Starship Asterisk*

Galactic centre is a very busy place. Does anyone know how far from centre is habitable?

Sam wrote:What kind of pictures can we expect to see? Will the action show up in radio and infrared?

It would be awesome to see a pair of energy beams eminating from the polar region. However, this small amount of matter should produce little more than a galactic belch.

soldier123 wrote:Galactic centre is a very busy place. Does anyone know how far from centre is habitable?

You can probably be quite close to the core and find things "habitable". But you might need to be outside the bulge completely to find stars where advanced life could form, since in the higher density center solar systems might be fairly unstable, meaning planets would not exist in their stars' habitable zones long enough to really give life a chance.

Okay, Chris, you just said what I was trying to say, but you said it better. So I'll have a bit of fun instead. In view of the fact that today's APOD is called The Diner at the Center of the Galaxy, I can't resist this...





















Ann

Very Intresting the picture and the description
Always something to learn ....

Regards
Sergio

When we see pictures of spiral galaxies, like the Milky way. It seems that in most every case, it is assumed there is a massive black hole in the centre. My impression of a spiral Galaxy is that the spiral is rotating like water going down a plug hole. If this is true. Does this mean that the black hole will eventually consume the entire Galaxy?

soldier123 wrote:When we see pictures of spiral galaxies, like the Milky way. It seems that in most every case, it is assumed there is a massive black hole in the centre. My impression of a spiral Galaxy is that the spiral is rotating like water going down a plug hole. If this is true. Does this mean that the black hole will eventually consume the entire Galaxy? :?

No. All the stars are in nearly circular orbits, and are not spiraling inward at all. The spirals are a phase effect.

Maybe dumb question, but where is the black hole in this picture?

Ricky wrote:Maybe dumb question, but where is the black hole in this picture?

In geometry, every ellipse has two foci (focus points). All the elliptic paths in the image have one such imaginary point in common. The invisible black hole should be right there in that center.

soldier123 wrote:When we see pictures of spiral galaxies, like the Milky way. It seems that in most every case, it is assumed there is a massive black hole in the centre. My impression of a spiral Galaxy is that the spiral is rotating like water going down a plug hole. If this is true. Does this mean that the black hole will eventually consume the entire Galaxy?

Think of it like this. Is the Earth falling into the Sun? Clearly not. Now imagine that the Sun suddenly turned into a black hole. (It can't do that, so you needn't worry, but let's imagine that it could, for the sake of the argument.) Let's imagine, too, that it could collapse into a black hole without disturbing the orbits of the planets.

Okay, now that the Sun had turned into a black hole, would the Earth fall into it? No, not at all. The Earth would continue orbiting the black hole in the same way it that it had orbited the Sun. There would be no difference. In fact, anything that is orbiting the Sun today without falling into it would continue orbiting it in the same way after it had turned into a black hole.

Is nothing falling into the Sun, then? Yes indeed, some comets are falling into the Sun. Amazingly, Comet Lovejoy plunged into the atmosphere of the Sun and nevertheless escaped. But possibly maybe, Comet Lovejoy may have fallen in and been eaten if it had been on the same path and the Sun had been a black hole.

In the same way, the black hole in the center of our galaxy doesn't eat anything that is in a stable orbit around it.

Ann

I found the simulation PR Video eso1151c that is one of the links brought up through the "Recent observations" link in the explanation to be most helpful in getting to understand what the APOD illustration and topic is about.

I have read that small or mini black holes can exhaust there their mass as escaping energy so my question is how massive does a black hole have to be to remain in existence and start growing?

Here's what confused me about the illustration: The central body of an elliptical orbit is at one of the ellipse's foci. Except for three of the orbits shown, possibly a fourth, the foci appear to be all over the map. Is this merely an indication of the accuracy of the orbit determinations for the individual stars? How were the observers able to obtain lateral positions or velocities in order to make the orbit determinations? Are the individual stars moving fast enough that changes in their positions would be apparent over, say, a few tens of years?

It occurs to me that a keen-eyed observer might be able to catch an actual indication of the presence of a central black hole (if one exists). One of the orbits is nearly edge-on. Some sort of lensing effect might be visible as the star in the orbit passes behind the black hole. If that event won't occur for another couple of hundred years, never mind, but still it might be something to watch for.

zbvhs wrote:Here's what confused me about the illustration: The central body of an elliptical orbit is at one of the ellipse's foci. Except for three of the orbits shown, possibly a fourth, the foci appear to be all over the map.

