Which direction is the Sun moving relative to the interstellar medium?

[HebaruSan]

Many of us will be familiar with illustrations such as this, showing the interactions between the heliosphere, termination shock, heliopause, and interstellar medium in popular science articles about recent measurements from Voyager 1 and Voyager 2:

(source)

This graphic shows the Sun moving "leftwards" through the interstellar medium, compressing the left edge of the heliosphere while the right edge extends out (speculatively, not measured yet) in a much longer tail. What I wonder is, what is "leftwards" in the real world? Both in terms of which constellations are in that spot as viewed from Earth, and in galactic orbit terms?

The naïve assumption would be that it is "galactic prograde", the direction of the Sun's orbital velocity around the galaxy (towards Vega, IIRC). But that implicitly assumes a stationary interstellar medium, and an interstellar medium without galactic orbital velocity would fall into the core of the galaxy and no longer be an interstellar medium. So the interstellar medium (or its constitutent particles) must also be in galactic orbit. If that's the case, then the orientation in the graphic is governed by the difference of two galactic orbits, that of the Sun and that of the medium, which could be anything depending on the specifics.

Anybody know more?

[Hotel26]

One has to be careful about extrapolating solar system rules out to this scale.

Spiral galaxies, for example, spiral as a disc with no or little relative movement in the spiral arms with distance.  This "anomaly" is what kicked off the "dark matter" speculation.

(Your post is a stimulant to further thought.)

[cubinator]

I think the shock front is pushed somewhat outward from the galactic core due to "stuff" coming from that direction.

[JoeSchmuckatelli]

I don't know.  We've exactly two probes that have gotten that far, and everything I've seen is speculation. 

I certainly would not assume anything like the illustration provided - as, yeah, the interstellar media would have to be relatively 'stationary' rather than co-orbiting. 

I've always perceived the heliopause as simply the region where the solar wind can no longer make the atmosphere exclusive to Sol - and if that's the case, the interstellar media is likely to be more dominated by the local environment of stars than some wind from the center of the galaxy... So I picture something more like a hazy bubble than a comet trail - we've all seen bubbles drifting with the wind: they're not perfect spheres, but they don't trail like that 

Edited September 9, 2020 by JoeSchmuckatelli

[JoeSchmuckatelli]

In an interesting twist - there's a new article on this very thing

Magnetic fields are fighting and pushing and tied up with each other. The image you should have is like the plunge pool under Niagara Falls

 https://www.bbc.com/future/article/20200908-the-weird-space-that-lies-outside-our-solar-system

 

Edit - Crud.  The image in the OP is from the article that I just discovered and cited. 

 

Edited September 9, 2020 by JoeSchmuckatelli

[K^2]

I don't know how much of it is going to be Sun moving through interstellar medium, and how much interstellar medium moving past the Sun. Hydrodynamics of galaxies is kind of weird.

What I personally find more fascinating, although only tangentially relevant, is that we can measure how fast the Sun is moving relative to the "rest of the universe", or as close to the "absolute" rest frame as we can find, the rest frame of Cosmic Microwave Background. And it so happens that Sun is traveling at about 369km/s roughly in the direction of Beta Sextantis or 264°, 48.25° in galactic coordinates, to be more precise. And you might as well call that the true velocity of our Sun, because there is no other reference frame nearly as universal, and I do mean that literally, as CMB.

[wumpus]

The milky way is spinning, and presumably the Sun with it.  Beyond that, I don't think there's a clear definition of 'interstellar medium'.  About the only thing clear is that the effects of the universe's expansion are much larger than any velocity the Sun might have (by measuring redshifts of other suns/galaxys).

[K^2]

1 hour ago, wumpus said:

About the only thing clear is that the effects of the universe's expansion are much larger than any velocity the Sun might have (by measuring redshifts of other suns/galaxys).

Actually, because space itself is expanding, everything in the universe is roughly at rest on average w.r.t. CMB. Granted, in some cases you have to average over huge distances, as even galactic clusters can be moving pretty fast relatve to neighbors, but expansion doesn't contribute either way. We aren't actually moving away from distant galaxies, just getting more space between us. And yes, it's a matter of choice of coordinate systems, but if you chose the one that's most at rest w.r.t. neighborhood, it actually ends up being the CMB rest frame, and in it, galaxies aren't really flying apart, just getting more space between them.

