If stars are traveling away from us at great speeds..then why are the constellations

[Iamme]

...still there, like nothing has moved? I have already heard speeds mentioned of one million miles an hour.

[jmercer]

They have moved and "distorted". The distances involved are so great that it takes millennia to see any difference.

[Terry]

To quote DNA: "Space is big. Really big."

[fribble]

...still there, like nothing has moved? I have already heard speeds mentioned of one million miles an hour.

1. Erm... you say "...Stars travelling AWAY from us..."
2. Even if they are moving sideways at one million miles an hour (which they might be; I'm no astronomer so I don't know) they're still light years away from us. So the distance they have to move to cause a noticeable (angular) effect in our night sky is enormous.

[Ziggurat]

...still there, like nothing has moved? I have already heard speeds mentioned of one million miles an hour.

The universe is expanding. Most galaxies are getting farther and farther apart. Each individual galaxy need not be expanding, however. The stars visible to the naked eye are all stars within our own galaxy, and the stars that make up constellations are all stars near us. They are moving WITH us in orbit around the galactic center. Their relative motion is therefore much slower than the relative motion of other galaxies in the universe. As pointed out, the other galaxies moving away from us are also moving mostly AWAY from us, so their angular position won't change as they receed. They are also VERY far away from us to begin with, and so even at large speeds, the relative change over the course of human history has been negligible.

But if you could wait long enough, you would indeed see the constellations slowly change shape. But "long enough" is quite long indeed, compared to a human lifespan.

[BobK]

Parallax (http://en.wikipedia.org/wiki/Parallax)

[ynot]

They have moved and "distorted". The distances involved are so great that it takes millennia to see any difference.
Distance delays when we see things but surley not rate that things change. In other words, a delayed "real time" of events.

[Jimbo07]

The stars are moving in whichever direction pleases them... :boggled:

Some are moving toward us.

[ynot]

...still there, like nothing has moved? I have already heard speeds mentioned of one million miles an hour.
As I understand it (and I could be very wrong), when things are uniformly expanding they are moving apart in all directions at the same rate. In other words, moving away at the same rate as moving apart. From an observers perspective therefore, they would be seen to be held in fixed relative positions because their paths of travel are on fixed radial lines from the observer. If they weren't moving apart as they were moving away, it would appear that they were moving together.

What I don't understand about the expansion is that as things get very distant they should travel beyond our field (length) of vision and we should be able to see stars getting progressively fainter then dissappearing (don't know if this happens). I saw a report once that the Hubble telescope was aimed at an "empty" area of the universe and set to a long exposure. The result was that a whole mass of stuff became visible.

[Soapy Sam]

Purely a question of scale.

[Ziggurat]

What I don't understand about the expansion is that as things get very distant they should travel beyond our field (length) of vision and we should be able to see stars getting progressively fainter then dissappearing (don't know if this happens).

Looking outwards in space is also looking backwards in time. The only fundamental limit (as opposed to just needing a bigger telescope) to how far away we can see is how far back in time the universe was transparent. If you get close enough to the big bang, energy and matter densities are so large that the whole universe was opaque. At that point, the only thing that will be visible is the cosmic microwave background, which is basically all that light which finally got released once the universe cooled/expanded enough to become mostly transparent.

But yes, distant galaxies that are moving away from us are indeed fainter.

I saw a report once that the Hubble telescope was aimed at an "empty" area of the universe and set to a long exposure. The result was that a whole mass of stuff became visible.

Yup. The first time they did it was the Hubble Deep Field image:
http://upload.wikimedia.org/wikipedia/commons/thumb/5/5f/HubbleDeepField.800px.jpg/594px-HubbleDeepField.800px.jpg
They did it again more recently, on a much smaller patch of sky with a longer exposure, to get the Hubble Ultra Deep Field image:
http://upload.wikimedia.org/wikipedia/en/thumb/4/4f/HUDF.jpg/600px-HUDF.jpg

[ynot]

Looking outwards in space is also looking backwards in time. The only fundamental limit (as opposed to just needing a bigger telescope) to how far away we can see is how far back in time the universe was transparent. If you get close enough to the big bang, energy and matter densities are so large that the whole universe was opaque. At that point, the only thing that will be visible is the cosmic microwave background, which is basically all that light which finally got released once the universe cooled/expanded enough to become mostly transparent.

