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dannyk
2002-Jan-15, 08:03 AM
are the stars near the centre of galaxies closer to each other than those at the fringes ?

Argos
2002-Jan-15, 10:13 AM
On 2002-01-15 03:03, dannyk wrote:
are the stars near the centre of galaxies closer to each other than those at the fringes ?


Brief answer: Yes.

Short answer: The galaxy's core is very dense. It's constituted of vast star clouds, dust and gas. The average distance between stars at the galaxy's core is far lesser than that at the fringes. The galaxy center is the realm of black holes, ionized particles, X-rays, far ultra-violet and other harmful radiations. The proximity between the bodies in there also turns it into a place where gravitational interaction triggers monster tidal effects. Stable orbits are harder to find there, making things difficult for the stablisment of planetary systems. It is uncertain that life, as we know it, could exist in such an agressive environment.

Long answer: Wait for the opinion of other members.

And, Dannyk, Welcome.

(PS) To me, this question has the right to be in the section "General Astronomy".



<font size=-1>[ This Message was edited by: Argos on 2002-01-15 10:44 ]</font>

Russ
2002-Jan-15, 02:27 PM
I'll echo the welcome to the board first voiced by Agros. I'll also agree that this post should be in the general astronomy area.

Although I'm no expert I'll take a little deeper stab at an answer to your question:

It depends. The answer is yes, on average. The stars near the hub of our galaxy average about 1/2 lightyear apart. They gather in clumps called clusters that tend to be closer to other clusters. BUT! That clustering also happens out in our neck of the woods as well. Out in the spiral armes there are groupings, such as the Pliades (subaru in Japan), where the stars are quite close also. It's just that the gaps between clusters out here are more common and bigger.

Our solar system seems to be somewhat unusual in that it does not appear to be closly associated with any other stars or clusters. This would be much less likely but not impossible near the center of the galaxy.

I'm not sure I helped you but hope so. /phpBB/images/smiles/icon_wink.gif

Argos
2002-Jan-15, 02:56 PM
On 2002-01-15 09:27, Russ wrote:

the Pliades (subaru in Japan)


That's something new to me. Now I understand the Subaru automobile brand logo. Thanks, Russ.

Russ
2002-Jan-15, 03:29 PM
On 2002-01-15 09:56, Argos wrote:
That's something new to me. Now I understand the Subaru automobile brand logo. Thanks, Russ.


Always glad to help, no matter how small the donation. /phpBB/images/smiles/icon_biggrin.gif

Argos
2002-Jan-15, 03:46 PM
I came across this article. Brand new info.

Info on the galaxy's core (http://www.nature.com/nsu/020107/020107-9.html)

Russ
2002-Jan-15, 10:12 PM
On 2002-01-15 10:46, Argos wrote:
I came across this article. Brand new info.

Info on the galaxy's core (http://www.nature.com/nsu/020107/020107-9.html)

Interesting article. I don't think it answered the original question but I'm glad you posted it for us.

It got me thinking....(a dangerous thing) I wonder if the event horizon for a supermassive BH would be so large that the gravitational tides would not be so fierce as on a stellar hole. I wonder if the reason we percieve our SMBH as being quiescent is because the matter doesn't get ripped apart at the atomic level the way it would on an EH of a dozen miles or so.

Anybody know the answer?

The Bad Astronomer
2002-Jan-15, 10:41 PM
Yes, that's correct. I have seen one calculation that shows for a sufficiently large black hole (around the mass of galactic core SMBHs) they can eat a star whole, without ripping it apart from tides. Just BLUP! and the star is gone. Yikes.

Argos
2002-Jan-15, 11:23 PM
On 2002-01-15 17:12, Russ wrote:

It got me thinking....(a dangerous thing) I wonder if the event horizon for a supermassive BH would be so large that the gravitational tides would not be so fierce as on a stellar hole. I wonder if the reason we percieve our SMBH as being quiescent is because the matter doesn't get ripped apart at the atomic level the way it would on an EH of a dozen miles or so.

Anybody know the answer?



As you know, there are many types of BH, but what you say does make sense. To my limited knowledge, for a non-rotational BH, the simplest one, the more massive it is the farther its EH is from the center of the system and the more quiescent it is. It means that when you stumble on a SMBH you will only know your fate when it gets too late. You could cross the EH without even noticing.

Things should work the same way for the matter in general. In a SMBH the matter only gets ripped apart after it penetrated too deep into the EH, next to the singularity, so the info on what happens within will never be able to leave the BH.

