Yet another thoughty question about the “big picture”.
Here 'goes …
The article contains this quote,
“Astronomers believe that many galaxies harbor black holes at their centers, where gas and dust tend to drain from the starry surroundings. As matter builds up at the center, its gravity compresses it ever more tightly.”
My questions are:
Why wouldn’t ALL galaxies harbor black holes at their centers? Don’t all galaxies revolve around a generally fixed point?
(likewise) Why don’t all black holes have galaxies revolving around them?
Why wouldn’t ALL galaxies harbor black holes at their centers? Don’t all galaxies revolve around a generally fixed
point?
2. (likewise) Why don't all black holes have galaxies revolving around them?
Answer to 1.
There may not be any reason otherwise - maybe all galaxies do have black holes at their centers. But revolving around a fixed point doesn’t imply that they do. They could revolve around the overall center of gravity without their being an especially high concentration of mass there. The behavior is a little different - if there is a black hole at the center, the orbital velocities of the stars you find ever closer to the center are high, whereas if there is nothing special at the center the orbital velocities of stars near there approach zero.
Some galaxies have a look more or less like star clusters, only much bigger (compare the eliptical galaxy type with a globular star cluster). There is no reason a star cluster has to have a black hole at its center - for all I know we may think the same way about galaxies. On the other hand they may be in the process of figuring out that all galaxies do have a black hole at the center, but if so I am pretty sure nobody is yet certain that it must be so.
Answer to 2.
All you need to create a black hole is to have a star collapse with more than a few times the Sun’s mass left over after the collapse. If it’s too massive to be self supporting as a neutron star, it falls the rest of the way down to a black hole. This horrible drama plays out all over every galaxy many times, as the biggest and brightest and shortest lived stars die. Most of the famous black holes and most of the first ones found are of this sort - somewhere near us in our own galaxy, within an order of magnitude or two of the Sun’s mass, spitting X-rays at us.
I don’t think that a galaxy has ever been found where there’s evidence that there’s not a black hole in the center, but we can’t rule out the possibility. Scientists have a strong tendancy to hedge their bets: You’ll hardly ever hear a scientist say “This is certainly true”; rather, we’ll usually say something like “The available evidence seems to indicate that this is probably true”.
The above paragraph was not deliberately written as an example of this style, but on looking back, it’s a pretty good one. Old habits die hard.
One question in regard to Napier’s response(“They could revolve around the overall center of gravity without their being an especially high concentration of mass there.”):
How can there be gravity without mass? Would this be from objects (stars) already in motion and pulling against each other?
To take this idea further, have scientists found galaxies that revolve around/with other galaxies?
Stars, gas and dust have mass. Galaxies are made of stuff like that, so all galaxies have mass. And yes, such a cluster of stars and gas can revolve around each other, because every star and every gas molecule pulls every other star and molecule by gravity. Even if you have just two stars floating in space, they can revolve around each other - in fact, they would have to, otherwise gravity will pull them together and they will collide.
Isn’t it true that there’s a possibility that many very small black holes were created by the big bang? I read somewhere that they wonder why we haven’t seen some. Also, one theory about what happened in Russia in the early 20th century was caused by a small black hole.
The “center of gravity” is an imaginary point whose position is calculated in such a way that if all the mass of the galaxy was concentred on this point, the effect of gravity on an object outside the galaxy wouldn’t be different.
For instance, some object close to the galaxy will be attracted at the same time by every star (or even planet, etc…) in it. If you want to calculate its move, you would have to take each of them into account. Once you have calculated where is the center of gravity, you don’t have to bother with that anymore. You just consider it is attracted toward the center of gravity, as if the whole mass of the galaxy was concentrated here, and the result is the same.
In the same way, if something is still, just at the center of gravity, it won’t move. Actually, some stars attract it to the “left” with exactly the same force others attract it to the “right”, some attract “up” with exactly the same force that others attract it “down”. The center of gravity isn’t the geometrical center of the galaxy. If there are more mass in a given part of the galaxy, the center of gravity will be closer to this part. If the mass is roughly distributed equally, the center of gravity will be roughly at its geometrical center. There could actually be something at the center of gravity (like a black hole), but there could be nothing as well, since once again it’s only an imaginary point used for convenience.