But yes, distant galaxies that are moving away from us are indeed fainter.



Yup. The first time they did it was the Hubble Deep Field image:
http://upload.wikimedia.org/wikipedia/commons/thumb/5/5f/HubbleDeepField.800px.jpg/594px-HubbleDeepField.800px.jpg
They did it again more recently, on a much smaller patch of sky with a longer exposure, to get the Hubble Ultra Deep Field image:
http://upload.wikimedia.org/wikipedia/en/thumb/4/4f/HUDF.jpg/600px-HUDF.jpg
WOW!!!!! - Thanks for the images Zigg. I have resisted getting in to astronomy because I know I would get so completely obsessed that I would never get any "real" work done.

Distant galaxies are fainter, or can be observed as getting fainter?

[Molinaro]

You can see what the big dipper will be doing by looking over here (http://astronexus.com/node/81?PHPSESSID=b3284fff305de85107e56cb1a25e610d), and selecting the animation.

Over the next hundred thousand years or so, the different motions of these stars will gradually change the familiar Dipper shape. In fact, the "bowl" will flatten out, and the "handle" will bend even more, so that after nearly 100,000 years the Dipper will have turned upside down and backwards! Additionally, after 100,000 years there will be an eighth star in the "Dipper". This bright interloper is the star Zeta Herculis, now very far away from the Big Dipper in the constellation Hercules.

[DanishDynamite]

Looking outwards in space is also looking backwards in time. The only fundamental limit (as opposed to just needing a bigger telescope) to how far away we can see is how far back in time the universe was transparent. If you get close enough to the big bang, energy and matter densities are so large that the whole universe was opaque. At that point, the only thing that will be visible is the cosmic microwave background, which is basically all that light which finally got released once the universe cooled/expanded enough to become mostly transparent.
I realize that your statement is "fundamentally" true, but I thought there was another fundamental limit before this one. I thought the farthest we could see would be to where things sent out "light" in our direction as many years ago as their distance in lightyears.

[DanishDynamite]

And, as far as I know, the size of the visible Universe in lightyears is not equal to the age in years of the Universe since it became transparent.

[blutoski]

WOW!!!!! - Thanks for the images Zigg. I have resisted getting in to astronomy because I know I would get so completely obsessed that I would never get any "real" work done.

Distant galaxies are fainter, or can be observed as getting fainter?

Not in our lifetime. They are redshifted, though.

[HeyLeroy]

If stars are traveling away from us at great speeds..then why are the constellations ...still there, like nothing has moved? I have already heard speeds mentioned of one million miles an hour.

Dey gots you hypmotized:hypnotize

[wollery]

Just to add, the vast majority of stars that you can see with the unaided eye are travelling round the Galaxy in roughly the same direction as us at roughly the same speed, which means that the constellations will remain fairly constant over the course of several millenia.

The Solar orbital velocity is 220km/s, which is 792,000km/hr, or a shade under half a million miles an hour. The local Galactic escape velocity is somewhere in the region of 525km/s, 1,890,000km/hr, or about 1.2 million miles an hour. This means that it is very unlikely for any star to be travelling much faster than 1 million miles an hour, and most travel at half that speed.

[jmercer]

Distance delays when we see things but surley not rate that things change. In other words, a delayed "real time" of events.

You misunderstand - it's not the distance between us and them I was referring to, it's the distances stars have to move before such movement becomes visible from earth.

The constellations today will look very different 10,000 years from now.

[ynot]

You misunderstand - it's not the distance between us and them I was referring to, it's the distances stars have to move before such movement becomes visible from earth.