In a stellar BH the tidal forces rip the matter apart before it gets into the EH. That's why we can detect the high energy emissions.

Some believe that exist other types of BH with electric charge and rotation. A rotational BH will drag the space-time around it, the Lense-Thrring effect. This effect will increase with the mass and rotation of the object.

In the rotational BH, or Kerr's BH, there is a static limit that envolves the EH. An astronaut who entered the EH would disappear from our universe. But if he only crossed the Static horizon, he would be thrown back into the common space. In this case his energy level would be higher than that he had when he went in. It is the ultimate "moto perpetuum". Such region, located between the SH and the EH, is known as Ergosphere, and was proposed by Roger Penrose.

But I only scratched the surface with these words, and still don't know if I got close to answer your question.



<font size=-1>[ This Message was edited by: Argos on 2002-01-15 19:04 ]</font>

IanB
2002-Jan-16, 01:50 AM
>It means that when you stumble on a SMBH >you will only know your fate when it gets >too late.

Must carry a torch with me so I don't "stumble" into a SMBH <grin>


>You could cross the EH without even noticing
Would it then be called an NEH ? (non-event horizon !)

Ian

James
2002-Jan-16, 12:44 PM
On 2002-01-15 18:23, Argos wrote:

Some believe that exist other types of BH with electric charge and rotation. A rotational BH will drag the space-time around it, the Lense-Thrring effect. This effect will increase with the mass and rotation of the object.

I don't know if this is appicable, but I was wondering...if a ship were to become stuck on the event horizon of a rotational BH, would time effectively stop aboard the ship? Or would it either end up smaller than a cockroach's brain or would it, given the right motion in the right direction, eventually leave the event horizon?


In the rotational BH, or Kerr's BH, there is a static limit that envolves the EH. An astronaut who entered the EH would disappear from our universe. But if he only crossed the Static horizon, he would be thrown back into the common space. In this case his energy level would be higher than that he had when he went in.

So, in effect, could this be used in a sort-of slingshot effect?

Argos
2002-Jan-16, 03:16 PM
On 2002-01-16 07:44, James wrote:

I don't know if this is appicable, but I was wondering...if a ship were to become stuck on the event horizon of a rotational BH, would time effectively stop aboard the ship?


Nope. For a Super Massive Black Hole everything onboard the ship would look the same, as it crosses the EH towards the very end in the singularity. An external observer would see the ship become slower and slower and then freeze when it reaches the EH, just before it disappears from the universe. The guys at the control of the ship would then start to notice a mounting shift in the trajectory. They would try to fight it by adding more power to the engines, but soon they will notice that no matter how much power they add the ship continues the same spiral path around something invisible. The tidal forces begins to act. The ship begins to tremble and shake and then just explodes into tiny pieces that go on getting crushed and crushed...

As a rotational BH drag the space-time more violently than others, the effect would be much more severe. The ship would slow down rapidly and the matter would be ripped apart long before the ship approached the EH. For the crew onboard it would be just like any explosion, followed by the grinding of the matter, being transformed into energy. Part of that energy would be emitted in the form of X-rays, gamma rays, etc. Part would be absorbed by the BH



Or would it either end up smaller than a cockroach's brain or would it, given the right motion in the right direction, eventually leave the event horizon?


As I wrote before, you could only leave the EH under certain conditions. One of them is the existence of an Ergosphere (i.e. the regions between the Stactic Horizon and the Event Horizon) sufficiently large. This could only happen in the case of a SMBH. For a stellar black hole, the Stactic Horizon coincides with the EH. As I said, the larger the BH's mass, the farther from the singularity is the EH, and the larger is the distance between the SH and the EH.



So, in effect, could this be used in a sort-of slingshot effect?


Dr. Roger Penrose says yes.

Others say that a BH must emit radiation. How?

Dr. Stephen Hawking says the answer, given by quantum mechanics, is that the particles don't come from within the BH; they come from the "empty" space just around it. The "empty" space may not be completely empty, because this implies that all fields, as the gravitational and the magnectic, would have to be null. Nevertheless, the value of a field and its ratio of variation in time are similar to the position and speed of a particle. The principle of uncertainty says that the more precisely you know one of those qualities, the less precisely you know the other. Hence, in the empty space the value of a field cannot be stablished as zero, else there would be either a known precise value (zero) or a precise ratio of variation in time (also zero). It is necessary to have a certain amount of uncertainty, or quantum fluctuation, in the value of the field. One may think of those variations as pairs of light (or gravity) particles which appear together in a given moment, separate , and right after join together and annihilate themselves. These particles are virtual, force carrier particles. In this case one will be a particle and the other, an anti-particle.

