Galaxy rotation

The non amateur stuff. Hawking, black holes, that sort of thing

Moderators: joe, Brian, Guy Fennimore

Post Reply
mark powell
Posts: 5
Joined: Tue Jul 24, 2007 6:47 pm
Contact:

Galaxy rotation

Post by mark powell »

Hi there. Can anyone explain to me, what makes / starts a galaxies rotation ? Ie, how is the rotation started in the first place ? I have searched various on line sources, but no one seems to be able to give me an answer
Kind regards mark
joe
Site Admin
Posts: 4382
Joined: Fri Dec 03, 2004 11:24 am
Location: Greenwich, London
Contact:

Re: Galaxy rotation

Post by joe »

The stars and dust, etc. that constitute a galaxy are in orbit around the central mass - assumed to be a massive black hole. Matter is attracted to the black hole through the force of gravity and that which does not fall into it, continues in orbit around - giving the appearance of a rotating object.

A bit of a basic explanation but that's how I understand it anyway.
200mm Newtonian, OMC140, ETX90, 15x70 Binoculars.
mark powell
Posts: 5
Joined: Tue Jul 24, 2007 6:47 pm
Contact:

Re: Galaxy rotation

Post by mark powell »

Thanks joe, I'm still trying to get my head round the problem.
I know that every galaxy has a rotating black hole at its Center, and I think I understand what your saying.
But I assumed the black hole at the centre formed initially as a result of the collapse of a star and the additional gravitational attraction of more mass being drawn into it from its surroundings. But this doesn't explain how the rotation starts in the first place ?
That's also a lot of mass to get rotating
But it also begs the question. How does a black hole start its rotation in the first place ?
Is a rotating galaxy just a non rotating globular cluster prior to a black hole forming at its Center ?
Is the rotation started due to gravitational forces ?
My heads still spinning, ( pardon the pun )
joe
Site Admin
Posts: 4382
Joined: Fri Dec 03, 2004 11:24 am
Location: Greenwich, London
Contact:

Re: Galaxy rotation

Post by joe »

I don't think the rotation of the black hole has any relevance, to be honest. The galaxy is not rotating because the black hole at the centre is rotating (not directly anyway).

If you imagine how the Solar System formed then it's basically the same process. Gravity from a large mass (a protostar) attracts surrounding mass. Some of it is drawn in to and adds to the greater mass but some of it ends up orbiting the central mass (and form planets in some cases). The planets and other material appear to be revolving around a central axis - we generally say that the planets and material are orbiting the Sun.

It's the same with galaxies, however in this case it is the star systems and galactic dust that are orbiting a central axis, which, as you know, is the black hole.
200mm Newtonian, OMC140, ETX90, 15x70 Binoculars.
mark powell
Posts: 5
Joined: Tue Jul 24, 2007 6:47 pm
Contact:

Re: Galaxy rotation

Post by mark powell »

Thanks you JOE, I understand now when you put it like that. :D
Regards Mark
brian livesey
Posts: 5442
Joined: Mon Jan 09, 2006 11:05 am
Location: Lancashire
Contact:

Re: Galaxy rotation

Post by brian livesey »

To tag a little on to Joe's explanation of galaxy rotation Mark, it was discovered that the stars in galaxies circled at more-or-less the same rate around the centre. This didn't seem to make sense, as the stars nearer the centre should be moving faster than the stars towards the edge. Also, galaxy clusters should be flying apart.
They don't do this, so it was theorised that there was some extra, invisible, mass keeping the galaxy clusters together. Nowadays, the missing mass is referred to as dark matter and it has been detected by radio telescopes.
Last edited by brian livesey on Wed Mar 30, 2016 1:59 pm, edited 1 time in total.
brian
mark powell
Posts: 5
Joined: Tue Jul 24, 2007 6:47 pm
Contact:

Re: Galaxy rotation

Post by mark powell »

Thanx Brian. All slotting into place nicely now
Regards Mark
Marty
Posts: 43
Joined: Mon Sep 17, 2007 9:44 am
Location: Australia
Contact:

Re: Galaxy rotation

Post by Marty »

If I might add in a bit more...

Everything in the universe is in some kind of motion, mostly rotating and orbiting, and other motions such as that due to expanding space. This motion had its origin in the Big Bang.

We are told that during inflation that slight inhomogenties introduced by random quantum fluctuations were magnified to significant proportions and motions. This created gravitational lumping of gas that would eventually condense into stars and galaxies.

So long as there is relative motion between atoms or of any individual lumps of matter (eg clouds of molecular hydrogen or dust etc) when they come into close proximity they are gravitationally attracted and their approach speed will accelerate and will either collide, or more frequently will glance past each other and be forced to a mutual orbit. If they spiral in toward each other they will eventually coalesce into rotating bodies (eg, stars, planets galaxies or black holes) because they conserve their combined angular momentum as rotation.

It's all due to the fact that all matter initially had motion imparted to it by the Big Bang.

Martin Lewicki
Post Reply