How can you tell this? From a single viewpoint, a low eccentricity orbit seen nearly edge-on is indistinguishable from a high eccentricity orbit seen face-on. So all you know is that the foci lie along one of the axes (you can't even distinguish the major and minor axes). If you look closely, you can pick an axis for each orbit such that there is a common intersection point for all of them- the black hole.

Is this merely an indication of the accuracy of the orbit determinations for the individual stars? How were the observers able to obtain lateral positions or velocities in order to make the orbit determinations? Are the individual stars moving fast enough that changes in their positions would be apparent over, say, a few tens of years?

Yes- the stars are moving very fast, and their orbits are therefore rigorously determined in the same way that orbits of planets or asteroids or double stars are determined. There is no need to resolve individual velocity components- given three or more positions, the orbit calculation is deterministic.

It occurs to me that a keen-eyed observer might be able to catch an actual indication of the presence of a central black hole (if one exists). One of the orbits is nearly edge-on. Some sort of lensing effect might be visible as the star in the orbit passes behind the black hole. If that event won't occur for another couple of hundred years, never mind, but still it might be something to watch for.

The orbit would need to be so close to edge-on that it's a near statistical impossibility. This image certainly doesn't show any orbits very close to edge-on.

Ann wrote:Think of it like this. Is the Earth falling into the Sun? Clearly not.

That's certainly one way to put it. But I think it also needs to be pointed out that the Earth very much is falling into the Sun- as are all the planets, and in some sense, everything in the observable Universe.

Free fall is called that for a reason: you experience no weight when you are falling freely towards the body that produces the gravitational field. The radial velocity of the Earth towards the Sun is exactly what Newtonian mechanics requires it to be. The important point is that we are not getting any closer to the Sun as we fall towards it, because we are also moving tangentially at just the right speed to keep our distance constant (well... not quite, since the orbit isn't perfectly circular, but that can be ignored for the purposes of this discussion).

Okay, now that the Sun had turned into a black hole, would the Earth fall into it? No, not at all. The Earth would continue orbiting the black hole in the same way it that it had orbited the Sun. There would be no difference. In fact, anything that is orbiting the Sun today without falling into it would continue orbiting it in the same way after it had turned into a black hole.

I've always like this example. It goes completely against most people's intuition, and is therefore a useful tool. The problem is, so many people have seen movies featuring great, sucking black holes that they have a totally unphysical model of them running in their minds.

Is nothing falling into the Sun, then? Yes indeed, some comets are falling into the Sun.

To be clear, there is nothing fundamentally different about the orbits of comets like Lovejoy and of planets like the Earth. In all cases, the bodies are falling into the Sun, but have a tangential velocity that keeps them from falling directly towards the center of the Sun. Comet Lovejoy is in a highly eccentric (oval) orbit; Earth is in a nearly circular orbit. So over a complete orbit of Earth, the distance to the Sun doesn't change much; for a complete orbit of Lovejoy, it does. If you were treating this example in a typical physics problem, nothing would ever be able to actually hit the Sun, because it would be treated as a dimensionless point. Of course, in reality, the Sun has a finite radius, so if a body has a sufficiently eccentric orbit, it can actually intersect some part of the Sun (not the center), which is what happens with sungrazing comets sometimes.

I presume that as well as heat and light, this event will release EM radiation at many wavelengths, inc. X-rays.
I hope that inverse squares will minimise the dose we get, so far away.
But this is an enormous event, with no doubt a LOT of radiation.

How far away from the BH will life have to be for the EMR be no longer dangerous?
That's life, Jim, as far as we know it.

John

Chris Peterson wrote:

soldier123 wrote:Galactic centre is a very busy place. Does anyone know how far from centre is habitable?

You can probably be quite close to the core and find things "habitable". But you might need to be outside the bulge completely to find stars where advanced life could form, since in the higher density center solar systems might be fairly unstable, meaning planets would not exist in their stars' habitable zones long enough to really give life a chance.

I have only posted two comments on here and I am amazed at the rapid response I received. You guys answered to a complete novice like me in very understandable terms. For that I must say thank you and wish you a very happy and prosperous New Year.

Where in that plethora of ellipses is the common focus that marks the location of the purported black hole?

zbvhs wrote:Where in that plethora of ellipses is the common focus that marks the location of the purported black hole?

