milky way today versus what we see

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rukie
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milky way today versus what we see

Postby rukie » Wed Mar 11, 2015 1:20 am UTC

I was just thinking when we view the stars they're in a position millions of years ago. When we see a map of the milky way, I assume that's based off of what we see today. But in reality these stars have had millions if not billions of years to move. Had anyone tried to redraw the galaxy as it exists in true time rather than the history we see in the sky?

Does my question even make sense?

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Re: milky way today versus what we see

Postby screen317 » Wed Mar 11, 2015 1:25 am UTC

What do you mean "in true time?"

You mean a picture of the entire galaxy in real time (Earth time)??

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Re: milky way today versus what we see

Postby doogly » Wed Mar 11, 2015 1:39 am UTC

It does not make sense in GR. In SR it's alright to take a coordinate system such that the earth is always at rest, and look at constant time slices. SR and Newtonian gravity probably isn't so bad, except for the center of the galaxy. (Which will not be at the center anymore. The earth will. Which doesn't sound like a good thing to want.)

The galaxy is "only" 100,000 light years across though. The million range is only for extra galactic stuff.
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Re: milky way today versus what we see

Postby FancyHat » Wed Mar 11, 2015 1:50 am UTC

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sevenperforce
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Re: milky way today versus what we see

Postby sevenperforce » Wed Mar 11, 2015 4:42 pm UTC

The furthest point from us in the Milky Way, the outer edge of the Scutum-Centaurus Arm, is roughly 75,000 lightyears away. But in the past 75,000 years, our solar system has only moved 63 lightyears in its orbit around the center of the galaxy, about a tenth of a degree. So the amount of change to the shape of the galaxy over this time period is extremely minimal.

Plus, keep in mind that the spiral arms of the galaxy don't actually move...at least, not at any meaningful rate. They are density structures rather than physical structures; stars orbit through them. So the physical appearance of the galaxy won't have changed at all in 75,000 years.

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Re: milky way today versus what we see

Postby SDK » Wed Mar 11, 2015 9:51 pm UTC

sevenperforce wrote:Plus, keep in mind that the spiral arms of the galaxy don't actually move...at least, not at any meaningful rate. They are density structures rather than physical structures; stars orbit through them. So the physical appearance of the galaxy won't have changed at all in 75,000 years.

Is this true? That doesn't really make logical sense. Whatever the spiral arms are made of, they're still affected by gravity, and therefore must orbit or they would be pulled in towards the center of the galaxy. Right?
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Re: milky way today versus what we see

Postby FancyHat » Thu Mar 12, 2015 12:03 am UTC

SDK wrote:Whatever the spiral arms are made of, they're still affected by gravity, and therefore must orbit or they would be pulled in towards the center of the galaxy. Right?

Imagine a Mexican wave of passengers on a moving train, where the train is moving as some particular speed, relative to the tracks, in one direction, and the Mexican wave is moving at the same speed, relative to the train, in the other, so that the Mexican wave moves from the front of the train to the back. Relative to the tracks and a watching observer standing next to them, what's the speed of the Mexican wave? If you were that observer, you'd see the Mexican wave standing still relative to you and the tracks. But the passengers are still moving with the train, and won't be falling out of the back of it when the Mexican wave reaches it.

Spiral arms are a bit like Mexican waves. The stars, dust and gas they're made of orbit the galaxy, which is why they don't just fall straight to the centre, while the arms can move much more slowly.

Another illustration is the way traffic can kind of bunch up on a busy road, so, as you're driving along with all the other traffic, you have to slow down for a bit, with the traffic getting more densely packed, and, after a while, traffic starts speeding up and spreading out again. That bunched up bit can stay at the same place on the road, while the traffic passes through. I gather spiral arms are a bit like those bunched up bits on busy roads.

