APOD: A Milky Way Dawn (2014 Mar 29)

[Nitpicker]

As if anyone would choose to interpret my statement in that way.

Apologies for the light banter. I thought I had already sewn it up with "you'd have to rely on ... a very good horizon".

[Nitpicker]

The sun is currently considered to be <100 ly away from the galactic plane, and the average outer disk thickness is many thousands of light years.

I'm often a little confused by this point. My understanding is that the galactic plane is a contrived human convention, which passes through the Sun and the galactic core, and that the Sun is the centre of the galactic coordinate system, with the core defining longitude zero.

I'm also aware that the thickness of the Milky Way can be estimated, as well as our position within the thickness, and we are thought to be not quite in the middle of the thickness. But I didn't think we had mapped the Milky Way so well as to define a "true" or "natural" galactic plane with any level of precision.

Are there correct terms for each of these two planes?

[Chris Peterson]

The sun is currently considered to be <100 ly away from the galactic plane, and the average outer disk thickness is many thousands of light years.

I'm often a little confused by this point. My understanding is that the galactic plane is a contrived human convention, which passes through the Sun and the galactic core, and that the Sun is the centre of the galactic coordinate system, with the core defining longitude zero.

I'm also aware that the thickness of the Milky Way can be estimated, as well as our position within the thickness, and we are thought to be not quite in the middle of the thickness. But I didn't think we had mapped the Milky Way so well as to define a "true" or "natural" galactic plane with any level of precision.

Are there correct terms for each of these two planes?

I think they are both called the galactic plane, and you just need to depend on context. Except in the specific case of discussing galactic coordinates, I would always take "galactic plane" to mean the physical plane that separates the galaxy's northern and southern hemispheres, which is clearly the intent in the above quote.

[Ann]

I have "always" believed that if I somehow managed to grow old enough (say, a couple of million years) I would be able to see the center of the Milky Way as the Sun dragged the solar system along with it as it rose sufficiently high above the galactic plane.

Ann

[Chris Peterson]

I have "always" believed that if I somehow managed to grow old enough (say, a couple of million years) I would be able to see the center of the Milky Way as the Sun dragged the solar system along with it as it rose sufficiently high above the galactic plane.

We don't ever get very far from the galactic plane. So even over millions of years, I'm afraid the view isn't going to change much. If you figure out how to live millions of years, you'd do better to invest the time aboard a spaceship heading out of the galactic plane. Then you might get a good view.

[neufer]

I have "always" believed that if I somehow managed to grow old enough (say, a couple of million years) I would be able to see the center of the Milky Way as the Sun dragged the solar system along with it as it rose sufficiently high above the galactic plane.

We don't ever get very far from the galactic plane. So even over millions of years, I'm afraid the view isn't going to change much. If you figure out how to live millions of years, you'd do better to invest the time aboard a spaceship heading out of the galactic plane. Then you might get a good view.

Folks who have somehow managed to grow old enough (say, a hundred years) might still be able to remember when they could see the Milky Way from their own back yards (i.e., before light pollution). We young whippersnappers will soon be able to see 3D computer renditions of the Milky Way thanks to Gaia.

[Chris Peterson]

Folks who have somehow managed to grow old enough (say, a hundred years) might still be able to remember when they could see the Milky Way from their own back yards (i.e., before light pollution).

I'm only halfway to a hundred, and I can see the Milky Way from my backyard. That just goes to show that I made better decisions in selecting my backyard than some.

[ems57fcva]

There is something about the contrast in this image that reveals something interesting: There appear to be two dust lanes in this image. What appears to be the foreground dust land starts on the upper right side of the Milky Way. As it come to the central bulge area, it heads downward and around the bulge, and comes back to the plane of the galaxy on the other side. Another dust lane in the background goes across the central bulge of the Milky Way. This can also be seen in http://apod.nasa.gov/apod/ap140212.html (where the second dust lane is above the central bulge) if you know what you are looking for.

I take this to indicate the the Sun offset from the plan of the galaxy a ways. so that at its closest to us we are viewing the foreground dust lane from a different angle than the background one. Another possibility is that the Milky Way is warped like may other spiral galaxies are. This could even be a combination of the two.

I believe that the answer is due to non-uniform matter distribution in the disk, maybe due to warping but not necessarily. The sun is currently considered to be <100 ly away from the galactic plane, and the average outer disk thickness is many thousands of light years. The perspective you're seeing is over 10's of thousands of light years. I think our displacement from the plane is too small to reveal the large structural differences in angle that you're referring to.

I think I have a clue to what is going on. See http://apod.nasa.gov/apod/ap110515.html, a Hubble picture of the Sombrero Galaxy. At the center of the picture, we are looking straight at the edge of this slightly warped galaxy, and you see dust lanes one above the other. So what is displaced from the galactic plane most likely is the dust lane itself, although a displacement on the part of the Sun could still contribute. That eliminates warping as an issue, and in hindsight it should not be an issue for a nearby dust lane.

