I was just looking at some wicked awesome pictures from the Hubble and came across this one.
It’s a picture of two galaxies colliding. Now, I’ve heard of the idea a bunch of times before, but a question occurred to me while I was looking at this picture. How is this possible? I thought one of the great mysteries of Astronomy was the fact that all of the galaxies out there were accelerating away from each other.
In that case, unless one was going faster than the other and caught up to it, how is this possible?
The Andromeda galaxy, for one highly relevant example, is approaching our own Milky Way galaxy and is expected to collide with us, oh, maybe, sometime after Wednesday next.
I’m not a physicist, but I think that gravity bears mentioning. If I have two powerful enough magnets, I can throw them in opposite directions and still have them end up swinging back to collide.
Isn’t a galaxy mostly empty space? If two galaxies collided would it be likely to be catastrophic? Or would it be like two atmospheric clouds colliding?
Are galaxies held together by their common gravitiational attraction? Do galaxies have something at the center, like a star factory or something that holds the whole works in place?
Yeah, from what I’ve read on the Astronomy Picture of the Day site, collisions of galaxies rarely result in collisions of individual stars. If any do occur, they’ll most likely happen at the galactic center, where the stars are especially dense. Apart from that, the most noticeable effect of a galactic collision is the disruption of the galaxies’ shapes.
It isn’t so much the danger of stars colliding as the gravitation pull of nearby stars causing damage to star systems. Planets would likely be pulled from their orbits. Multiple star systems would be disrupted. It would be very slow-moving chaos, but over hundreds of millions of years the two galaxies would be a mess.
This is one of the biggest issues in astronomy today. But your two questions have to be looked at separately.
Most astronomers believe that giant black holes sit at the middle of galaxies. They are not star factories per se but neither do they suck the rest of the stars into the center. It’s a much more complex and ongoing balance. See the article “Black Hole Blowback” in the March 2007 Scientific American.
Galaxies are thought to be held together by gravitational attraction. But the attraction of what? Simple Newtonian physics shows that the rotation speeds of galaxies should decrease as the stars get farther from the galactic center. Yet they don’t, they remain constant. This has led some astronomers to postulate that galaxies are surrounded by huge amounts of “dark matter,” matter than doesn’t shine in the electromagnetic spectrum yet has mass that creates gravity. Other astronomers have postulated a change to Newton’s laws of gravity (called Modified Newtonian dynamics (MOND)) which accounts for the data without needing extra matter. There are other hypotheses as well. The answer will change pretty much everything we know so everybody’s racing to be the one to figure it out.
I love that I can drop off a few rambling hijack questions that have low-level bugged me for years and get sourced answers with guides to further reading in less than an hour.
At least it’s more precise than the paper which gave the dimensions in a mathematical problem a value between 6 and Graham’s Number.
“Clearly, there is some room for improvement here.” I’ll say.
Actually, most astronomers do believe that dark matter is the answer to the “what holds galaxies together” question. The galactic rotation curve problem that you mention can be solved by MOND, but nothing else can; to explain things like gravitational lensing and the behaviour of groups and clusters of galaxies, dark matter has to be invoked. Even MOND invokes dark matter to explain the interaction of groups and clusters of galaxies; thus making dark matter only the most logical explanation. Granted, we have no idea what dark matter actually is, but a model wherein a universe evolves governed by standard physics and dark matter matches the observations rather well.
As to what a galaxy actually is, the standard answer is dark matter + stars + more or less gas depending on what sort of galaxy it is (spiral galaxies have more, elliptical galaxies have less). Galaxies tend to collide near centres of groups or clusters of galaxies. Here, the gravitational potential of the cluster, that we believe is mainly provided by dark matter, is strong enough to overcome the universal expansion, and galaxies can collide with each other.
In the case of the Antennae galaxies that the OP posted, is unusual in this respect; the Antennae are an ‘isolated pair’ of galaxies rather than being at the centre of a group or cluster, but they could be the end result of a very small, compact group of galaxies having merged together to make two large galaxies that are now merging.
