Science Questions

Why don't galaxies that are not rotating collapse?

Fri, 25th Feb 2011

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Question

Colin Strand asked:

I have a question for the Naked Astronomers - Why don't galaxies that are not rotating all collapse?

 

I believe that Spiral Galaxies rotate but ellipticalgalaxies do not.

 

I understand the universe can / is expanding because of the energy of the big band, but would expect all non-rotating galaxies to aggregate and eventually form massive black holes. Without centrifugal force what stops this from happening? Dark energy, is the gravity too weak or will they given enough time?

 

Hope you can straighten this our for me, it has puzzled me since you explained on the shows that elliptical galaxies where relatively static, it suprised me.

 

Love your programme, I never miss a podcast - keep up the good work.

 

Colin Strand

Ashford, Kent

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Colin Strand asked the Naked Scientists: Dear Chris I have a question for the Naked Astronomers - Why don't galaxies that are not rotating all collapse? I believe that Spiral Galaxies rotate but ellipticalgalaxies do not. I understand the universe can / is expanding because of the energy of the big band, but would expect all non-rotating galaxies to aggregate and eventually form massive black holes. Without centrifugal force what stops this from happening? Dark energy, is the gravity too weak or will they given enough time? Hope you can straighten this our for me, it has puzzled me since you explained on the shows that elliptical galaxies where relatively static, it suprised me. Love your programme, I never miss a podcast - keep up the good work. Colin Strand Ashford, Kent What do you think? Colin Strand , Sat, 12th Feb 2011

Jesus, we live a hundred years at most and you want us to define this? Using 'dark energy' no less. We should then use what we never seen to define what we do not know :)

He*, why not?

Any takers? yor_on, Sun, 13th Feb 2011

Large elliptical galaxies are not showing coherent rotation because they are assemblies of bits of other smaller galaxies that have merged in all sorts of different directions.  The stars themselves have elliptical orbits in all sorts of different directions.  This does not cause any problems because stars are so tiny compared with the distance between them, even in very dense clusters the probability of a collision between two stars is almost negligibly small.  Spiral galaxies are in effect mostly much younger and simple structures  that have formed as a single unit and have not had time to have many collisions with other galaxies.  Unlike stars, galaxies collide quite frequently (on a very big timescale of billions of years) because their separation compared with their size is much smaller.  Two galaxies can collide and pass through each other.  They are distorted and fragmented a bit but their basic structures usually remain intact. Soul Surfer, Sun, 13th Feb 2011

Found this interesting.

"The researchers over at Galaxy Zoo have determined that the orientation of the spin axes galaxies in the universe is indeed random, meaning that the total angular momentum of the universe is zero.

While it is true that planets move around their respective stars in an orderly fashion most of the time, the spin axes of star systems have nothing to do with the spin axis of their parent galaxy and are indeed random. "

Now, this is interesting, if true, as some state that "Kurt Godel demonstrated in 1949 that in a Universe with a non-zero net angular momentum it is possible to have time travel."

Although "Its definition is somewhat artificial (the value of the cosmological constant must be carefully chosen to match the density of the dust grains)"
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And this for those wondering about 'shapes'

"Astronomers classify galaxies into three major categories. Spiral galaxies look like flat disks with bulges in their centers and beautiful spiral arms. Elliptical galaxies are redder, more rounded, and often longer in one direction than in the other, like a (American) football. Galaxies that appear neither disk-like nor rounded are classified as irregular galaxies." yor_on, Sun, 13th Feb 2011

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