Even so, the galactic center is more evident on one side of the central bulge than the other, and that is evidence for either a slight warping or a small displacement of the Sun from the physical central plane. (Either must be small. Otherwise we would be able to see the other side of the galaxy.)

[neufer]

Folks who have somehow managed to grow old enough (say, a hundred years) might still be able to remember when they could see the Milky Way from their own back yards (i.e., before light pollution).

I'm only halfway to a hundred, and I can see the Milky Way from my backyard. That just goes to show that I made better decisions in selecting my backyard than some.

But could you see the Milky Way from the backyard that you grew up in as a child half a century ago?

(In any event, I was referring to the majority of the population in the developed world).

[Chris Peterson]

But could you see the Milky Way from the backyard that you grew up in as a child half a century ago?

Not very often.

[Nitpicker]

The sun is currently considered to be <100 ly away from the galactic plane, and the average outer disk thickness is many thousands of light years.

I'm often a little confused by this point. My understanding is that the galactic plane is a contrived human convention, which passes through the Sun and the galactic core, and that the Sun is the centre of the galactic coordinate system, with the core defining longitude zero.

I'm also aware that the thickness of the Milky Way can be estimated, as well as our position within the thickness, and we are thought to be not quite in the middle of the thickness. But I didn't think we had mapped the Milky Way so well as to define a "true" or "natural" galactic plane with any level of precision.

Are there correct terms for each of these two planes?

I think they are both called the galactic plane, and you just need to depend on context. Except in the specific case of discussing galactic coordinates, I would always take "galactic plane" to mean the physical plane that separates the galaxy's northern and southern hemispheres, which is clearly the intent in the above quote.

Thanks Chris. The "physical plane that separates the galaxy's northern and southern hemispheres" does not appear to be well defined, other than as a self referential definition. Or perhaps I have misunderstood. Is there a basis for defining it? I agree there is no confusion in alter-ego's statement, but there is often confusion. Perhaps it should be stated as the "natural galactic plane", versus the "fundamental plane of the galactic coordinate system" (which is quite a mouthful). There is not a huge difference between the two planes (less than a quarter of a degree if we are 100ly from the natural plane and 25,000ly from the core). Another term which is easily confused is "galactic poles". The polar axis of the Milky Way, projecting from the core, normal to the natural plane, appears as a line or arc in the sky to us. But there are also the poles of the galactic coordinate system, which appear as points in the sky to us. The line joining these points is normal to the "fundamental plane of the galactic coordinate system" and passes through the Sun (and us within a sensible tolerance). Quite a difference.

The current Wikipedia article on the topic seems rather poor, and mangles the helio-centric and core-centric contexts:
http://en.wikipedia.org/wiki/Galactic_plane

[alter-ego]

Thanks Chris. The "physical plane that separates the galaxy's northern and southern hemispheres" does not appear to be well defined, other than as a self referential definition. Or perhaps I have misunderstood. Is there a basis for defining it? I agree there is no confusion in alter-ego's statement, but there is often confusion. Perhaps it should be stated as the "natural galactic plane", versus the "fundamental plane of the galactic coordinate system" (which is quite a mouthful). There is not a huge difference between the two planes (less than a quarter of a degree if we are 100ly from the natural plane and 25,000ly from the core). Another term which is easily confused is "galactic poles". The polar axis of the Milky Way, projecting from the core, normal to the natural plane, appears as a line or arc in the sky to us. But there are also the poles of the galactic coordinate system, which appear as points in the sky to us. The line joining these points is normal to the "fundamental plane of the galactic coordinate system" and passes through the Sun (and us within a sensible tolerance). Quite a difference.

The solar system has an analogous definition which conceptually is the same, but maybe not estimated the same way. The solar system's "natural plane" is referred to the Invariable Plane. In this reference system, the ecliptic is off by 1.57°. However, determining the galaxy's "invariable plane" and it's precision depends on the estimations and assumptions. It seems a lot more difficult to determine it, e.g it appears to be defined based on visible matter and not dark matter (?). Also, defining the solar system's invariable plane and barycenter seems at least tenable based on it's relatively near-zero finite size compared to the galaxy.

... The invariable plane of a planetary system, also called Laplace's invariable plane, is the plane passing through its barycenter (center of mass) perpendicular to its angular momentum vector. In the Solar System, about 98% of this effect is contributed by the orbital angular momenta of the four jovian planets (Jupiter, Saturn, Uranus, and Neptune). The invariable plane is within 0.5° of the orbital plane of Jupiter,[1] and may be regarded as the weighted average of all planetary orbital and rotational planes.

In the picture below, the galactic equator (as we are used to thinking of it) is equivalent to the Earth's ecliptic which passes through the Sun's center. The "Galactic Plane" is what I conceptually associate to the Invariable Plane. Other than looking at snapshot of observable mass distribution, I don't know how it is exactly determined.