Or water swirling round and down a plug hole. The water's swirling round, but the shape of the swirl itself could rotate much more slowly, not rotate at all, or even rotate the other way to the way the water's swirling.
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Re: milky way today versus what we see

Postby sevenperforce » Thu Mar 12, 2015 2:22 pm UTC

SDK wrote:
sevenperforce wrote:Plus, keep in mind that the spiral arms of the galaxy don't actually move...at least, not at any meaningful rate. They are density structures rather than physical structures; stars orbit through them. So the physical appearance of the galaxy won't have changed at all in 75,000 years.

Is this true? That doesn't really make logical sense. Whatever the spiral arms are made of, they're still affected by gravity, and therefore must orbit or they would be pulled in towards the center of the galaxy. Right?

As FancyHat explained, the spiral arms aren't "made of" anything different than anything else in the galaxy; they're density waves. His example of traffic on a highway is an excellent one, because it's exactly what we're dealing with here.

Suppose you're driving down the highway and traffic suddenly begins to thicken. You realize that there was an accident earlier that day which slowed traffic, but the accident has been cleared and so there's no reason why traffic should still be bunched up in that area. Why is there still a traffic jam?

The answer is simple: as you approach the denser portion of traffic, you slow down to avoid hitting the cars in front of you. Simply because the traffic is already dense, you are forced to move more slowly, which means you contribute to the density. After a while, though, the flow of traffic begins to thin and you gun the engine, leaving the traffic jam behind. The traffic jam is not composed of individual vehicles moving slowly, but of the overall flow of vehicles.

This page has a neat animation and a lot of interesting information about dealing with traffic jams, which may be useful to you if you have a long commute.

It's the same thing in a galaxy. The galactic arms persist because stars and nebulae and dust clouds slow down as they pass through them.

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Re: milky way today versus what we see

Postby SDK » Thu Mar 12, 2015 3:18 pm UTC

Okay, so the stars, dust etc slows down when it hits a high density region. Why? As it's appoaching the higher density area, shouldn't it speed up due to the added gravitational pull from the dense region?

And then the stars, dust etc leaves the high density area... and speeds back up again? Maybe this isn't exactly like a traffic jam?

I'm reading that Wikipedia page, but it jumps right into pretty high level stuff. The "corotation radius" seems to say that material does speed up towards the center of the spiral arm, but then I don't see how that could result in a region of higher density. Am I reading that wrong?
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Re: milky way today versus what we see

Postby sevenperforce » Thu Mar 12, 2015 3:22 pm UTC

Technically I suppose it's the reverse of a traffic jam.

In a traffic jam, you slow down as you approach and speed up as you exit.

In a galaxy, you are pulled forward by the increased gravity of the arm ahead of you, so you enter it more quickly. However, as you cross the center of the arm and continue through, it pulls on you and slows you down. This has the same effect, just in the opposite way.

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Re: milky way today versus what we see

Postby SDK » Thu Mar 12, 2015 3:26 pm UTC

But then I don't see how it can be self-sustaining. The material making up the higher density region is moving more quickly than the material that's progressing towards the high density region, so I'd think that it would lose mass as material leaves the region faster than it's being added.

Though I guess that might be the whole reason why it's a wave in the first place... :o
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Re: milky way today versus what we see

Postby sevenperforce » Thu Mar 12, 2015 3:50 pm UTC

SDK wrote:But then I don't see how it can be self-sustaining. The material making up the higher density region is moving more quickly than the material that's progressing towards the high density region, so I'd think that it would lose mass as material leaves the region faster than it's being added.

Though I guess that might be the whole reason why it's a wave in the first place... :o

That's the tricky part -- the density waves are not radial, like the spokes of a wheel, but spiral, like the ripples in a whirlpool.

Take a look at this animation and try following the paths of individual stars. Because of the spiral shape, stars pass through the arms almost parallel to the length of the arm at that point, rather than perpendicularly. They are accelerated as they approach the arm, maintain speed as they travel down and through the arm (still moving tangent to the disc of the galaxy) and then are decelerated as they exit the arm. The end result is that the shape is maintained.