Since energy cannot be created out of nothing, in a pair one of the elements will be positive and the other negative. The one which has negative energy is condemned to be a short living virtual particle, because a real particle has positive energy in normal situations. It should look for its pair so both get annihilated. However, a real particle, close to a massive body has less energy than if it were apart from it, because it would have to spend energy to counter balance the gravitational pull. Normally, the energy of a real particle is always positive, but the gravitational field in a black hole is so strong that even a real particle may present negative energy. It is possible, therefore, in the presence of a BH, that a virtual particle with negative energy falls within it and becomes a real particle or anti-particle. In this case it won't need to annihilate itself with its counterpart, which, being abandoned, may, in the same way, fall within the BH, or, having positive energy, escape from its vicinity as a real particle or anti-particle. An observer in the distance may think that it was emitted from within the BH. The lesser the BH, the lesser is the distance it has to cross until it becomes a real particle and, so, the higher will be the emission ratio and the apparent temperature of the Black Hole.

Ugh! Did I make it any clear?:D







<font size=-1>[ This Message was edited for correcting spelling errors by: Argos on 2002-01-16 18:34 ]</font>

<font size=-1>[ This Message was edited by: Argos on 2002-01-16 18:45 ]</font>

Argos
2002-Jan-16, 11:01 PM
On 2002-01-15 20:50, IanB wrote:
>It means that when you stumble on a SMBH >you will only know your fate when it gets >too late.

Must carry a torch with me so I don't "stumble" into a SMBH <grin>


Darn! The purists...

Read as: "It means that when you come across a BH...":wink:




<font size=-1>[ This Message was edited by: Argos on 2002-01-16 18:03 ]</font>

IanB
2002-Jan-17, 01:54 AM
No one took the bait. I suspect it would be difficult to see a SMBH with a torch anyhow !

It is not like the light will reflect back...

For newbies like me, would anyone care to define the two horizons ? i.e. the SH and EH.

Ian

Argos
2002-Jan-17, 10:22 AM
On 2002-01-16 20:54, IanB wrote:


No one took the bait. I suspect it would be difficult to see a SMBH with a torch anyhow !



BH's can act like lenses in certain circumstances, due to the warping they cause on the space-time frame. If you had a big big torch (i.e. a galaxy) lighting it from behind, you would probably see a ring of light around a void. That void would be a Black Hole.



It is not like the light will reflect back...


Read my post right up to see a brief note (based on the findings of S. Hawking) on how a BH could radiate. What is described here is what's called now The Hawking Radiation.



For newbies like me, would anyone care to define the two horizons ? i.e. the SH and EH.


Everyone is a newby when it comes to BH. Right now they're saying that a black hole has no singularities, Static or Event Horizons. See

A new theory on Black Holes (http://www.badastronomy.com/phpBB/viewtopic.php?topic=444&forum=1&1)


The question of the "Static Horizon" is just a theoretical proposition by Roger Penrose, and doen't belong to the hard core of the general BH theory (at least to my knowledge.). I'm not the most prepared to answer your question. But i'll try to prepare an explanation within my reach. Wait a few hours (or days, depending on my precious /phpBB/images/smiles/icon_smile.gif time).





<font size=-1>[ This Message was edited by: Argos on 2002-01-17 13:35 ]</font>

DStahl
2002-Jan-17, 11:22 AM
"No one took the bait. I suspect it would be difficult to see a SMBH with a torch anyhow ! It is not like the light will reflect back..."

You mean they're not a nice shiny glossy black? *grin*

Over on physicsforums.com we were having a discussion on detecting black holes from a hypothetical starship. This (http://www.physicsforums.com/topic.asp?ARCHIVE=&TOPIC_ID=759) is the thread.

Don Stahl

2002-Jan-17, 04:45 PM
AhhA time for LIE ?
anyway looking that way, um twards where
Sagattarius lies {um lays?} [er has been lain]
you are looking thru an optical lenze.
I think Astronomers call them "ARMS"
I think one Arm's named ORiON or something.
anyway according to ME [& no one else]
when you look past an arm [ Lenze ] like
that you can see double so intead of just
one lump you can see two. of coures there are
More arms than just OR. so you get Mult,Mult. and see a K if you look, OK.