black holes

[Avia]

Hi. I'm very interested in black holes but I don't know much about them and was wondering if anyone who does know something, or who has any ideas, could answer a question for me.

 

Looking at the curved spiral arms of our galaxy swirling around the supermassive black hole that is believed to be at it's centre, I am often reminded of a liquid going down a plug hole which also has a curved path veering to the right or the left depending on which hemisphere you are in before it goes down the plug hole. It does this because the Earth is rotating, as far as I understand, so would it be reasonable to hypothesize that black holes might also be rotating?

[Airbrush]

Take a look at wikipedia.org "black holes" which will answer most of your questions.

 

Sure they may spin, in fact most of them must spin to conserve angular momentum of the star that collapsed. It is hard to imagine something with no volume spinning. What is spinning? The space and matter surrounding it. I recall that black holes may spin up to about 1,000 revs per second. I don't believe spin of the supermassive black holes (SBH) at the center of galaxies has anything to do with the galaxy's spiral shape. Can anyone explain why many galaxies have a spiral shape?

[granpa]

density waves:

 

[Martin]

...Looking at the curved spiral arms of our galaxy swirling around the supermassive black hole that is believed to be at it's centre, I am often reminded of a liquid going down a plug hole ...

 

You are asking about two different things---black hole spin, and on the other hand, spiral galaxy form.

 

They are unrelated, but both are interesting topics. Lots of galaxies have supermassive black holes and yet the galaxy is not of the spiral type.

There is no connection.

 

Airbrush gave you a good lead if you want to read up on black holes.

 

So the other question is about spiral structure----why do some galaxies have it? How does it work? (It is not about stars spiraling down a plug hole! You can have spiral structure in a galaxy without a black hole. The stars in a spiral galaxy are not spiraling in towards the center. There is a different explanation for what maintains the pretty pattern.)

 

So how does it work? Here is a comparatively hard Wikipedia discussion

http://en.wikipedia.org/wiki/Density_wave_theory

Here is a much easier (dumb-down) version by an astronomer named Alice.

Alice likes to explain things so everybody feels they get it.

http://alicesastroinfo.wordpress.com/2008/10/29/density-waves/

 

I audited a course by Frank Shu years ago, the guy who discovered how spiral arms of galaxies work, I'm pleased to say. It wasn't about this topic specifically, just a general astro course. He's an encyclopedia.

 

The story goes like this. Stars orbit the galactic center. Inner stars take less time to go around, they cover more angle per unit time (just like in solar system inner planets orbit quicker). Stars further out from center take longer to go around, cover less angle per unit time.

 

Suppose that just by random motion a region of higher than average density develops in the outer galaxy. It will tend to progress around faster than the stars at that radial distance because its gravity pull slows down the stars ahead of it that it is overtaking. And then since they are slower once they are inside the (proto-arm) region they tend to fall behind. Then once they are out and it has moved ahead of them the arm's pull speeds them up again.

 

So the region of hightened density has passed thru a neighborhood of stars, momentarily concentrating them, and then left them behind, still traveling at the same speed as they had before.

 

Something similar happens in the inner part of the galaxy except there the stars are going faster than the arm (the region of higher density) and they catch up with it. Spend some time in it, and then move on out in front.

Like you are driving on the freeway and you catch up to a clog-traffic section that is moving slower, and you gradually get thru it, and then you speed up again and are in the clear.

 

So an arm is constantly gaining some stars and losing other. It does not stay comprised of the same stars. It is a temporary concentration that forms a stable pattern.

 

There is no net effect on the stars. they just keep on orbiting around the center of the galaxy for ever and ever, as if the spiral arm pattern weren't even there. Periodically getting caught up in heavy traffic, like on the freeway, and then getting out, and then after another cycle getting caught in it again, and so on.

 

You can learn more detail from Alice Astro Info and from Wikipedia.

Edited May 31, 2009 by Martin

[Avia]

So black holes may well be spinning but the spiral shape of our galaxy is related to stars closer to the center taking less time to orbit the center than ones further towards the edge of the galaxy. That makes sense. Thanks guys that's cleared a lot of things up.

[Airbrush]

Fascinating explanation of motion of the spiral arms, thanks Martin. Nice density waves Granpa.