[Chris Peterson]

The "Galactic Plane" is what I conceptually associate to the Invariable Plane. Other than looking at snapshot of observable mass distribution, I don't know how it is exactly determined.

I don't believe it is exactly determined. Indeed, I'm not sure what value there would be in doing so, even if it were possible. It's really a qualitative concept, not a quantitative one.

[alter-ego]

I believe that the answer is due to non-uniform matter distribution in the disk, maybe due to warping but not necessarily. The sun is currently considered to be <100 ly away from the galactic plane, and the average outer disk thickness is many thousands of light years. The perspective you're seeing is over 10's of thousands of light years. I think our displacement from the plane is too small to reveal the large structural differences in angle that you're referring to.

Even so, the galactic center is more evident on one side of the central bulge than the other, and that is evidence for either a slight warping or a small displacement of the Sun from the physical central plane. (Either must be small. Otherwise we would be able to see the other side of the galaxy.)

Interestingly, the asymmetry of our galaxy's galactic bulge does lend itself to a reasonable estimate of how far the Sun is displaced from the galactic plane. I used this Milky Way panorama in the picture below and chose a central plane axis based on the primary dust lane (Great Rift) and extended disk. I assumed the Great Rift distance estimate to be 300 ly and, as shown in my picture, I measured the asymmetric bulge component (a parallax) to be ~5°. From these numbers, the Sun would be +26 ly above (northward) of the galactic plane. This is correct in direction and agrees well in magnitude. Independent estimates based on several hemispheric comparison methods (e.g. star and molecular cloud counts, dust and gas distribution) yields a weighted average displacement of 53 ly northward. If your interested also see: The Sun's Distance Above the Galactic Plane

I think for the purposes here, using the apparent asymmetry of the galactic bulge against the Great Rift is valid. I'd never done that analysis before and a little surprised the simple model was that close.

[Nitpicker]

Thanks. I think the terms Galactic Plane and Galactic Equator are better than the terms I suggested. I will stop using the former for the latter.

[alter-ego]

The "Galactic Plane" is what I conceptually associate to the Invariable Plane. Other than looking at snapshot of observable mass distribution, I don't know how it is exactly determined.

I don't believe it is exactly determined. Indeed, I'm not sure what value there would be in doing so, even if it were possible. It's really a qualitative concept, not a quantitative one.

I was just thinking , given all the methods that go into determining the Sun's displacement, e.g star count asymmetry (see my link above), there is some quantitative definition. However, it may be a single point definition, i.e. we can accurately determine our distance from the Galactic Plane but we can't accurately establish the plane's orientation except a two points: where we are and maybe the galactic center with greater uncertainty.

[Chris Peterson]

I was just thinking :), given all the methods that go into determining the Sun's displacement, e.g star count asymmetry (see my link above), there is some quantitative definition. However, it may be a single point definition, i.e. we can accurately determine our distance from the Galactic Plane but we can't accurately establish the plane's orientation except a two points: where we are and maybe the galactic center with greater uncertainty.

We can identify the galactic center with high certainty, but not its invariable plane, and therefore not our distance from it.

[Nitpicker]

Looks to me like the Galactic Plane and the Galactic Equator both fit wholly within the nebulous stuff which gives the Milky Way its name. I like the description of the Galactic Plane as a qualitative one.

I have inadvertently read a bit of "dodgy looking" conjecture about the significance of crossing the Galactic Plane, in terms of triggering changes to the gravitational balance of the Solar System, extinction events on Earth, blah, blah, blah. I've always been pretty sceptical about it. And yet, I do still maintain a slight suspicion that the Coriolis effect may very occasionally tip the balance in setting the direction in which a bathtub drains. No one has yet been able to quantify how far I am from the scientific Plane of Consensus, though a few have been qualitative.

[alter-ego]

I was just thinking , given all the methods that go into determining the Sun's displacement, e.g star count asymmetry (see my link above), there is some quantitative definition. However, it may be a single point definition, i.e. we can accurately determine our distance from the Galactic Plane but we can't accurately establish the plane's orientation except a two points: where we are and maybe the galactic center with greater uncertainty.

We can identify the galactic center with high certainty, but not its invariable plane, and therefore not our distance from it.

Well, I guess what's in question is whether the algorithms defining hemispherically-symmetric mass distributions (e.g. star counts) define the invariable plane. The definition of the (local) galactic plane and our distance from it is defined and measurable with an uncertainty of ~15%. Does the invariable plane fall within this uncertainty? I'd bet it might, and, ideally, the more stars and their displacements that could be measured, the better the average plane could be determined. This average plane (or surface) should suffice to define the invariable plane more accurately. But alas, this is purely academic at this point. The Sun's local displacement as defined by hemispherically-symmetric mass distributions is all we got, and the concept of invariable plane works for me to visualize the galactic plane.