Why are galaxies shaped like a disc?  Why are they not spherical like planets?
821714
Galaxies come in a variety of shapes.  Ours is a spiral, but only about 25% of galaxies are spirals.  There are also barred spirals (about 20%), which tend to be a bit smaller than normal spirals.  There are ellipticals (about 50%), a category with the widest range of sizes, from the dwarf ellipticals that accompany the Andromeda spiral to giants with up to a trillion stars.  These, although classed as ellipticals, are usually spherical in shape.  There are also irregular galaxies, and rare ones like Seyferts and ringtails.\nAs to why, the best we can say at the moment is that this is the consequence of their evolution and gravitational interaction with other galaxies, including collisions and mergers.
791325
Most are actually more ellipsoid in shape (i.e. not perfect spheres).\n\nI'm no astronomy/physics expeert, but I think it has something to do with gravity and rotational forces.
171073
Rephrase your question sweety, the Milky Way & the Galaxy are the same, but there are other galaxies than ours. then... different than what? are you smoking something?
793198
Because they exhibited ENORMOUS red shifts, implying huge distances from us. At those distances, it was inconceivable that they were stars, which was the earlier interpretation. They were then renames quasi-stellar objects, or quasars for short. Now, current theory holds that they are the nuclei of some of the earliest formed galaxies.
663722
The largest galaxies that we know of are giant elliptical galaxies in the centres of clusters of galaxies. They are sometimes called cD galaxies (for giant diffuse galaxies) or BCGs (for brightest cluster galaxies). These galaxies are about 10 times more luminous that a typical galaxy (like the Milky Way) and about 100 times more massive. They can have diameters of more than 6 million light years (compare to about 100 thousand light years for the Milky Way). A good example is the central galaxy in the cluster Abell 2029. \n\ncD galaxies are thought to grow by accreting any galaxy that comes anywhere near them, and in the centre of a cluster this can be a lot of galaxies over the age of the universe - so their mass can grow by a large factor. They usually have an area around them in which no smaller galaxies are found (as they've all been 'eaten') and a diffuse halo of gas and dust which is thought to be the debris of the smaller galaxies.
1090455
They start out as an enormous gas cloud. Gravity causes the parcicles in the cloud to spin around the point of center mass, and the velocity of the spin subsequently flattens it into a disk. That's how galaxies, and on a much smaller scale, planetary systems form.
1164830
Oh Boy! got it right. The larger asteroids are also basically spherical. This is ultimately due to the fact that they have enough mass for the gravity to force them into this shape. Of course, there are irregularities, but they are fairly small except for the bulges caused by rotation. The smaller asteroids don't have enough mass for the gravity to have much effect.
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.
112797
Galaxies are constantly colliding with eachother because of gravitational attraction.  In fact, most galaxies are in clusters of galaxies that stay in clusters because of a mutual gravitational attraction,(including our own milky way), so in a sense, these clusters of galaxies have somewhat of a jumbled orbit around eachother untill they collide and form new galaxies.  These orbits are much different than planetary orbits in that they are not as uniform.
341419
Yes, Planets are generally spheres, not cubes or cylinders or odd shapes. Some small galactic bodies such as asteroids comets do have odd shapes, but planets, suns and stars and even moons look like spheres. That's because, when they were formed millions of years ago, they were essentially molten liquid.  Gravity and meniscus forces (surface tension) cause the object to take on a rounded shape--and it cools in that position.  In addition, the nature of gravity will pull uniformily toward the center--as the distance from the center increases, so does the gravitational force. You can think of gravity as a force that points inward toward the center of all things that have mass-- so that every part of the surface (especially liquids)is pulled evenly toward the center, resulting in a spherical shape.\n\nPlanets are not perfect spheres because there are other forces acting on the planets too which are trying to stretch them.  Because they were once hot moltent liquid, they buldge in the middle.  Earth and the moon reflect this phenomina.   Although gravity keeps planets close to spherical, there are other forces that cause deviations from the basic spherical shape. For example, the rotation of the earth once every 24 hours, causes an apparent centrifugal force which creates a bulge at the equator. In fact the earth's diameter at the equator is 7,926 miles while the diameter between the poles is only 7,900. \n\nNote that planets and moons had to be a liquid or they would not have been able to escape the gravitational force of the nearest larger body.  Remember that liquid will seek the path of least resistance as it moves.  In otherwords, an impact on the earth would cause a splash of moltent material that could escape the earth's atmosphere at the exact angle (path of least resistance) to be able to exit the planet's atmosphere without burning up.  Once it exits into space, the gravitational pull of the larger body will pull it into orbit. It will stay in orbit until it is acted upon by a larger force.   Fortunately, we haven't seen any larger forces since the earth was created and therefore, the conditions will remain for millions of years into the future.
