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.

[Strategia_In_Ultima]

Again, *fizzle*

[EvilGenius4ABetterTomorro]

I've always been amazed at how we can't see the dense center area of stars (sans black hole of course). You'd think that with all those stars, the brightness would be obvious. Someone told me we can't all the light from the center because of all the interstellar dust in the way. I hope he wasn't B.S.ing me.

[Renegade 13]

He wasn't BSing you, that is the currently accepted and most plausible theory.

[cshank2]

The dust and because of how Earth is placed, the milky way we see is actually the converging point of the... Persius and... Some other arm for the galaxy. Sol is right between those arms

[Renegade 13]

And the Orion arm I believe. Or was it Saggitarius...

[Kamog]

Renegade 13, thanks for the explaination about galaxies.

[Renegade 13]

No problem at all, I love talking about stuff like that But bear in mind, it might be partly wrong!

[Karibu]

What makes things more interesting, the matter we see is only smaller part of the matter of the galaxy. Dark matter is unseen matter, which has about 10 times more mass in our own galaxy than we have matter we can see.

A quotation from Wikipedia article:

For comparison, the Milky Way is believed to have roughly 10 times as much dark matter as ordinary matter.

For those who say it is/may be untrue: This same conclusion about dark matter you can make from various different places in the internet, including BBC site and Berkeley university.

[Renegade 13]

I love astronomy. Look at all the fascinating things you learn!

Dark matter is still, technically, a theory, but it is a theory that is nearly universally accepted today. There simply isn't enough baryonic ("normal") matter to account for all the gravitational effects we observe. If the visible matter was the only stuff there was, the large scale structures would not/could not exist, since the gravity required for them to form and stay formed simply would not be present. By the way, large scale in this context means larger than the solar system. Without dark matter, structures like galaxy clusters and superclusters wouldn't exist, even galaxies may not have enough gravity to stay together.

Scientists recently discovered a particle that makes up part of dark matter; neutrinos. Neutrinos are a by-product of nuclear fusion, among other things I'm certain, and so are hugely abundant. However, until recently, scientists didn't think they had any mass at all, that they were a "massless" particle. However, they recently have determined the mass, which although it is vanishingly small, it does exist. Neutrinos do make up a percentage of dark matter, but it is a quite small percentage.

Another candidate for dark matter is the Higgs Boson particle. This one hasn't even been detected, only theorized. But if it does exist, it should account for a large portion of dark matter, since it would have a large mass. Higgs Boson Link The Large Hadron Collider, a particle accelerator that is, I think, already under construction at CERN, should prove or disprove the existance of the Higgs.

Things get a lot more strange however, when you think of things like dark energy, a theoretic form of energy that is sort of "anti-gravitational", and so is causing the expansion of the universe to accelerate. It appears dark energy rules the vast inter-galactic spaces, and gravity rules the on the galactic scale. In other words, dark energy is powerful enough to continue to force galaxies apart from one another, but not powerful enough to rend the galaxies apart.

Scientists now predict that "normal" baryonic matter, of which you and I are all made, composes merely 4 or 5% of the universe. Dark matter, either baryonic dark matter, or hot/cold dark matter makes up another 25%, with dark energy taking the other 70%. Dark Energy Link

But I'm sure I've totally gone beyond the point of interest here, so I'll shut up And I just realized most of what I said about dark matter is already in the link Karibu posted But hopefully the links I provided are interesting