The constellations today will look very different 10,000 years from now.
Sorry but I still don't get it. Obviously if we observe an object that is x light years away we are seeing it x years after the event. We don't see it in freeze-frame or slow motion however, we see it at the speed it originally happened. As I explained in post #9, I don't see that uniform expansion would ever cause us to view any change in the relative positions of distant objects. Any change would be the relative different movements of the objects that would be observed if the universe wasn't uniformly expanding. One change we should observe however (I think) is the furthermost objects fading out as they go beyond our field of vision. With uniform expansion, the further on object is away, the faster it is moving away from the observer. When AxBxCxD uniformly expands to AxxBxxCxxD, A and B have moved apart xx and A and D have moved apart xxxxxx. Sorry that this is kindergarten stuff but I can't think of any other way of expressing it. I don't know the distances involved when things would leave our field of vision but they must be immense (even with naked eye vision).

[Jeff Corey]

Ynot,
Your point about uniform expansion would be valid if all the stars in a constellation were about the same distance from us. However, since they can be at greatly different distances, the previous post about motion parallax explains some of the distortions to be expected.

[drkitten]

As I explained in post #9, I don't see that uniform expansion would ever cause us to view any change in the relative positions of distant objects.

Expansion isn't uniform. The distance between the earth and the moon, for example, is controlled, not by the expansion of the universe but by orbital mechanics.

That's why constellations shift. The stars that you see as part of constellations are gravitationally bound to this galaxy, and they're moving (slowly) around the center of the galaxy in a very large, very slow orbit.


One change we should observe however (I think) is the furthermost objects fading out as they go beyond our field of vision.

Not on a human-observable scale. Even something moving at "a million miles an hour" is still only moving away at about 300 miles a second, maybe 0.0003 times light speed.

This means that an object 10 light years away today would be 10.003 light years away next year, and thirty years from now would be 10.1 light years away. By cosmological standards, 10 light years is practically in our hip pocket. To put this kind of proportional change in perspective, imagine that there is an object 10 yards away from you, and it's moving at a rate that would make it 10 yards and three inches away when you retire. Would you notice that the object is moving at all?

[jmercer]

Sorry but I still don't get it. Obviously if we observe an object that is x light years away we are seeing it x years after the event. We don't see it in freeze-frame or slow motion however, we see it at the speed it originally happened. As I explained in post #9, I don't see that uniform expansion would ever cause us to view any change in the relative positions of distant objects. Any change would be the relative different movements of the objects that would be observed if the universe wasn't uniformly expanding. One change we should observe however (I think) is the furthermost objects fading out as they go beyond our field of vision. With uniform expansion, the further on object is away, the faster it is moving away from the observer. When AxBxCxD uniformly expands to AxxBxxCxxD, A and B have moved apart xx and A and D have moved apart xxxxxx. Sorry that this is kindergarten stuff but I can't think of any other way of expressing it. I don't know the distances involved when things would leave our field of vision but they must be immense (even with naked eye vision).

It's ok - let me try to explain a bit further. It seems like you're thinking in terms of the expansion of the universe; that's not the effect that causes the constellations to change, however.

Don't think of the stars in the constellations as two-dimensional points painted on the inside of a bubble with the earth at the center. Instead, think of them as three-dimensional locations at different distances embedded in the space between the bubble's surface and the earth.

They're so distant from the earth, however, that we have no depth-perception of them with the naked eye - that's why the "patterns" of the constellations appear two-dimensional to us. (And before anyone quibbles, yes - I know the earth is NOT the center of the universe!)

Once you've visualized that, you can consider the following:

All of these stars (at different distances from the earth) are all moving relative to us in different directions, at different velocities. Some are moving away from us; some are moving toward us; some are moving at angles to us. But all of them are moving, and at different speeds.

The "patterns" don't appear to change to us because the stars are so far away that they have to move enormous distances before we can detect any difference - and the distances are so great that it takes thousands of years for us to notice the changes. (It's like watching the hour hand on a clock - you know it's moving, but you can't detect it directly. However, you see that it's moved once enough time has gone by.)

Over time, the stars move in these random directions, "distorting" the constellations as we know them. The night sky 50,000 years ago, for example, would be unrecognizable for people today... just as it will be 50,000 years from now.

Does that help?

[wollery]

Sorry, just to reiterate, most of the stars that make up the constellations are not moving in random directions, but are moving in roughly the same direction as we are around the Galaxy. Most of them do have some movement towards or away from us and across our line of sight, but these movements are relatively small compared to their overall movement.

