat instead of round?
« Reply #8 on: 23/08/2006 19:07:12 »

 

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