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Porl

Possibly a dumb question about the observational speed of DSO's

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Hi everyone,

I've had a look over the thread names for a few pages in here because I didnt know how to word a Search.

My girlfriend asked me this question the other night and I tried to answer how I thought it worked but I dont think i have the right end of the stick and the more I think about it the less im sure of the slight idea I have being right. confused yet? haha

She asked this while we were looking at an image of the Pillars of Creation. " why do we not see all this cloud and dust moving and changing if its moving so fast?? " and its something I've often wondered. She also asked " why has the horse head nebula not changed shape over hundreds of years if its all moving so quickly? "

I watch shows like Wonders x2 and The Universe and they all say how fast the banks of dust and gases are moving, especially with say a super nova etc which is how some of these nebulas are formed i think. So how come we dont actually see them moving?

Is it because of the scale of the things? They are so massive and so far away that for some reason the speed doesnt seem to be obvious when observing them?

Is it like when you watch a plane fly over fairly low down it moves pretty quickly across the sky, then when you see a plane really high up with its cloud trail behind it moves really slowly. And again on top of that when you see the ISS move across the sky at something like 10000 miles an hour (its something like that right?) it takes a good couple of minutes to go from one side to the other. The more distance to the moving object the slower it appears to move?

Trying to type this out the more confused I've become haha.

Is there an easyish way of explaining it so that I can explain it to my girlfriend without maths?

Thanks guys :D

Paul''

Edited by Porl
spelling mistake in title

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" why has the horse head nebula not changed shape over hundreds of years if its all moving so quickly? "

Is it because of the scale of the things? They are so massive and so far away that for some reason the speed doesnt seem to be obvious when observing them?

Yes you're exactly right - it's because these things are so big. Any obvious change would have to be over an enormous distance, and that would take thousands or millions of years.

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I agree. It the size of these objects. I have often pondered the same thing looking at pictures of M51, the Whirlpool Galaxy. The interaction of two galaxies makes the area look so dynamic. Come back in a thousand years, and I guess it would look the same.

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The more distance to the moving object the slower it appears to move?

That's it ... look out of the window of a jet cruising at 500 mph at 35,000 feet and the world outside seems pretty static. OTOH if you look out of the window of a car going 50 mph with objects only a few feet away you appear to be moving pretty fast.

Now the Pillars of Creation are 6,500 light years away ... if something nearby to them was moving at right angles to our line of vision at the speed of light, it would move about 2 arc minutes in a year ... or the same speed as objects on the ground would appear if observed from 35,000 feet by an aircraft flying at 5 feet per year rather than 750 feet per second. And the speed of the turbulence in the Pillars of Creation is, of course, a few orders of magnitude less than the speed of light.

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