The fastest moving star is Barnard's star, which is going across our field of view at a little over 10 arcseconds/year (this movement is called Proper Motion). At that rate it takes 180 years to move a distance equivalent to the diameter of the Moon. By comparison the majority of stars have proper motions on the order of 0.01 arcseconds/year, and would thus take around 180,000 years to travel one Moon diameter. The constellations were first described about 5000 - 6000 years ago, so they haven't had enough time to change much.

[Jimbo07]

The constellations were first described about 5000 - 6000 years ago, so they haven't had enough time to change much.

Which is remarkably similar to the exact age of the Earth! Therefore, evolution isn't correct.

It's true. :)

...

:D

[Overman]

...still there, like nothing has moved? I have already heard speeds mentioned of one million miles an hour.


How on Earth does someone post over 2000 times here and not alrealdy know the answer to this question!?

[Foster Zygote]

They did it again more recently, on a much smaller patch of sky with a longer exposure, to get the Hubble Ultra Deep Field image:

Just sitting and looking at that image gives me goose bumps. Just think how many forms of life may be in that field of view, how many civilizations. There might be a few, thousands, or even millions of thinking species in those galaxies. There might be creatures with civilizations so old and powerful that they would seem like gods to us. There could be varieties of life unlike anything we could ever imagine. Then again it might be sterile, but that seems the most extraordinary possibility to me.

Steven

[jmercer]

Sorry, just to reiterate, most of the stars that make up the constellations are not moving in random directions, but are moving in roughly the same direction as we are around the Galaxy. Most of them do have some movement towards or away from us and across our line of sight, but these movements are relatively small compared to their overall movement.

Thanks, Wollery - quite true. (Actually, I knew that but didn't want to confuse things by trying to explain that during my simplification.)

However... they are random in the sense that they just happen to be where they are at this time. :)

[slipknotmcfadden]

I realize that your statement is "fundamentally" true, but I thought there was another fundamental limit before this one.

It's called the Hubble Distance I believe. Where the recessional velocity is equal to the speed of light. (Note that it's not the object itself moving at the speed of light, it's just receding at that speed because of the expansion of space.)

I thought the farthest we could see would be to where things sent out "light" in our direction as many years ago as their distance in lightyears.

Not quite, but you're on the right track. I think the Bad Astronomer explained it pretty well... let me look it up...

Hey I can't post links anyway. But do a search on Phil Plait's blog for "Hubble Distance" and do one on Google for good measure. Wikipedia, unfortunately, lacks a comprehensive article.

[LawnOven]

Just sitting and looking at that image gives me goose bumps. Just think how many forms of life may be in that field of view, how many civilizations. There might be a few, thousands, or even millions of thinking species in those galaxies. There might be creatures with civilizations so old and powerful that they would seem like gods to us. There could be varieties of life unlike anything we could ever imagine. Then again it might be sterile, but that seems the most extraordinary possibility to me.

Steven

I saw that picture in a book at a bookstore, and I felt both exileration and melancholy at once because it reminded me of everthing I'll never know and see or understand. It does make our stupid day-to-day problems seem petty to me, by compairison. Just thinking about the picture can cause me to space out for 5 minutes.

I suppose what I'm saying is; that is a powerful image for me too.

[KingMerv00]

I saw that picture in a book at a bookstore, and I felt both exileration and melancholy at once because it reminded me of everthing I'll never know and see or understand. It does make our stupid day-to-day problems seem petty to me, by compairison. Just thinking about the picture can cause me to space out for 5 minutes.

I suppose what I'm saying is; that is a powerful image for me too.

Makes a good desktop wallpaper.

[ynot]

Expansion isn't uniform. The distance between the earth and the moon, for example, is controlled, not by the expansion of the universe but by orbital mechanics.

That's why constellations shift. The stars that you see as part of constellations are gravitationally bound to this galaxy, and they're moving (slowly) around the center of the galaxy in a very large, very slow orbit.




Not on a human-observable scale. Even something moving at "a million miles an hour" is still only moving away at about 300 miles a second, maybe 0.0003 times light speed.

