OT: Lost in the Galaxy? No wonder.

[Renegade 13]

Kamog said:
I don't understand galaxies. Especially elliptical ones. I can sort of see how a spiral galaxy works, it rotates around so the spiral arms orbit around the center and the rotation and the gravitational pull towards the center balance each other out. So the galaxy doesn't collapse or fly apart.

Pretty much, yeah. That sums it up. Also, elliptical galaxies are believed to form from mergers between two or more galaxies. These mergers could disrupt the spiral arms of either galaxy, due to the new gravitational forces that have to be compensated for. Thus, in many cases the spiral arms collapse, unless the merger is between a really small galaxy and a much larger one. Mergers like this have taken place many times in the Milky Way's history, but the galaxies that have been absorbed have seemingly been small enough to not significantly disrupt the spiral arm structure of our galaxy. Though it is difficult to tell, since we can't exactly take a nice little picture of our galaxy from the outside looking in to see what the structure actually is like.

So why doesn't an elliptical galaxy collapse under its own gravity? If it's spinning, that will stop it from collapsing in the plane of rotation but not at right angles to the rotation.

An elliptical galaxy won't collapse under its own gravitational attraction due to the fact you mentioned; spin. What stops the collapsing at right angles (or any angle) to the rotation is the fact that a lot of galaxies are essentially two-dimensional. They may be hundreds of thousands of light years across, but they're only a few thousand ly thick. So there basically aren't many stars at right angles to the rotation, and ones that are, yes could be pulled in...but bear in mind we're talking distances of thousands of light years, and stars travelling at a very low percentage of light speed. It would take a LONG time for them to reach the galactic center. Also, just because they're in the same galaxy as the majority of the stars that are spinning one way, doesn't mean they aren't rotating contrary to the rest of the stars. So even if they are at a right angle to the plane of the galaxy, the stars that aren't in the plane could have a different rotation that results in a stable 'orbit' around the galactic nucleus.

Also, I wonder how a barred spiral galaxy can form. What happened when the galaxy was forming? The bars full of stars extended outwards from the middle, straight, in opposite directions, and then after the bars grew to a certain length, the whole galaxy started to rotate so that the arms bent into a spiral? How can that happen, I just don't understand!

Barred spirals are probably, again, the result of galactic mergers disrupting things. For one thing, galactic mergers can cause "bow shocks" that compress interstellar gases in an area, causing a burst of star formation. This would result in stars of approximately the same age, and sort of "strung out" in a line. I suppose that strung out line of stars could form the bar of the barred spiral, but I'm really not very familiar with the finer points of galactic formation. The bar of the barred spiral galaxy could actually even be the remnants of the smaller galaxy that was absorbed!

Please bear in mind that I'm by no means an expert in astronomy or cosmology, all of what I have said is merely from the viewpoint of a very interested amateur. Most of it is taken from what I can remember having read in astronomy books, etc. and since my memory is fallible, the information is probably at least partly incorrect. But hopefully I could clear a little confusion.