Q:
Do we know which direction the Milky way "pancake" is spreading out relative to the position of Earth?
In other words: Does our North pole point toward the "top" of the pancake, the south pole toward the "bottom" of the pancake and East and West toward the edges of the pancake?
I find it most common for us to think of North as up, South as down and East and West as left and right. How are we actually situated in our Galaxy even if the rest of space is isotropic
A:
Within the solar system, most of the planets are rotating fairly close-ish in line with their orbit around the sun. The Earth's north pole isn't exactly at right angles with our orbit though - it's out by about 23°. The north pole always points in the same direction, even though we are orbiting around the sun. So half the the north pole is pointing slightly towards the sun, and half the time it's pointing away - that's actually what causes the seasons.
The plane that the sun, the moon and all the planets are on is called the "ecliptic". From Earth it looks like a great big arc. If you can see the moon and a couple of planets at night or the sun and the moon at day you can actually trace vaguely where it goes. Also, all of the signs of the zodiac are along this curved line - so if you find a starchart and look up a few of those constellations (Scorpius, Libra, Gemini, Taurus etc) then you at night you can find where the ecliptic is.
The galactic plane isn't the same as the ecliptic though. But that's actually easier to see at nigh (if it's dark enough): it's the Milky Way, the faint stripe of stars going across the sky.
See if you can find both the galactic plane (the Milky Way) and the ecliptic in the same night: it really helps to see our place in the universe.
But if you like angles: The plane of the solar system is about 23° from the Earth's equator, and the plane of the galaxy is about 63° from the Earth's equator.
Wednesday, 2 December 2009
Tuesday, 1 December 2009
Do stars spin?
Q:
Do stars spin?
A:
Yes.
You can see an animation of the sun rotating here. It rotates every 25 days or so. This rotation comes from angular momentum left-over from the gas cloud that collapsed to form the solar system. Actually, this initial gas cloud had too much angular momentum - the sun had to get rid of it by firing out huge jets of gas before settling into its present state.
Other stars rotate too - some of them are so fast that they are "oblate" - the centrifugal force pulls them out in the middle, so they're a bit fatter and a bit shorter than a slower rotating stars.
Do stars spin?
A:
Yes.
You can see an animation of the sun rotating here. It rotates every 25 days or so. This rotation comes from angular momentum left-over from the gas cloud that collapsed to form the solar system. Actually, this initial gas cloud had too much angular momentum - the sun had to get rid of it by firing out huge jets of gas before settling into its present state.
Other stars rotate too - some of them are so fast that they are "oblate" - the centrifugal force pulls them out in the middle, so they're a bit fatter and a bit shorter than a slower rotating stars.
Monday, 30 November 2009
Space is 3D
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!
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!
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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