1113779
Look up "disk accretion", a popular theory about the formation of our solar system.  This theory explains that as the cloud of dust and gas that formed our system was condensing, it's rotational motion caused an effect similar to centrifugal force which flattened the spinning cloud into a spinning disk.  \n\nI should note that the system isn't exactly flat, a couple of the outer planets have taken on orbits that are quite askew to the disk that the inner planets are aligned with.\n\nThe following web site has some simplified explanations of solar system formation:\nhttp://www.windows.ucar.edu/tour/link=/our_solar_system/formation.html\n\nand more in-depth info at:\nhttp://www.dtm.ciw.edu/akir/Seminar/seminar.html
892719
It's radial motion away from us - orbital would be around some central point, which the universe doesn't have (a gravitational center, at least) - although galaxies usually have some particular velocity within their clusters.
968297
The gravity is pretty much even all the way around, just like it is on Earth. The flat accretion disk is just the result of angular momentum of nearby matter. It is also the reason Saturn has flat rings and not just a cloud of stiff orbiting it.
563730
Galaxies are clusters of stars and its planetary systems found in groups across the universe. List of Galaxies can be found in link below. \n\nThey are really far... measured in light years and the distance is increasing as per our expanding universe.
34520
Gravity shapes objects.  If an objects mass is over a certain threshold then gravity will be strong enough to overcome molecular forces to "mold" an object.  Most asteriods have masses that are too low for this however planets and most moons are massive enough.  \n\nWhy a sphere?  This is due to the inverse radial dependance of the gravitational force.  In order words at a given distance from the center of mass distribution the force of gravity is the same.  This type of shape is a sphere.
1130599
The planets in this solar system are in stable orbits and get no farther away from each other than their orbits allow.\n\nYou are thinking of things like galaxies moving away from eachother, indicating that the universe is expanding.\n\nFirst off, to correct a misconception, some galaxies are moving towards one another, for example, the Andromeda Galaxy, and our own galaxy the Milky Way. \n\nCertain galaxies also seem to be "clumping" together, traveling towards something called "the geat attractor".\n\nDistant galaxies and quasars seem to be moving away.\n\nTo answer your question, some speculate that the expansion of the universe will continue forever and that gravity will lose in the long run. Eventually clusters of matter will become so seperated from eachother that the possability of any productive interactions approaches zero and stars will eventually die, and their remnants will be spread to thin to form into new ones. The universe, will essentially be a cold, dark, corpse. This is called the "cold death" theory. Depressing isn't it?\n\nBut never fear! There are other theories out there! Perhaps the universe will eventually implode into a big crunch, and the force of this implosion will create a new universe! Scientists aren't yet sure exactly how the universe will end. Hopefully none of us will be around to find out by example.
465388
The Earth is round for the same reason that things don't fall off it.  Gravity is a force in the universe that attracts all forms of matter equally, this means that pieces of matter will eventually move together and form clumps, if this is happening from every direction then the most likely shape it will form is a sphere.\n\nThis gravitational attraction is pretty small.  However, the Earth is pretty big, so the combined effect of all that gravity is just enough to keep your feet firmly planted on the surface :)\n\nThere are a few other related effects that come into play, such as centrifugal force, which keeps everything spinning and moving.