This means that an object 10 light years away today would be 10.003 light years away next year, and thirty years from now would be 10.1 light years away. By cosmological standards, 10 light years is practically in our hip pocket. To put this kind of proportional change in perspective, imagine that there is an object 10 yards away from you, and it's moving at a rate that would make it 10 yards and three inches away when you retire. Would you notice that the object is moving at all?
Thanks - now I "get it" (99.99% at least). All the answers were helpful (so thanks to all) but this one "nailed" it for me. Have made the Hubble Ultra Deep Field photo my desktop wallpaper. Hope seeing it everyday doesn't dull the buzz.

[The Atheist]

How on Earth does someone post over 2000 times here and not alrealdy know the answer to this question!?Some swine actually has an infinite number of monkeys. Where's the ****ing Shakespeare? That's what I want to know.

[The Atheist]

Have made the Hubble Ultra Deep Field photo my desktop wallpaper. Hope seeing it everyday doesn't dull the buzz.Now, as a fellow Kiwi, I'm going to make sure nobody takes advantage of your revelation about being distracted by astronomy pics and I'm going to insist that nobody posts the url for APOD (http://antwrp.gsfc.nasa.gov/apod/astropix.html), because you'd never get out.

[jmercer]

How on Earth does someone post over 2000 times here and not alrealdy know the answer to this question!?

Oh.

Well.

He's a very, very, very good troll, is all. :D

[ynot]

Now, as a fellow Kiwi, I'm going to make sure nobody takes advantage of your revelation about being distracted by astronomy pics and I'm going to insist that nobody posts the url for APOD (http://antwrp.gsfc.nasa.gov/apod/astropix.html), because you'd never get out.
Thanks "Dorklander". Any more crap out of you and I'll cut the cable :D. Just as well that I have several computers so I don't have to choose one over the other.

[Foster Zygote]

I saw that picture in a book at a bookstore, and I felt both exileration and melancholy at once because it reminded me of everthing I'll never know and see or understand. It does make our stupid day-to-day problems seem petty to me, by compairison. Just thinking about the picture can cause me to space out for 5 minutes.

I suppose what I'm saying is; that is a powerful image for me too.

Imagine what the Total Perspective Vortex must be like.

Steven

[Iamme]

The universe is expanding. Most galaxies are getting farther and farther apart. Each individual galaxy need not be expanding, however. The stars visible to the naked eye are all stars within our own galaxy, and the stars that make up constellations are all stars near us. They are moving WITH us in orbit around the galactic center. Their relative motion is therefore much slower than the relative motion of other galaxies in the universe.

Ahhhh. I never thought of that. They are in our own galaxy, eh? The big Dipper? The Southern Cross? The scorpion one, and all those other ones that they name astrological signs after? They are all in the Milky Way? I never really gave that a thought.

[Iamme]

Some are moving toward us.

Are you sure about that?

I always have heard about them all moving away from us at break-neck speeds... and there has been this auro of mystery cast on this by some, while others say this is a true sign that there really was a Big Bang. I always thought that space was supposed to be like the outside of an expanding balloon... and that ALL stars and galaxies have to be moving away from us.

How do they explain that some are heading towards us?

[Iamme]

What I don't understand about the expansion is that as things get very distant they should travel beyond our field (length) of vision and we should be able to see stars getting progressively fainter then dissappearing...

I have company. :)

Like...after a while, it just be our galaxy we would see, and that is it?

[Jimbo07]

Are you sure about that?

I always have heard about them all moving away from us at break-neck speeds... and there has been this auro of mystery cast on this by some, while others say this is a true sign that there really was a Big Bang. I always thought that space was supposed to be like the outside of an expanding balloon... and that ALL stars and galaxies have to be moving away from us.

How do they explain that some are heading towards us?

Others have already given more detailed answers than I gave.

Distant galaxies which are not part of the local group of galaxies (emphasis galaxies) appear to be moving away from us. This occurs in every direction. However, the stars are not distant galaxies. Some are moving away from us, some toward us, and some across the sky (this 'across' is 'Proper Motion'). Wollery gave a good example. Those moving toward or away from us have a high 'radial' motion. This radial motion is measured using Doppler Shift. Stars moving toward us have a blue shift.

The galaxies moving away have a red shift. That is why people will talk about the red-shift when discussing universal expansion. In short... different rules for distant galaxies vs. nearby stars.

