Can anyone offer details to the theory that each galaxy has a black hole at its centre?
359590
The short answer is that whenever we take a good hard look at any given galaxy, we find good evidence that there is, or could be, a black hole at it's core.\n\nThe long answer is:\nIt is not so much a theory as something being increasingly noticed. Because black holes are the latest and coolest thing in astronomy (just look how many questions about them appear here!), everyone is always looking for them. They are not so easy to find because, well, they are black. You can't see one directly. One way to tell if there is a black hole there is by the effects of it's gravity on nearby matter. So far, the biggest effect observed on nearby matter has been high energy radiation caused by the rapid acceleration of that matter as it falls into the black hole. Some galaxies show that in their core, indicating a massive black hole there.\nAnother effect is that stars could orbit the black hole, just like the Earth orbits our Sun. We have known for a long time that all stars in a galaxy are orbiting around the center of that galaxy. IN the past it was assumed that there were enough regular stars and dust and gas in the center of the galaxy to make a mass big enough to orbit around. But recently observations with more powerful telescopes of stars closer and closer to the center of our Milky Way galaxy have shown stars VERY close to the center of the galaxy orbiting VERY fast. In that case it is easy to calculate the mass of whatever it is that they are orbiting, just like we can calculate the mass of the Sun by knowing how fast the Earth orbits it. Those calculations show the mass is millions of times more than the sun. Now we know all that mass has to be in an area smaller than the orbits of those stars, and the size of those orbits is measured as being quite small, so we know the volume far too small to be a big star cluster or other ordinary concentration of mass. The only known way to get so much mass in such a small volume is a black hole.\n\nI recall many years ago, when computers were just becoming powerful enough to do big simulations, that someone tried to simulate a globular star cluster by programming a few thousand stars to orbit in a big ball with no mass other than the stars themselves. It didn't work. The stars kind of just all dispersed at random and did not stay in a nice ball shaped globular cluster. Now it should be obvious that if there was a massive black hole in the center of the globular cluster, then all the stars could orbit it like the planets do the Sun, only not all in one plane like the planets do. And I believe some evidence of medium large black holes has been found in some globular clusters recently by the same method of using powerful telescopes to look at the orbits of stars very close to the core. And after all, what is a globular cluster but a really small galaxy.\n\nSo giant telescopes, some of them is space, are providing data never before available that just makes it more and more reasonable to assume that all galaxies have a massive black hole at the core.
277935
A galaxy is a huge gravitationally bound system of stars, interstellar gas and dust, plasma, and (possibly) unseen dark matter. Typical galaxies contain 10 million to one trillion (107 to 1012) stars, all orbiting a common center of gravity. In addition to single stars and a tenuous interstellar medium, most galaxies contain a large number of multiple star systems and star clusters as well as various types of nebulae. Most galaxies are several thousand to several hundred thousand light years in diameter and are usually separated from one another by distances on the order of millions of light years.\n\nAlthough so-called dark matter appears to account around 90% of the mass of most galaxies, the nature of this unseen components is not well understood. There is some evidence that supermassive black holes may exist at the center of many, if not all, galaxies.\n\nIntergalactic space, the space between galaxies, is filled with a tenuous plasma with an average density less than one atom per cubic meter. There are probably more than a hundred billion (1011) galaxies in the visible universe.
