It seems to me that it would be perfectly possible to get stars with planets like ours that aren’t in a giant group of millions of other stars, and as far as I can see the rest of the galaxy isn’t of any particular benefit to us (in the broad sense of “us”). But I don’t know a lot.
What’s the reason there are so many galaxies and are there in fact stars that form outside such large groups?
I agree, but that’s not what I meant. Let me try again:
What’s the reason there are galaxies at all and not just stars spread around more individually? I gather stars tend to form all close and cozy, but why?
As for the “do we need them” question: would there be any positive or negative influences on the formation or planets if stars just formed “individually”, and the same question for potential life on those planets.
Once gravity brings two stars close together, the two of them start pulling harder on other stars. Lather, rinse, repeat.
We actually seem the same pattern, from the atom to planet systems to solar systems to galaxies. It’s my guess that the big bang started off unbalanced and clustered, and the universe has followed suit ever since.
The large-scale structure of the universe resembles a sink full of soap bubbles. There are enormous voids separated by by thin walls and filaments of higher density. Galaxies are found in the high-density walls and filaments.
The vacuum of space inside our galaxy seems pretty empty compared to Earth, but actually it contains lots of gas and dust. But space in the enormous voids between galaxies is REALLY empty … on the order of one atom per square meter.
The universe is like this mostly because of gravity. If you start out with matter roughly evenly distributed throughout the universe, random chance will result in some areas that are slightly denser than others. A slightly denser area will have a slightly higher gravitational pull, which will suck more material in from surrounding areas. Over millions of years dense areas will get much more dense and sparse areas will get much less dense.
It’s unlikely that a star could form in the intergalactic voids. The atoms there are just too diffuse.
To add another factor: it’s widely believed that star formation is at least partly driven by the influence of nearby stars and galactic events. The spiral arms of the galaxies are essentially pressure waves moving through dust and gas and sometimes that pressure gives a little extra nudge to stars that are slowly collapsing due to gravity. Supernovae can also create pressure waves to with star formation. At the very least, galaxies help keep dust and gas in relative proximity, rather than evenly spread throughout millions of light years.
And terrestrial planets absolutely require galaxies or other star clusters. If elements heavier than hydrogen require stellar fusion to form, then a terrestrial planet requires multiple stars to form. You need one star to form and die so that you can get the carbon, oxygen, iron, etc. Then you need a second star to form and collect up the remnants of the old star. If you’re deep in intergalactic space, you’ll be too far away from any second star.
Galaxies do sometimes crash into each other. (Andromeda is on a collision course with our Milky Way, in fact) It might be possible, even probable, for some stars to get flung off into the intergalactic voids when that happens.
and then comes dark matter.
while certainly it is likely that stars and galaxies formed from small lumps getting bigger, there is a fundamental question as to where the small lumps came from. This figure supplies the answer: http://map.gsfc.nasa.gov/media/080999/index.html
That is one impressive plot! As I understand it, that plot shows that the quantum uncertainies that must exist everywhere at all times including during there expansion phase of the universe have been stretched out to cover the entire visible universe. When you are looking at the WMAP image, you are looking at the uncertainty principle writ large.
As for dark matter, realize that galaxies can’t exist as is. The rotation of galaxies would spin out many of the stars in short order. Gravity is too weak to hold the galaxy together. The inconvenient fact that galaxies do exist in spite of what Sir Newton says is strong evidence that dark matter exists. So, yes gravity pulls things together (most of the time), but a careful examination of the details shows it isn’t the whole answer.