Here's a good little lesson on star motion:

http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit1/motions.html

[Iamme]

How on Earth does someone post over 2000 times here and not alrealdy know the answer to this question!?

Did you say this right? Or do you really mean to say that since I am here as much as I am, that I should have perhaps read from other posts and threads, enough to know the answer?

Regardless,... I don't know, or can't remember, as my brain is filled up with so many unrelated things. And such a thread surely must be enlightening for newcomers here, as well.

[Iamme]

Purely a question of scale.

Fish and music were my first two thoughts. :)

[Iamme]

Just to add, the vast majority of stars that you can see with the unaided eye are travelling round the Galaxy in roughly the same direction as us at roughly the same speed, which means that the constellations will remain fairly constant over the course of several millenia.

The Solar orbital velocity is 220km/s, which is 792,000km/hr, or a shade under half a million miles an hour. The local Galactic escape velocity is somewhere in the region of 525km/s, 1,890,000km/hr, or about 1.2 million miles an hour. This means that it is very unlikely for any star to be travelling much faster than 1 million miles an hour, and most travel at half that speed.

Well...I wasn't too far off then. 1/2 a million...a million...that's still pretty darn fast.

[Iamme]

The constellations today will look very different 10,000 years from now.

Why didn't that article use 10,000 then, rather than 100,000 years, I wonder?

[Iamme]

Oh.

Well.

He's a very, very, very good troll, is all. :D


But I'm a well intentioned one. I really need to get my own computer and spring for the money to pay for a hookup so I can get one of these in my own house, instead of using Ecman's for the last 6 years or so.. Then maybe I'd post here more till wee hours of the night or early in the morning, or on miserable rainy or snowy days, or play hooky from work.

[drkitten]

Like...after a while, it just be our galaxy we would see, and that is it?

"A while."

At three-thousandths of the speed of light, it would take a star three hundred years to move one light-year.

The Andromeda galaxy -- which we can still see -- is about three million light years away. So it would take a star about 900,000,000 years to move that far away.

So yes, after a while. A rather long while.

[jmercer]

But I'm a well intentioned one. I really need to get my own computer and spring for the money to pay for a hookup so I can get one of these in my own house, instead of using Ecman's for the last 6 years or so.. Then maybe I'd post here more till wee hours of the night or early in the morning, or on miserable rainy or snowy days, or play hooky from work.

I must admit you're fun to play with. Kinda like a favorite cat-toy for a cat. :)

[Ziggurat]

Ahhhh. I never thought of that. They are in our own galaxy, eh? The big Dipper? The Southern Cross? The scorpion one, and all those other ones that they name astrological signs after? They are all in the Milky Way? I never really gave that a thought.

Almost everything you can see with the naked eye is in our own galaxy. One of the few notable exceptions is the Andromeda galaxy, the closest galaxy to the milky way of comparable size (there are a few dwarf galaxies closer to us). It is visible to the naked eye, but it's far enough away that it just looks like a star itself if you don't use a telescope.

[hgc]

Are you sure about that?

I always have heard about them all moving away from us at break-neck speeds... and there has been this auro of mystery cast on this by some, while others say this is a true sign that there really was a Big Bang. I always thought that space was supposed to be like the outside of an expanding balloon... and that ALL stars and galaxies have to be moving away from us.

How do they explain that some are heading towards us?
So much information spilled in this thread that may be of use to you in clearing up some misunderstandings. Yet you insist on sticking to "I always heard" and "auro (sic) of mystery cast by some." It's one thing to come here not knowing, but quite another leave here in the same condition. Ignorance is indeed bliss. Nirvana, apparently.

[Yahzi]

...still there, like nothing has moved? I have already heard speeds mentioned of one million miles an hour.
If airplanes fly so fast, why does it take them so long to fly just a few inches across the sky? When you hold your fingers apart, it takes whole seconds for the plane to fly between them.

:D

I know they say there is no such thing as a dumb question, but when your question can be answered by looking at an everyday object, it's a dumb question. I can only hope Iamme was joking...

[jmercer]

Nah, Yahzi, he's been doing this for years. He's one of the more fun trolls in the forums. :)