[JoeSchmuckatelli]

51 minutes ago, K^2 said:

Actually, because space itself is expanding, everything in the universe is roughly at rest on average w.r.t. CMB. Granted, in some cases you have to average over huge distances, as even galactic clusters can be moving pretty fast relatve to neighbors, but expansion doesn't contribute either way. We aren't actually moving away from distant galaxies, just getting more space between us. And yes, it's a matter of choice of coordinate systems, but if you chose the one that's most at rest w.r.t. neighborhood, it actually ends up being the CMB rest frame, and in it, galaxies aren't really flying apart, just getting more space between them.

I always struggle with this.  Questions like:

• Is it only expanding between galaxies, and if so, why?
• If it's not only between galaxies - why don't we see the expansion in our local environment?
• And why stop there?  Why not expansion between atoms as well?

...arise.

And then you go and throw in something I've never heard - heck, never even thought about, like being at rest w.r.t. the CMB... and I'm floored.  Because, if space is expanding, and a galaxy in a given neighborhood can be at rest with the CMB... what the heck is the shape of the universe?  Doesn't this imply the expanding balloon is a pretty good analogy?

[YNM]

Wasn't there some observation that showed that stars on the galactic disc actually have some inclination, and that the Sun is currently about as far out-of-plane as it gets ? Would this imply the velocity vector is roughly in the galactic plane ?
Though it'd be interesting if there's a way to measure it, maybe wrt the galactic "center" ?

Though yeah the real shock would depend on the ISM flow as well, it's a relative thing rather than a position-dependent thing I suppose.

 

Spoiler

9 hours ago, JoeSchmuckatelli said:

• If it's not only between galaxies - why don't we see the expansion in our local environment?
• And why stop there?  Why not expansion between atoms as well?

1. Yes. But they're really small, proportional to the smaller sizes we're applying it to. Hubble's constant** is expressed in inverse second (or speed over distance), not in acceleration (change in velocity/speed over time) or velocity/speed.

2. Yes. But they're really small that in general other forces (strong, weak, electromagnetic and gravitational forces) overwhelms it.

The often-used example of marks on a balloon's surface, which were marked when they were deflated, then inflated after, is a fairly good example - the marks themselves do get lighter as well. You have to realize that there's actually no force involved at all in the expansion - it's just the very surface (and the very space, in our real case) that expanded by it's own. Everything will feel to themselves that they're at rest, if no other forces are acting on it.
It's just that, in the real universe, what would be the mark particles have forces between them, and the marks are waaaaaay smaller than the balloon.

CMB rest frame is simply the actual rest frame that we should have - because the CMB is equally redshifted (equally going away) to all radial directions, the average is simply zero; like if a horse were to be arranged neatly then asked to pull equally on a rope with the same distance to all horses (equal force magnitude, vector always point away from center), then the horses (and the rope, and the rope center) won't get anywhere because no net force vector is acting on it*

* NOTE : I'm meaning this in the way of vector summation, not the forces itself or velocity or whatever. Couldn't think of anything else that naturally want to go separate ways so uniformly and not having two things happening at once (ie. centrifugal/centripetal forces).

** NOTE : see post below. No longer a proper constant.

 

Edited September 10, 2020 by YNM

[magnemoe]

7 hours ago, JoeSchmuckatelli said:

I always struggle with this.  Questions like:

• Is it only expanding between galaxies, and if so, why?
• If it's not only between galaxies - why don't we see the expansion in our local environment?
• And why stop there?  Why not expansion between atoms as well?

...arise.

And then you go and throw in something I've never heard - heck, never even thought about, like being at rest w.r.t. the CMB... and I'm floored.  Because, if space is expanding, and a galaxy in a given neighborhood can be at rest with the CMB... what the heck is the shape of the universe?  Doesn't this imply the expanding balloon is a pretty good analogy?

Space is getting larger everywhere, its pretty slow however. Inside a galaxy or even galaxies who are gravitational bound to each others the gravity is more than enough to hold things together however as expansion is constant it adds up over long distances like many million light years. 

Expansion rate is 74 km for 3.3 million light years /s 
https://www.nasa.gov/feature/goddard/2019/mystery-of-the-universe-s-expansion-rate-widens-with-new-hubble-data
or 22.4 mm /light year/ s 
or 706 km /light year/ year 
In short on galactic scale its something gravity has no issues handling. 

