Two questions from a non astrophysicist :)

[Loshmi]

Hello, just a couple of questions
I stumbled across the term "galaxy plane", and i got confused about it..

How do you define galaxy plane? Or better, WHERE do you define it? Im thinking... if the galaxy is 1000 light years thick, where would a galaxy plane be? In the statistical middle? Cause planes should be infinitely thick, right?

Second question: Are we (the human race) absolutely sure that the Sun is moving up and down the so called galaxy plane on its orbit around the galaxy center. Can you tell me what experiments confirmed this, and who did those experiments, and when.

Thanks in advance!

[loglo]

Here is a good potted history.

Mathematical planes may be infinitely thin but astronomers are more practical than mathematicians. Since the galaxy is 100 000 times as wide is it is thick the disk is essentially as good as a mathematical plane. Which is why the galactic coordinate system is centred on the Sun.

[Hornblower]

Here is a good potted history.

Mathematical planes may be infinitely thin but astronomers are more practical than mathematicians. Since the galaxy is 100 000 times as wide is it is thick the disk is essentially a mathematical plane. Which is why the galactic coordinate system is centred on the Sun.

Since when is the galaxy 100,000 times as wide as it is thick?

[loglo]

Since when is the galaxy 100,000 times as wide as it is thick?

Since I got carried away with the zeros. Should have been a 100 of course.

[grant hutchison]

How do you define galaxy plane? Or better, WHERE do you define it? Im thinking... if the galaxy is 1000 light years thick, where would a galaxy plane be? In the statistical middle? Cause planes should be infinitely thick, right?

Infinitely thin.
The "galactic plane" would be the mid-plane for the mass distribution of the galactic disc. Since the exact mass distribution is unknown, its precise location is unknown, but by observing the distribution of stars and gas in the sky we know that the Sun lies a little north of the galactic plane, by a few tens of lightyears.

Second question: Are we (the human race) absolutely sure that the Sun is moving up and down the so called galaxy plane on its orbit around the galaxy center. Can you tell me what experiments confirmed this, and who did those experiments, and when.

If we look at the stars in the Sun's neighbourhood, we see them all moving with different velocities. If we average out those velocities, we can recover the Sun's own movement relative to the Local Standard of Rest (we assume that the local stars on average are moving around the galaxy at the local orbital velocity, in the plane of the galaxy). The exact result depends on how we sample the local stars, but all the calculations show that the Sun has a velocity component that points north out of the galactic plane (as well as moving a little towards the galactic centre, and a little faster around the galaxy than the local mean). That means we are moving away from the galactic plane at present, but not fast enough to escape from the galaxy. Therefore, we'll climb north for a while, then fall south through the galactic plane, move southwards for a while, then fall north again, oscillating backwards and forwards with a period of tens of millions of years. The first such calculations were carried out by an astronomer called Frank Bash; as I recall, in the 1960s or 1970s. They've been refined since.

So we know the Sun's velocity from statistical studies of local stars; we know the mass distribution of the galaxy by observations of distance stars and gas/dust clouds. The movement of the Sun in its galactic orbit is then a matter of calculation.

Grant Hutchison

[Loshmi]

Wow, thank you all for answering so quickly, you guys are great.

Yes i meant infinitely thin

If the precise location of galaxy plane is unknown, how do we use that plane to calculate positions of other objects???

This article:

http://en.wikipedia.org/wiki/Galactic_coordinate_system

says: "The symbols ℓ and b are used to represent the galactic longitude and latitude, respectively. The galactic longitude is measured in the plane of the galaxy using an axis pointing from the Sun to the galactic center. The galactic latitude is measured from the plane of the galaxy to the object using the Sun as vertex."
So if we are uncertain about the position of the galaxy plane, we are uncertain about all the positions, using that coordinate system, correct?

And also shouldn't Sun be in the galaxy plane, for a calculation like that?
Maybe im missing something in "using sun as a vertex" part.

[grant hutchison]

This article:

http://en.wikipedia.org/wiki/Galactic_coordinate_system

says: "The symbols ℓ and b are used to represent the galactic longitude and latitude, respectively. The galactic longitude is measured in the plane of the galaxy using an axis pointing from the Sun to the galactic center. The galactic latitude is measured from the plane of the galaxy to the object using the Sun as vertex."
So if we are uncertain about the position of the galaxy plane, we are uncertain about all the positions, using that coordinate system, correct?

And also shouldn't Sun be in the galaxy plane, for a calculation like that?
Maybe im missing something in "using sun as a vertex" part.