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Re: milky way today versus what we see

Postby SDK » Thu Mar 12, 2015 4:11 pm UTC

Well that makes sense. Thanks for that. Very interesting stuff.
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Re: milky way today versus what we see

Postby Aelfyre » Sun May 10, 2015 1:22 am UTC

rukie wrote:I was just thinking when we view the stars they're in a position millions of years ago. When we see a map of the milky way, I assume that's based off of what we see today. But in reality these stars have had millions if not billions of years to move. Had anyone tried to redraw the galaxy as it exists in true time rather than the history we see in the sky?

Does my question even make sense?


no it totally makes sense.. everything we see out there exists somewhere aside from where we see it now do to light travel time. The problem with trying to draw a map of current positions is that it would require predicting the future location of billions of bodies up to 100000 years in advance as well as their behavior when we know full well that stars will explode and scatter their mass out into the galaxy within a timeframe but with almost no degree of accuracy.

Betelgeuse for instance.. might have already exploded and we could be in for a spectacular supernova any day now or it could be another million years before it blows, just no way to know with any certainty and that redistribution of mass would affect the future positions of all nearby objects in a chaotic fashion.

Basically we can't even predict the weather on earth more than a week or so out due to the effects of chaos, trying to project the positions of stars.. millions of which could explode at any time, as far as 100k years in the future. Simply impossible with any amount of computational power even theoretically available.
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Re: milky way today versus what we see

Postby brenok » Sun May 10, 2015 3:27 am UTC

Aelfyre wrote:~snip~

I think you're vastly overestimating the size of our galaxy, while underestimating the lifespan of stars.

The Milky Way is 100,000 lightyears in diameter, so that's the largest distance between any two stars on the galaxy. Our sun is over 4,500,000,000 years old, and will probably stil burn for a similar amount of time. For comparison, to a human with lifespan of 80 years, the time it takes for the light of the most distant stars to reach us would be equivalent to less than 8 hours. Now, I could say "by the time this message reach you I could be long dead", but that would be only on the most pedantic technicality way.

Also, even if they were to explode, that wouldn't do any difference on the positioning of the stars, or the gravitational influence between them. Momentum is conserved, the center of mass of the debris would remain of the same trajectory the former star was, and, because of the distances involved, stars are pretty much point masses, so nothing would change on the timescale.

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Re: milky way today versus what we see

Postby Wildcard » Sun May 10, 2015 7:20 am UTC

brenok wrote:~snip~
In that case, it sounds like the OP's imagined map is definitely feasible. The question remains, has it ever been done? (Possibly.)

If so, has it ever been done accurately?

I highly doubt it.*

Such a diagram, with even close to perfect accuracy (say each star or planet within one diameter from its actual location), would only be necessary if we had something like near-instant teleportation. Not to mention that without teleportation, I don't see how you could verify a diagram's accuracy even if you had one.

If you gave a perfect satellite-confirmed globe of Earth as of 10000 B.C. to a native of Earth of that time period, (a) Would he have any use for it? (b) Would he have any way to know if it was fully accurate?

*on this planet. I have no data on what may or may not exist in the wide vast universe beyond our atmosphere.
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Re: milky way today versus what we see

Postby waltmck » Sun May 10, 2015 10:50 pm UTC

What time are you referencing your time frame off of? From my understanding of physics, time and space is all relative.

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Re: milky way today versus what we see

Postby Xanthir » Thu May 14, 2015 5:53 am UTC

It's generally understood that, unless otherwise specified or if you're specifically talking about some heavy spacetime distortion, whenever people refer to astronomical times they're talking about times in the comoving frame.
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Re: milky way today versus what we see

Postby Tyndmyr » Thu May 14, 2015 8:10 pm UTC



If you shoot geese, you *do* have to lead them. This is more a result of the slow speed of shot rather than the speed of light itself, but the principle would be similar for particularly lengthy space missions.

But yes, predictions on that kind of scale are gonna have errors.


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