955030
The dark matter hasn't gone anywhere.  The problem is we can't see it directly because it doesn't or reflect any light (hence the name!).  It is defninitely there, but we don't know what form it takes - is it normal matter that just happens to be really really dark (dark dust, very faint brown dwarfs, isolated black holes)?  Or is it some form of weird, exotic matter that we have yet to discover (I know that sounds far-fetched on the surface, but it actually isn't).\n\nWe suspect that dark matter exists for various reasons.  One is by observing how spiral galaxies rotate.  By watching any object orbit around another (like our Sun or other stars orbiting around the center of our galaxy) we can calculate the mass of the thing being orbitted.  But with all spiral galaxies (and others, too) the mass we calculate is far greater than the mass of all the stuff (stars, gas, dust) that we can see - therefore there must be stuff in our galaxy that we can't see, but that still has a significant gravitational effect.\n\nWe also see galaxies orbiting each other.  Here again, but watching them we can calculate how much mass there is, since mass determines gravity.  If we only had the mass of the visible stuff inside and outside the galaxies, those galaxy clusters should break apart - the gravitational force from all the visible stuff isn't strong enough to hold those clusters together.  But those clusters do stay together, so there must be more dark matter there between the galaxies.\n\nNow you may just be asking yourself "maybe gravity doesn't work the same on the galactic scale as it does here on Earth", which is a question scientists have asked, as well.  But recent evidence suggests that that is not the case - that we really do need dark matter to explain our observations.\n\nIt gets even weirder - we now think there's something called "dark energy" that is causing our Universe to epand at a faster rate, instead of being slowed down by gravity the way we used to expect.\n\nThere are quite a few good books about this stuff written for laymen - just puruse your local book store in the science section.  And if it all seems too confusing, don't worry - it's confusing to the scientists, too!  As I tell my students, if it all seems to make sense, you're not paying attention!
429063
The Milky Way is moving at about 1,342,161 mph towards a galactic cluster known as Abell 3627\nhttp://answerway.com/expertans.php?category=243&expertname=Locii&catnam=Physics\n\nOur galaxy is NOT moving towards the center (or centre) of the universe. The universe has no center!\nhttp://sciastro.astronomy.net/sci.astro.9.FAQ\n\nGod should be spelled B-I-G B-A-N-G\nPBrax@netscape.com
270562
This has been asked maybe 137 times but instead of researching, please, keep asking.  The latest research shows that it is expanding faster than ever.  So it will probably keep going.  There is no 'edge' to the galaxy and nobody understands why.  I personally think it has a finite size but it is expanding at the speed of light.
1075536
No. There is no center of the expansion. Every place in the universe would see redshifts acting exactly the same way we see them. So if you were in a galaxy 4 billion light years from us and looked around,  the other galaxies would be moving away from you at a rate that is proportional to their distance from you. This happens no matter where you are in the universe.\n\nRemember that motion is relative. In other words, we don't say that some galaxy is moving at 400 km/sec. We say it is moving at 400 km/sec *with respect to us*. But if you look at relative motion with respect to any other galaxy, the expansion looks the same.\n\nOne problem with the term 'Big Bang' is that it encourages people to think of the universe expanding like the debris from a bomb. That is *not* how it is. There really is no center of expansion!\nNeat huh?
475165
Us.  The Milky Way is the name of the galaxy in which we currently reside.  Aside from us, there are a lot of stars and planets and such.  There is a widespread belief that the center of the galaxy contains a black hole.
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Black Holes are strongly suspected to be responsible for a few powerful X-ray stars, and also to be at the center of most Galaxies - including ours.\n\nBut, while these are all much to far away for us to get to now - probably ever. You can see what the trip would be like with the Virtual Trips to Black Holes and Neutron Stars web page:\nhttp://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html\nand with the animations on the Falling into a Black Hole page:\nhttp://casa.colorado.edu/~ajsh/schw.shtml
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A black hole does not necessarily pull in everything in the universe. It's gravitational field is no different from any other object of the same mass, until you get very close.\n\nAccording to Stephen Hawking, black holes will evaporate over time. However, a black hole the mass of the Sun would take many times the current age of the universe to disappear. So the billion-solar-mass black holes at the centers of giant elliptical galaxies will probably still be around when everything in the universe has gone cold.
