APOD: Milky Way Motion in 3D from Gaia (2022 Jul 06)

[APOD Robot]

Milky Way Motion in 3D from Gaia

Explanation: Our sky is alive with the streams of stars. The motions of 26 million Milky Way stars are evident in the featured map constructed from recent data taken by ESA's Gaia satellite. Stars colored blue are moving toward us, while red indicates away. Lines depict the motion of the stars across the sky. The large blue on the left and red areas on the map's right give the overall impression that stars in the Milky Way are rotating around the center. However, there is a region near the middle -- caused by our own Sun's motion relative to a rigidly-rotating central Galactic bar -- that seems to reverse it. Understanding details about the motion of stars is helping humanity to better understand the complex history of our Milky Way galaxy and the origin of our Sun.

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[bystander]

viewtopic.php?t=42438

[XgeoX]

So it seems according to the image that the central bar is rotating in a direction opposite the rest of the galaxy but the caption implies this is an illusion caused by the Sun’s motion.
How does that work?



Eric

[orin stepanek]

MilkyWayMotion_Gaia_1080.jpg

I watched the feature video and was surprised that so many stars
were going helter-sklelter in so many directions at once! I guess I
pretty much presumed they were going around the galaxy in an
orderly fashion like the planets do around the sun!
https://cdn.pixabay.com/photo/2019/09/0 ... 3_1280.jpg
Pretty photo of Kitty! I couldn't transfer pic, so I sent URL instead!

[Chris Peterson]

MilkyWayMotion_Gaia_1080.jpg
I watched the feature video and was surprised that so many stars
were going helter-sklelter in so many directions at once! I guess I
pretty much presumed they were going around the galaxy in an
orderly fashion like the planets do around the sun!

Aside from a few than might have been perturbed into hyperbolic orbits (and are heading out, never to return), all of the stars are orbiting the central part of the Milky Way, quite orderly! (Consider that the planets of the Solar System, as viewed from the Earth, look like they're going all over the place, too.)

[Chris Peterson]

So it seems according to the image that the central bar is rotating in a direction opposite the rest of the galaxy but the caption implies this is an illusion caused by the Sun’s motion.
How does that work?

We're not seeing the motion of the stars here in the galaxy's frame of reference, but in our own. Compare this with the observed motion of a planet in the Solar System, where Earth's motion creates retrograde movement, and planets appear to reverse direction in the sky.

[orin stepanek]

MilkyWayMotion_Gaia_1080.jpg
I watched the feature video and was surprised that so many stars
were going helter-sklelter in so many directions at once! I guess I
pretty much presumed they were going around the galaxy in an
orderly fashion like the planets do around the sun!

Aside from a few than might have been perturbed into hyperbolic orbits (and are heading out, never to return), all of the stars are orbiting the central part of the Milky Way, quite orderly! (Consider that the planets of the Solar System, as viewed from the Earth, look like they're going all over the place, too.)

Thanks Chris; Now that makes sense.

[RJN]

An email from the Gaia team has indicated that our Galaxy's bar is not evident in the featured map. Therefore, that sentence has now been removed from the main NASA APOD. We apologize for the mistake. A link from Gaia that more fully explains some of the map's more unusual features is given here: https://www.cosmos.esa.int/web/gaia/dr3 ... -move-away . This link has now been added to the main NASA APOD.

- RJN

[guestaroni]

I think these images would be much easier for the layperson to understand if they could be shown with a 360­° viewer, with the image projected on the inside of a sphere, and the viewpoint at the center of the sphere. Is this a valid idea? Does anyone know of where this can be found?

[guestaroni]

I think these images would be much easier for the layperson to understand if they could be shown with a 360­° viewer, with the image projected on the inside of a sphere, and the viewpoint at the center of the sphere. Is this a valid idea? Does anyone know of where this can be found?

After some more searching for the right lingo, I think I'm asking for a conversion from Mollweide projection, to an interactive spherical projection app with viewpoint at the sphere's center. Or for a real treat, dynamic parameters for projection types, shapes and viewpoints! I can dream, right? An app for this would be epic, as there are some many of these Mollweide projection images out there.

[johnnydeep]

I'm not understanding why there are disjoint areas of red and blue in this map. At first I thought that was caused by the Sun not being at the exact center of the Milky Way, but apparently that's not right. The cosmos link says:

The sample from Gaia DR3 extends further out and the resolution is much improved, thanks to the large number of measurements. The cloverleaf pattern of negative-positive-negative-positive galactic radial velocities (that is, contracting and expanding motions with respect to the Galactic center) due to the stars orbiting within the Galactic bar, is also clearly seen. The change in sign is aligned with the bar axes. A bar angle of 20 degrees is inferred from the Gaia measurements – on the small end of past determinations. The angular rotation rate of the bar is also estimated and is found to be in good agreement with the most recent determinations using less direct methods.

I'm sure the math works out, but my obsolete 3D software just can't make sense of it.

[Ann]

Wouldn't it be great to see an animation of this and actually see the stars move?

Except that... yeah. You'd need a 3D screen for that. Or red/blue glasses or squinting, and I can't do either.

Ann

[VictorBorun]

I think these images would be much easier for the layperson to understand if they could be shown with a 360­° viewer, with the image projected on the inside of a sphere, and the viewpoint at the center of the sphere. Is this a valid idea? Does anyone know of where this can be found?

I am not happy with the posted infographics, too.
White lines trace the tangential movement of 26 million stars in the nearest part of the galaxy, that's ok.
Background fill colors should show the radial velocities of that stars, should they not?
So off with the Large and Small Magellanic Clouds and field globular clusters, that's ok.
But why leave the two spots that white lines have to cross near the center of the map and that are supposed to correlate with the Milky Way's bar?
All those two spots do is messing with the map of the stellar flows.

Now what did I expect before I get to see this map?
Some stars must follow some path passing by Sun; half of the path would be with radial velocity component of coming closer towards Sun and the other half of the path would be populated with red-shifted stars.
Particularly, a lower galaxy orbit would make a path of stars orbiting a little faster than Sun (though the dark matter halo make Sun's galactic orbital velocity almost equal to a lower orbit's one). A higher orbit makes for a path in the other direction. Sun's bias toward the Northern surface of the galactic disk should make the two flows look a little lower than the galactic equator; each flow should span almost 180° horizontally.

I don't think this is what I see. I can see a 180° flow crossing the galactic center in the background; that should be the lower orbit flow.
If it orbits at a higher velocity than Sun, then the galactic core is paradoxially doing the opposite thing: spinning slower than Sun.
As for the higher orbit's flow the map is quite unfriendly and I just can't see any heads or tails.

Just how unfriendly a 360° map can be is illustrated here

Pearl Harbor was an inside job.jpg

[XgeoX]

So it seems according to the image that the central bar is rotating in a direction opposite the rest of the galaxy but the caption implies this is an illusion caused by the Sun’s motion.
How does that work?

We're not seeing the motion of the stars here in the galaxy's frame of reference, but in our own. Compare this with the observed motion of a planet in the Solar System, where Earth's motion creates retrograde movement, and planets appear to reverse direction in the sky.

Thank you Chris!

Eric