 

[JoeSchmuckatelli]

@YNM - this is interesting: . Yes. But they're really small, proportional to the smaller sizes we're applying it to. Hubble constant is expressed in inverse second (or speed over distance), not in acceleration (change in velocity/speed over time) or velocity/speed

... because I've always read it as acceleration - so I'll have to dig in and see if I can learn how inverse second differs.  Thanks! 

I love physics and cosmology - but with no math background I feel like I'm discussing Japanese poetry, (I enjoy the form, structure and translations) all the while missing the nuances because I don't speak Japanese. 

 

Which is to say - I appreciate the patience from you guys who do get the math! 

[YNM]

14 minutes ago, JoeSchmuckatelli said:

no math background

Astronomy is mostly just observing the nature as it goes - Edwin Hubble started off as a lawyer.
However we've kinda run out of stuff to observe on it's own (plus it's getting real expensive) that most of the time people do wait for the theoretical guys to come up with something to seek out.

EDIT : I should point out that Hubble's 'constant' only a "constant" for a given time. We've since proved through observation (and testing against models) that the expansion of the universe is indeed accelerating.
it has been 5 years since I dabbled in anything mathematical in cosmology, I think I can't really do it anymore XD

Edited September 10, 2020 by YNM

[JoeSchmuckatelli]

4 hours ago, YNM said:

Astronomy is mostly just observing the nature as it goes - Edwin Hubble started off as a lawyer.
However we've kinda run out of stuff to observe on it's own (plus it's getting real expensive) that most of the time people do wait for the theoretical guys to come up with something to seek out.

EDIT : I should point out that Hubble's 'constant' only a "constant" for a given time. We've since proved through observation (and testing against models) that the expansion of the universe is indeed accelerating.
it has been 5 years since I dabbled in anything mathematical in cosmology, I think I can't really do it anymore XD

So there is hope for for me, yet!

(like Hubble, I've a J. D.)

The 'constant' is actually a particular interest of mine. Every time I run into an article that finds a different measure I eat it up.  There are a few questions regarding the 'standardness' of the candles we use; different teams of researchers finding different measurements when looking at 1a SN or TRGB flares etc.

https://www.google.com/amp/s/www.sciencealert.com/we-can-t-figure-out-how-fast-the-universe-is-expanding-here-s-why/amp

- Back in '05, people questioning this stuff would be shouted down and mocked, but now they're publishing - - which shows there's still quite a bit to be discovered and learned. (when even the constant can be questioned - how much more is there to discover?!?)

Anyway - the implications of what @K^2 wrote are kind of mind blowing: it's a matter of choice of coordinate systems, but if you chose the one that's most at rest w.r.t. neighborhood, it actually ends up being the CMB rest frame

As I read this, you can pick any galaxy that is "most at rest" with its neighbors and if it is thus the CMB rest frame - does not that mean the galaxy you picked is the center of its own universe? 

8o

Edited September 10, 2020 by JoeSchmuckatelli

[Scotius]

The Great Attractor should be the center of our local 'universe' then After all, it influences entire Superclusters, including our own Laniakea.

[NFUN]

On 9/9/2020 at 5:23 AM, Hotel26 said:

One has to be careful about extrapolating solar system rules out to this scale.

Spiral galaxies, for example, spiral as a disc with no or little relative movement in the spiral arms with distance.  This "anomaly" is what kicked off the "dark matter" speculation.

(Your post is a stimulant to further thought.)

Spiral arms actually have nothing to do with galactic rotation curves. The popular theory is that they're density waves--basically traffic jams of dust--that aren't created by groups of co-orbiting stars and gas, but rather are made up of starbursts created when increased density of gas triggered star formation. Stars don't remain in the arms forever, and they tend to be bluer as they're made of new stars, while the smaller, older stars that last long enough to escape them are left behind

[K^2]

2 hours ago, JoeSchmuckatelli said:

As I read this, you can pick any galaxy that is "most at rest" with its neighbors and if it is thus the CMB rest frame - does not that mean the galaxy you picked is the center of its own universe?

Best we can tell, every point in the universe can be considered its center, because they all started being literally the same point at the big bang, or at least, as close as it's physically possible. Unfortunately, both the theory and our ability to make relevant measurements with sufficient precision break down when we are talking about the universe's very, very, very beginning. So there's a bit of wiggle room on whether universe started out as a literal point or just something incredibly tiny, but everything indicates all of space starting out at least from a very close neighborhood of the same point.