609154
Your question has two parts. First, there isn't a "center" to the universe. I know that sounds strange, but it is true nonetheless.\n\nAs to what pulls, as you noted, everything pulls. However, some things are more massive and pull harder. There is a good book that shows the history of the discovery of what was called the "Great Attractor" IIRC, the book was Journey to the Great Attractor. \n\nThe Great Attractor is a supercluster of galaxies. Since then, we have discovered more about what is around us and exerting the stronger gravitional pull.\n\nA quick Internet search for galaxies + "great attractor" should give you plenty of reading material.\n\nIf you are interested in space, try\nhttp://groups.yahoo.com/group/lunar-observing/\n\nHope this helps\n\nChuck
59422
Our “address” is: \n\nThe third planet of the star “Sol”, which is the name of the sun. \n\nSol is about 27,000 light years from the center of the Milky Way galaxy, on the inner rim of the “Orion Arm”. The Milky Way galaxy—our galaxy—has four “spiral arms”. An “arm” of a galaxy is a very huge arc of stars that extend out from the center of the galaxy in a long spiral. \n\nThe "Milky Way" galaxy is in the “Local Group”. Galaxies are clustered into groups of galaxies, with wide areas of semi-nothingness between the groups. We have named the group of which our galaxy is part the “Local Group” for obvious reasons. \n\nThe “Local Group” is part of the “Virgo Supercluster”. A supercluster is a cluster of groups of galaxies. \n\nThe “Virgo Supercluster” is one of many clusters and superclusters in the known universe. There is no “official name” of our universe, since it is the only universe of which we know. It is therefore usually just called “the universe”. \n\nOur address is therefore the third planet of the star Sol, on the inner rim of the Orion Arm of the Milky Way galaxy, in the Local Group of galaxies, within the Virgo Supercluster, in "the universe". \n\nNow that you know the address, from whom are you expecting mail?
590743
The most interesting fact I know?  That they don't exist and that they're some stupid idea somebody came up with to help explain a few things we don't currently understand.  Singularities have infinitely dense mass?  Riiiight.\n\nYou'd better start reading up on the Electric Universe, buddy.  There are powerful magnetic fields all over the Universe, and the stupid idiot "scientists" running the show right now don't like to acknowledge that maybe there are electric currents shaping our galaxies.
711640
Sorry to Sun of Samsa.  I meant to give you a thumbs up!  The Black Hole Singularity is infinitely small (infinite density and ZERO volume).  The effects of the hole can be miles or light years across!
495737
Smartie pants?\nEvidence points to a massive black hole at the center of the galaxy (Milky Way).  The black hole, with a mass more than three million times that of our sun, would appear to be in the constellation of Sagittarius.
1057606
Most people *don't* believe galaxies are black holes.  A black hole is a region of space from which nothing can escape, even light.  \n\nSince you can't see a black hole (that light that isn't escaping), one has to rely on indirect evidence.  \n\n If you've found a large mass concentrated in a small volume, and if the mass is dark, then it's a good guess that there's a black hole there.  \n\nIt'll be quite some time before we can get "hard evidence" of a black hole.  We don't even have "hard evidence" that those are stars; it's just indirect evidence.
138404
Often when people are told that galaxies are receding from us, they assume that means we are at the center of the Universe.  However, remember that the Universe is homogeneous and isotropic.  No matter where one is, it looks the same in all directions.  Thus, all galaxies\nsee all other galaxies receding from them.  Hubble's relationship is compatible with a Copernican view of the Universe: Our position is not a special one.\n\nSo where is the center?  *There isn't one*.  Although apparently nonsensical, consider the same question about the *surface* of a sphere (note the *surface*).  Where's the center of a sphere's surface?  Of course, there isn't one.  One cannot point to any point on a sphere's surface and say that, here is the center.  Similarly, because the Universe is homogeneous and isotropic, all we can say is\nthat, in the past, galaxies were closer together.  We cannot say that galaxies started expanding from any particular point.
23482
In astronomy, an object so massive that nothing, not even light, can escape its gravitation. Black holes were given their name because they absorb all the light that falls on them. The existence of black holes was first predicted by the general theory of relativity. Supermassive black holes have been found in the centers of many galaxies. Stellar black holes are thought to arise from the death of very massive stars. Astronomers expect to find many stellar black holes in the Milky Way. \n\n\nFiguratively, the term black hole is used to refer to a total disappearance: “They never saw the man again — he might as well have fallen into a black hole.”