Well, Wikipedia has never been renowned for its clarity.
"In the plane of the galaxy using an axis pointing from the Sun to the galactic center" means that the galactic coordinate system is based on a plane passing through the sun, parallel to the plane of the galactic disc. So the coordinate system is offset northwards from the mean plane of the galactic disc, by whatever distance the Sun is positioned north of the mean plane. Since galactic coordinates are based on our own location, they're precise.

Grant Hutchison

[jfribrg]

BTW,

Welcome Loshmi from a fellow non-astrophysicist.

I don't know if this works with all galaxies, but with a spiral galaxy, the galactic plane would be normal to the axis of rotation. It would seem reasonable that the galactic plane would intersect the center of mass of the galaxy. I suspect that gravitational dynamics would impose as certain high level symmetry, so that the above definition would be usable in most cases, but in reality I'm just taking a guess.

Hope this helps.

[AndreasJ]

Well, Wikipedia has never been renowned for its clarity.
"In the plane of the galaxy using an axis pointing from the Sun to the galactic center" means that the galactic coordinate system is based on a plane passing through the sun, parallel to the plane of the galactic disc.

Hm? Since the Sun is above the galactic plane, pressumably the plane in which the Sun-galactic centre line lies is inclined compared to the galactic plane?

[grant hutchison]

Hm? Since the Sun is above the galactic plane, pressumably the plane in which the Sun-galactic centre line lies is inclined compared to the galactic plane?

Only if we actually used the physical galactic centre as an anchor point for the coordinates.
But in 1958 the position of the galactic centre was not well known, so we defined galactic coordinates using our best guess at the position of the galactic rotation poles in the sky, and then ran a plane at right angles to that axis to define the galactic equator. The intention implicit in that construction is to produce a plane parallel to the disc midplane, passing through the sun. Which would place the centre of the galaxy at a small southerly latitude in our coordinates.
And, in the time since the coordinates were invented in 1958, we've discovered that the black hole at the galactic centre does indeed lie at a small southerly latitude in our chosen coordinates, so the original pole estimates must have been pretty good.

Grant Hutchison

[peter eldergill]

BTW,

Welcome Loshmi from a fellow non-astrophysicist.

I don't know if this works with all galaxies, but with a spiral galaxy, the galactic plane would be normal to the axis of rotation. It would seem reasonable that the galactic plane would intersect the center of mass of the galaxy. I suspect that gravitational dynamics would impose as certain high level symmetry, so that the above definition would be usable in most cases, but in reality I'm just taking a guess.

Hope this helps.

And for other non-astrophysicists "normal" means perpendicular

Pete

[sirius0]

And for other non-astrophysicists "normal" means perpendicular

Pete

I know from my own history that some "normal" people just want to go and lie down when it comes to understanding perpendicular on a spinning object. So I like to add "like a wheel spinning on it's axel"

[Loshmi]

Thank you all for great answers, it would take me a lot of reading and searching to find out all that.

Only if we actually used the physical galactic centre as an anchor point for the coordinates.
But in 1958 the position of the galactic centre was not well known, so we defined galactic coordinates using our best guess at the position of the galactic rotation poles in the sky, and then ran a plane at right angles to that axis to define the galactic equator. The intention implicit in that construction is to produce a plane parallel to the disc midplane, passing through the sun. Which would place the centre of the galaxy at a small southerly latitude in our coordinates.
And, in the time since the coordinates were invented in 1958, we've discovered that the black hole at the galactic centre does indeed lie at a small southerly latitude in our chosen coordinates, so the original pole estimates must have been pretty good.

Grant Hutchison

So according to those coordinates galactic center has a southern latitude. Finally everything makes sense. Thanks fro the clarification. Thank all of you, i really appreciate it.

[ShinAce]

When I think of normal, I think of a line projecting from a surface.

The surface is not normal to the axis, it is the axis that is normal to the surface. So I feel it's incorrect to say the galactic plane is normal to anything. It is perpendicular. The axis of galaxy rotation should be normal to the galactic plane.

Which is to say I wouldn't normally use normal in the context of this thread.

/nitpic

[Jeff Root]

When I think of normal, I think of a line projecting from a surface.

Right.

The surface is not normal to the axis, it is the axis that is normal to the
surface.

That isn't right. It isn't even left.

They are normal to each other.

So I feel it's incorrect to say the galactic plane is normal to anything.
It is perpendicular.

The terms are synonymous in this useage.

The axis of galaxy rotation should be normal to the galactic plane.

It is, and thus the galactic plane is normal to the axis of rotation.

Which is to say I wouldn't normally use normal in the context of this thread.

That's nonstandard, thus abnormal.

-- Jeff, in Minneapolis