Did you check out the link called orbits of central stars in the caption? The link takes you to an animation that I found helpful.

I have reproduced the link here.

Ann

Many thanks for trying to square me away on the orbits of the stars. I still have a question however: If the nebula is orbiting on a plane affected by the gravitational pull of the black hole, I would assume the plane of it's orbit was established by this gravity. If so wouldn't the stars tend to align themselves on the same PLANE.? Just can't seem to get it through my thick head how the stars would orbit TOWARDS the gravitational pull, and then opposite AWAY from.??? Sorry if I'm bugging you. I really enjoy this site and have convinced many of my friends to enjoy it as well. THANKS.!! Charlie

Charlie Patriot wrote:Many thanks for trying to square me away on the orbits of the stars. I still have a question however: If the nebula is orbiting on a plane affected by the gravitational pull of the black hole, I would assume the plane of it's orbit was established by this gravity. If so wouldn't the stars tend to align themselves on the same PLANE.? Just can't seem to get it through my thick head how the stars would orbit TOWARDS the gravitational pull, and then opposite AWAY from.??? Sorry if I'm bugging you. I really enjoy this site and have convinced many of my friends to enjoy it as well. THANKS.!! Charlie

Every orbit defines a plane, with the central body (black hole in this case) lying on that plane. But the inclination of the plane is random- it isn't determined by gravity. So all the stars, as well as the nebula, are on independent planes determined by the history of those objects.

All closed orbits are elliptical- some nearly circular, some very eccentric. Obviously, any body in an eccentric orbit will spend half its time moving toward the central body, and half moving away from it. As it falls inwards, it picks up speed, as it moves outwards, it slows down again. That is just the basic mechanics of an orbit.

Ann wrote:Isn't it true that the bulge of any spiral galaxy could be regarded as a miniature elliptical galaxy? Isn't the bulge of our own galaxy pretty similar to a small elliptical galaxy? And aren't elliptical galaxies elliptical because the stars in them haven't settled into a plane of rotation, but instead they swarm around like bees?

Come to think of it, isn't that what globular clusters are like, too?

Check out this APOD from 2002, which shows a simulation of the motions of the stars inside a star cluster.

Ann

It seems to me that the net angular momentum of a galaxy is highly correlated with its classification, "globular", "elliptical", "spriral"... in a globular (spherical) galaxy, since the inclination of the orbits of individual stars are random, the galaxy's net angular momentum must be (relatively) extremely small. As you proceed from globular to elliptical to spiral, the individual stars' orbital inclinations become less and less random, and so the galaxy's net angular momentum becomes larger. In spiral galaxies the orbit inclinations are nearly identical (at least outside of the central bulge?) and so the net angular momentum of such a galaxy is large.

My point here is that I don't see how a galaxy can evlove from one type to another without a significant change in angular momentum. Going from globular to elliptical to spiral... Where would this angular momemtum come from? How can this occur?

flash wrote:It seems to me that the net angular momentum of a galaxy is highly correlated with its classification, "globular", "elliptical", "spriral"... in a globular (spherical) galaxy, since the inclination of the orbits of individual stars are random, the galaxy's net angular momentum must be (relatively) extremely small. As you proceed from globular to elliptical to spiral, the individual stars' orbital inclinations become less and less random, and so the galaxy's net angular momentum becomes larger. In spiral galaxies the orbit inclinations are nearly identical (at least outside of the central bulge?) and so the net angular momentum of such a galaxy is large.

My point here is that I don't see how a galaxy can evlove from one type to another without a significant change in angular momentum. Going from globular to elliptical to spiral... Where would this angular momemtum come from? How can this occur?

Some observations that seem relevant:

1. In a spiral galaxy, most of the (visible) mass is in the bulge, where stellar orbits have random inclinations (i.e. do not lie on the plane of the galaxy). Therefore, the (visible) matter in a spiral galaxy has a lower net angular momentum that you might expect.

2. In any galaxy, the vast majority of the mass is distributed in a spherical halo of dark matter, which probably has a net angular momentum near zero. Therefore, the net angular momentum of any galaxy is much less than you would expect just considering what we can see.

3. Galaxies, on their own, don't "evolve" from one major classification to another. The main "evolution" that is observed is the conversion of spiral galaxies to elliptical or irregular galaxies, and that process occurs due to collisions between galaxies- a process which can obviously have a big effect on the net angular momentum of the final system.