# Observing stuff in space over various distances

1. Apr 12, 2013

### wendal

I know you all hate hypotheticals (long time lurker), so I'll try to keep them to a minimum. I understand, on a very basic level, the concepts of time dilation, relative time and distance, and redshift, but get confused about one thing: why does everything in deep space always appear stationary?

For example, if I were to get close enough to observe a supernova with my naked eye (and was wearing a supersuit to protect myself), I would see a massive explosion. Let's pretend that it lasted for ten minutes from my point of view. If I were to observe that same event from a thousand light years away, from the same T=0, would it last the same 10 minutes, or would I see a slow motion explosion? What about if I were 100,000 light years away?

I just don't understand why our Hubble photos are not videos. All of the cool stuff we see is some amount of photons, emitted at some point in the past. The photons that we see at T=1 are followed by photons at T=2, so doesn't NASA just keep the lens pointed there and the tape rolling when it finds a star being born or a black hole forming?

I always think I'm going to find my answers in threads about observing aliens a billion light years away, but the hypothetical nature of those thought experiments usually causes the conversation to devolve into arguments over telescope resolving power. So I pose this question with a little more reality seasoning. Any thoughts are very much appreciated. Thanks!

Last edited: Apr 12, 2013
2. Apr 12, 2013

### Bandersnatch

You don't see Hubble movies, because the light of the faraway objects is so dim that it takes up to hours of continuous exposure to create a single frame(i.e., a photo).
It's the same principle as when making family/cat/whatever photos with your handheld camera. If the scene is dark, you need to expose the film for a longer amount of time to get a bright picture.

As for this one, really faraway supernovae explosions are stretched in time due to the expansion of the universe.

3. Apr 12, 2013

### wendal

I was hoping the answer wasn't as simple as that. Maybe I just didn't want to know. So, a related question: in a thousand years, when we expect the destruction of the Pillars of Creation to (possibly) become observable to us, will we be able to point the telescope and watch a super slow motion supernova blow through that dust?

4. Apr 12, 2013

### Bandersnatch

No.

Here's why:
1.The expansion of the universe does not affect small-scale structures, like galaxies. It becomes noticeable at distances of some 1 billion light years and more. The Pillars of Creation are merely 7kly away, which not only within our galaxy, but quite close(compare the diametre of the Milky Way: 100kly).

2.Even assuming that the interpretation of the dust formations is correct, you won't be seeing any supernovae there in 1000 years(or at least not the one responsible for the cloud dispersal).
The whole idea is that the supernova already happened some time ago(in the Earth reference frame; i.e., it has been already "observed"), and it's just the effects(the shockwave) that are lagging behind.

Imagine you see a supernova today in a cloud 7000 ly away. Due to the finite speed of light, you could say that technically it happened seven millenia ago, and it's just now that the light is reaching us.
Now imagine it's 6000 years later, and people see that the shockwave from that event has travelled some distance through the cloud. Using the same argument as before, they can say that what they see had already happened 7000 years ago, and predict that in another 1000 years people would see the shockwave finally reaching some pretty gas structure and diperse it.

edit:
To clarify,
1. is the reason why there wouldn't be any slow-mo supernovae;
2. is the reason why there won't be any supernovae at all(unless an unrelated, new one pops up)

Last edited: Apr 12, 2013
5. Apr 12, 2013

### Staff: Mentor

I'm sorry but that answer was wrong. The reason we don't see motion isn't because the exposures are too long, it is because the motion is too slow compared with our resolving power. If the resolution of a photo is a hundred million miles, then the object has to move/expand by a hundred million miles for us to notice.

When I get home from work, I'll post some numbers about the Ring Nebula and Crab Nebula (unless someone else looks them up first...), two examples where motion is visible on human timescales, but barely.

6. Apr 12, 2013

### Bandersnatch

I admit that I had singlemindedly focused on recording supernovae, which should not have anything to do with the resolving power. In their case, the observed change is the varying luminosity, not anything "moving" through space.
I believed this to be the focus of the OP question, perhaps wrongly, and certainly narrowly.

7. Apr 12, 2013

### Staff: Mentor

Varying luminosity happens relatively fast, yes. On a scale of hours. But supernovae are so bright you don't need very long of an exposure to view them. And yes, the OP asked about motion, not brightness change.

8. Apr 12, 2013

### Staff: Mentor

http://en.wikipedia.org/wiki/Crab_Nebula

The Hubble photo of the Crab Nebula has its width at about 4,000 pixels. With a diameter of 11 light years, that's .00275 LY/pixel. 1,500 km/sec is .005 C, so the Crab Nebula expands at a rate of about 2 pixels per year. So over the course of a few decades, we could make a 1 second video clip that shows it expanding just a bit (1% in 20 years).

As the Crab Nebula is reasonably close and a supernova remnant (fast expanding), it represents about the fastest "video" we could record of an event outside our solar system. The Ring Nebula, by comparison, is a third as far away and is expanding at ~25 km/sec. It is the result of a much slower disintegration of a red giant (it just blasts away its outer layers).

9. Apr 13, 2013

### davenn

And referring to viewing SN expansions. The expansion of the shock wave of SN 1987A over the last 26 years has been easily observable with its interaction with interstellar dust and gas

Dave

10. Apr 14, 2013

### wendal

WOW! All the hours of Discovery Channel, the DVDs, the Carl Sagan videos, the read posts... nothing dropped the apple on to my head like Mr. Watters' resolution analogy. A few months ago, I asked my (smart, but totally uninterested in science) brother the same question about the Pillars of Creation that I asked above. He guessed, "because, it's so big that we can't see it move. It's like how you can't see bacteria dancing around on your hand."

I sort of got it, but couldn't resolve that idea at the time. I dismissed his answer as too simple and figured that if I were a hand bacteria, I could easily observe the motion in my carrier's eyes. I feel dumb now. I'm not the first to be unable to wrap my head around the scale of **** in space, right?

I am extremely grateful for not only the knowledge, but for your assistance in removing this rock from my shoe. It has been sporadically bothering me for a long time. After years of trying to piece it together from chunks of random forum spewings, I'm glad I finally decided to ask. Thank you!