Q:
It seems like we only ever speak in horizontal terms when referring to space. What happens when we go straight up? Are there other galaxies way above us?
- Graeme S.
A:
Space is (more or less) "isotropic" - that is, on a large scale it's pretty much the same in every direction. "Large scale" means "huge groups of clusters of huge numbers of galaxies" - on that scale, if you point in any direction you'll see about the same number of galaxies.
On a smaller scale it's a bit different: our solar system is actually very flat. That comes from the way our solar system was formed: we started off as a big blob as gas which contracted into a small blob of gas. The initial big blob of gas is going to be spinning a little - just because if you take a bunch of random gas the odds that it's perfectly still is pretty much zero. As it contracts, it starts spinning faster and faster - this is just the conservation of angular momentum. The spinning makes the gas blob stretch out into a thin pancake, which eventually turns into the Sun and the planets, which stay in a nice flat plane as well. Something kinda similar happens with the Milky Way, our galaxy, as well - so both our galaxy and our solar system have most of their material in a fairly flat shape. Once you go beyond our galaxy though, all the other galaxies are pretty much all over the place - in fact, two of the closest galaxies, the Magellanic Cloud are pretty close to straight "down" from the plane of the Milky Way.
One more thing - even though most of the stuff in the Milky Way is in the flat plane, that plane is thousands of light years thick. There's also a good number of "halo" stars - stars in a more sphere-type shape, nowhere near the flat plane. The dark matter is also supposed to be spread out in a pretty much spherical shape as well.
That help? :) Keep asking questions!
Monday 30 November 2009
First Question
Q:
Hey Dave, are the suns/ galaxies on the outer edge of the universe moving outward faster than the inner suns/ galaxies?
A:
sorta.
The universe is expanding, with every galaxy in the universe getting further away from every other galaxy. The image that is most appropriate is stretching a rubber sheet - every point on the sheet is getting further away from every other point.
To answer your question: consider the rubber sheet again and pretend your galaxy is somewhere on it. If a galaxy is right night to you, then the stretching is only going to move it a little further away. If another galaxy is a lot further away then the stretching is going to make it move away much faster.
Spacetime is stretching in a similar way: every bit of space is expanding, so the more space between us and an object, the faster it seems to be going away. Note this doesn't mean we're in the centre of the universe: anybody in any galaxy anywhere will notice the more distant galaxies are moving away faster than the closer galaxies. Does that help? :)
Q:
Yep thx. So does than mean all the galaxies than people r studying r red shifting?
A:
Almost always yes - in fact, astronomers use redshift as a measure of distance. The exception is that when galaxies are real close, this effect is quite small, and so the proper motion of galaxies (due to gravity etc) is stronger at short ranges than the expansion of the universe, hence it's possible for nearby galaxies to be heading towards us, i.e. blueshifted.
Q:
Ah ok, cool. Thx 4 that.
Hey Dave, are the suns/ galaxies on the outer edge of the universe moving outward faster than the inner suns/ galaxies?
A:
sorta.
The universe is expanding, with every galaxy in the universe getting further away from every other galaxy. The image that is most appropriate is stretching a rubber sheet - every point on the sheet is getting further away from every other point.
To answer your question: consider the rubber sheet again and pretend your galaxy is somewhere on it. If a galaxy is right night to you, then the stretching is only going to move it a little further away. If another galaxy is a lot further away then the stretching is going to make it move away much faster.
Spacetime is stretching in a similar way: every bit of space is expanding, so the more space between us and an object, the faster it seems to be going away. Note this doesn't mean we're in the centre of the universe: anybody in any galaxy anywhere will notice the more distant galaxies are moving away faster than the closer galaxies. Does that help? :)
Q:
Yep thx. So does than mean all the galaxies than people r studying r red shifting?
A:
Almost always yes - in fact, astronomers use redshift as a measure of distance. The exception is that when galaxies are real close, this effect is quite small, and so the proper motion of galaxies (due to gravity etc) is stronger at short ranges than the expansion of the universe, hence it's possible for nearby galaxies to be heading towards us, i.e. blueshifted.
Q:
Ah ok, cool. Thx 4 that.
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