To expand on what Angua said, it’s really only one astronomer, not plural, who thinks that MOND is true. I’ve never seen the hypothesis put forward by anyone but Milgrom, who developed it. To be fair, he has found some interesting patterns in the rotation curves of disparate galaxies, but most astronomers think that just reflects some as yet undiscovered pattern in the dark matter distribution. The evidence is not yet extraordinary enough for the extraordinary claims.
I thought that as well, but I keep reading article after article on the subject in New Scientist and other science magazines, so I assumed I was wrong.
They are a tiny minority of astronomers though. And until MOND can explain large scale structure, CMB afterglow and everything else that dark matter can explain, and can make predictions that can be experimentally verified and that would distinguish MOND from dark matter (without invoking dark matter in MOND), then its very much on the fringe.
That probably wouldn’t happen to that many multiple star systems or planetary systems in a galactic collision. Stars in galaxies are really, really far apart (except at the galactic center), and you have to get a star pretty close to a planetary or multiple star system for something like that to happen.
The questions have already been answered, but that’s never stopped me before. While the galaxies may be “mostly empty space” as far as matter is concerned, they’re full of gravitational fields which can disrupt both structures; here are some beautiful animations of the simulation of colliding galaxies. You can see how the overall structure is distorted and stars are flung outward. It may also have the effect of subjecting areas of the respective galaxies to very high fluxes of high energy cosmic rays and fusable material, resulting in modification of stellar evolution of affected stars, the so-called starburst effect. In addition, the diffuse dark matter which makes up the bulk of the galactic disks will presumable have interactions that will further disrupt existing structure. This is all going to take millions of tens of millions of years, so it’s not something to get uptight about; the actual accelerations are low enough that it probably won’t cause more than a slight hiccup, though the radiation might require you to dig in and live like a mole. (We should be prepared, as we don’t want to have a Minshaft Gap issue.)
Spiral galaxies have some large anchoring mass in the center which is widely thought to be a supermassive black hole. Unlike normal stellar mass singularitie, the object in the galactic center is thought to be of low average density with a very low gradient field such that relatively little matter falls inward. The radio source coming from the direction of Sagittarius A is thought to eminate from the galactic center; however, the intervening clouds of dust prevent us from making any direct observations of the core or areas in the arc beyond. Until someone invents a hyperdrive that will allow us to flit above the ecliptic plane of the galaxy and check things out from above, we’re going to have to make a lot of inferrences as to what is going on there. We can observe other spiral galaxies that have an axis-on aspect but they’re very distant and are little more than blurry spots of light and radiation.
Modified gravity theories suffer from the essentialy problem of answering a question that nobody asked. It’s a rather complex logical articulation to explain what otherwise appears to be a collection of unseen, nonradiating, and perhaps weakly electromagnetically interacting mass. There is no particular reason to suspect that gravity behaves differently in different regions (which would be a violation of the strong equivilence principle) and while there are theories of “composite gravity” (where gravitons are composed of smaller fundamental fermions which dictate its strength and interactions) or gravity being composed of large attractive and repulsive components that almost cancel each other out at planetary distances, there is really no way currently to test these theories. Invoking Lord of Ockham’s razor suggests that hidden mass is a more likely solution than complex gravity theories, but we can’t really validate one over the other until we find some real evidence or predicted lack thereof.
Well, technically that’s not just gravitational lensing. The link explains it rather well, but we needed a combination of gravitational lensing, a decent X-ray telescope and computer simulations to show that the Bullet Cluster does in fact need dark matter to explain it. In fact, some of my collaborators in Germany have shown that with a dark matter + hot gas model, one can accurately re-create the Bullet Cluster in computer simulations.
The link explains it rather well, but we needed a combination of gravitational lensing, a decent X-ray telescope and computer simulations to show that the Bullet Cluster does in fact need dark matter to explain it. In fact, some of my collaborators in Germany have shown that with a dark matter + hot gas model, one can accurately re-create the Bullet Cluster in computer simulations.