 

Anyone know how close we are to the nearest high-density spiral arm? When will we pass thru another arm and how close will our neighbor stars get to us when we do? I will try to look these up, unless someone knows offhand.

 

When spiral galaxies collide does that mix them up so much that they either form irregular shapes or become ellipticals?

---

Merged post follows:

Consecutive posts merged

So black holes may well be spinning but the spiral shape of our galaxy is related to stars closer to the center taking less time to orbit the center than ones further towards the edge of the galaxy. That makes sense. Thanks guys that's cleared a lot of things up.

 

Yes and the motion of density waves.

 

I bet that if the SBH at the center of our galaxy is spinning that it is spinning in the same direction as the galaxy is rotating.

[Martin]

Fascinating explanation of motion of the spiral arms, thanks ...

 

Airbrush I was trying to do several things and messed up a detail in that explanation. Only saw it later when too late to edit and correct.

 

I'll try to give the key idea. (It doesn't really matter, or change the picture of density waves very much, but it's good to get right)

 

A kilo mass orbiting at earth distance from sun has less energy*, but orbits faster, than a kilo orbiting at Jupiter distance. If you take a mass, say a one ton craft in circular orbit at earth distance, and slowly try to speed it up with an ion thruster boost you will end up making it swing out farther and actually go slower.

 

Earth goes 30 km/s around sun. If you have a craft doing that same thing and try to boost it faster you can have it end up going 25 km/s.

 

If you put an ion thruster on the front of the craft try to slow it down, it will end up spiraling in closer to the sun and going faster.

===================

 

Now the arm has to rotate as a whole, to keep its shape. So close in it will be going slower than standard circular orbit speed. And out away it will going faster than normal orbit speed at that distance.

 

So take an inner star. It will be going faster and catch up. But while it is catching up and closing in it will be being pulled---like the craft with the ion thrust booster. By the time that sucker has merged into the arm it will have swung out slightly and be going slower. So it spends more time in the arm, because it is temporarily slowed down. That is what makes the traffic heavy in the arm. But it still is not completely slowed down to the same speed as the arm so it gradually passes thru, and comes out the front. OK now it is being dragged back by the gravity of the arm (the higher density)----like the spacecraft with a braking thruster---and that actually brings it back into its old orbit and speeds it up.

 

So the inner track stars catch up with the arm and pass thru, and are slightly slowed while they are in the arm.

 

The outer track stars, the opposite happens. The arm catches up with them, and temporarily they get speeded up, while they are in the arm. And so they stay longer in the arm than they would otherwise. And this also contributes to the congestion.

 

The arm is essentially some self-reinforcing traffic congestion.

 

I think we all have the right density wave picture and what I just did was repeat the story over again, but with that little gravity mechanism detail fixed.

 

(A textbook explanation might say that orbital energy is the kinetic energy of the speed the thing has PLUS the potential energy of how far out it is. If you inject orbital energy into the system it can actually slow the thing down because the energy is flowing into the potential, pushing it out in a wider orbit and wider orbits are slower. The earth goes 30 and Jupiter, if I remember, goes 13, etc etc.)

 

*the total energy of an orbit is the sum of the kinetic energy and the potential energy related to distance from center.

Edited June 1, 2009 by Martin

[purintjp]

It would seem that black holes would also have to rotate to create the

'black hole eddy' effect shown in so many scientific programs and articles.

It would appear that eddy is a twisting of space-time itself due to the

rotation of the large mass singularity.

Could a non-rotating black hole even exist ?

[Airbrush]

It would seem that black holes would also have to rotate to create the

'black hole eddy' effect shown in so many scientific programs and articles.

It would appear that eddy is a twisting of space-time itself due to the

rotation of the large mass singularity.

Could a non-rotating black hole even exist ?

 

This is the first I heard of the "eddy effect" of a black hole. Would you care to elaborate in more detail?

 

It seems like all black holes must spin, because motion is the norm in the universe, nothing seems static or stationary. But consider the conservation of angular momentum of a collapsed star. Like the example of a ice skater spinning with arms out, then when they bring their arms in their spin increases. What must happen with black holes? It must be far more of an effect. It seems like the spin would reach relativistic speed very easily.