A center of gravity can be calculated for any system. For instance, if there are two stars with an equal mass, the center of gravity of this system is right in the middle. If one of these stars has a greater mass, the center of gravity is closer to it. A celestial body would be attracted toward their center of gravity (or would revolve around it). Even isolated bodies have a center of gravity. The sun has one. The earth has one. Even your own body has one, somewhere inside your belly. Where the latter is situated exactly has not much importance for an astronomer, but it has a lot of importance if you’re practising a martial art and your opponent is trying to throw you to the ground.
And yes, galaxies commonly revolve around other galaxies. Or more exactly, they revolve around the center of gravity of other galaxies.
I.e., even if the Earth had a 50 mile diameter hollow cavern in it’s exact center, we and everything else on the surface would still be attracted to that (now empty) center point, because, although there is no mass there with gravity, the sum total of gravitational attractions of all other earthly matter averages to attract us to that point. whew!
It is now thought that extremely small black holes cannot exist. Or rather, that they evaporate. Stephen Hawking proposed that black holes evaporate, and it seems now to be widely accepted. We know that particle-antiparticle pairs appear out of vacuum all the time. But they almost always annihilate each other, and thus no matter or energy is created or destroyed. The pairs come out of nothing and dissolve into nothing, leaving nothing.
But, imagine a particle pair created very close to the event horizon of a black hole. One particle goes into the event horizon, while the other particle zips away. They cannot annihilate each other, and so the remaining particle continues to exist. Of course, matter cannot be created from nothing, and so the surviving particle has to get its matter/energy from somewhere. That somewhere is the black hole. The black hole gets a negative amount of energy while the surviving particle gets a positive amount.
To an outside observer, it will look like the black hole is radiating particles. Now, the amount of Hawking radiation of this kind is dependent on the surface area of the event horizon. A very small event horizon will radiate a lot compared to the mass of the black hole, while a very large event horizon will radiate very little compared to the mass of the black hole.
I can’t remember exactly how fast black holes evaporate, but stellar mass black holes take hundreds of billions of years to evaporate, while asteroid size ones take a few minutes. So, the big bang could very well have created lots of very small, sub-stellar mass black holes. But they would have all evaporated by now.
Not all of the primordial black holes would have evaporated, only the smaller ones. In fact, (whipping out calculator), any black hole of mass greater than about 10[sup]24[/sup] kg would be totally stable in the current Universe, as inflow from the Microwave Background Radiation (if nothing else) would balance the energy lost to Hawking radiation (by comparison, the Earth is about six times this). Black holes smaller than this are also not out of the question, since they’d only radiate slowly. In fact, there’s no mass which would be impossible for a current black hole: It could be the final stages of a primordial hole which had just the right initial mass and environment.
As an interesting aside, one of the pages I stumbled across in searching up the numbers for this was right here on the Straight Dope.
This sounds amazing. Has this been observed by astronomers? Can it actually be seen? What is the possibility of scientists actually witnessing this and being able to study this occurrence? Does this ever occur in our galaxy?
Quick & dirty answers…
1a. Maybe they do. But so far, we’re detecting the supermassive black holes (i.e., easier to detect).
1b. Sort of. Not all galaxies have the symmetry of a spiral galaxy (e.g., the Small Magellanic Cloud). The center of mass (point) of a galaxy (system) need not be an actual object.
There are different sizes & sources of black holes. Many are formed from “dead” stars that are within a galaxy. And has been stated, small black holes may have been formed from the Big Bang. Keep in mind that from a distance, the gravity of a black hole is like the gravity from anything else, so there’s no special reason why a galaxy should form around a black hole any more than around every big star. Also keep in mind that black holes from dead stars are a few stellar (our sun) masses whereas the supermassive black holes being found at the centers of galaxies are millions or billions of stellar masses.
I’m not familiar with the experimental evidence for this (I’m sure someone else here has that info) but yes, this occurs in our galaxy. In fact, it is supposed to be occuring just about everywhere in space throughout the universe.
upperdeckfan asked about particle-antiparticle pairs appearing out of the vacuum
Yes, it has been observed, but not by astronomers, but rather in your mundane earth-bound science lab. Because the virtual particles can have any wavelength, two metal plates which are very close will exclude those wavelengths which do not fit a whole number of times between the plates. This creates an attractive force, since all wavelengths are pressing on the plates from the outside but only a subset of the possible wavelengths are counteracting that force in the gap between the plates.