# The Naked Scientists Forum

### Author Topic: A Galaxial Dilemma?  (Read 4352 times)

#### Roy P

• Sr. Member
• Posts: 103
##### A Galaxial Dilemma?
« on: 11/05/2006 21:41:57 »
Imagine travelling in a spaceship towards a spiral galaxy similar to our own Milky Way. You eventually get close enough that it fills the window of the spacecraft you're looking though. Based on views we currently have of these galaxies, one would think the view would be quite blinding.

Now go closer and closer until you reach a position similar to the position we occupy in the Milky Way galaxy. Will the sky be as black as the sky here on Earth?

If so, why? Why is it that the closer you get to a galaxy the dimmer it becomes? It certainly seems that way if our galaxy *is* the same as those we see in the sky from here on Earth.

Roy P

#### another_someone

• Guest
##### Re: A Galaxial Dilemma?
« Reply #1 on: 11/05/2006 23:30:04 »
quote:
Originally posted by Roy P

Imagine travelling in a spaceship towards a spiral galaxy similar to our own Milky Way. You eventually get close enough that it fills the window of the spacecraft you're looking though. Based on views we currently have of these galaxies, one would think the view would be quite blinding.

Now go closer and closer until you reach a position similar to the position we occupy in the Milky Way galaxy. Will the sky be as black as the sky here on Earth?

If so, why? Why is it that the closer you get to a galaxy the dimmer it becomes? It certainly seems that way if our galaxy *is* the same as those we see in the sky from here on Earth.

The issue is that in space (including within a galaxy) there is no atmosphere.

On Earth, if you look directly at the Sun, it can be blinding, but as you turn away from the Sun, you still get reflections from the Earth itself, and light scattered by the atmosphere, so the Sun appears to light up the whole sky.

In space, there is no atmosphere, so there is no scattering of light.  If you get real close to a single star, and look at the star, it will indeed blind you, but as soon as you look even slightly away from the star, you will just see the black of space, because the light of the star is not being scattered.

George

#### ukmicky

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##### Re: A Galaxial Dilemma?
« Reply #2 on: 12/05/2006 01:03:16 »
yeah

Michael

#### Roy P

• Sr. Member
• Posts: 103
##### Re: A Galaxial Dilemma?
« Reply #3 on: 12/05/2006 11:17:54 »
Thanks for the responses, but AS's answer is to a different question. If I'd have asked, 'Why is the sun so bright during the day?' I'd have gone away a happy bunny. But, applying that answer to my original question makes that question even more valid. I very much doubt that the atmosphere has much to do with the strength of the Sun. In fact, I'd imagine the atmosphere actually *decreases* its strenght. But, either way, looking at it will blind you!

Perhaps my question isn't worded well? Shall I try again?

Roy P

#### another_someone

• Guest
##### Re: A Galaxial Dilemma?
« Reply #4 on: 12/05/2006 14:51:37 »
I think you missed my point.

The fact is that a galaxy is not a solid mass of light, it is a collection of individual stars, with lots of gaps between them (in fact, more gaps than stars).

What I was saying is that as you look at a galaxy close up, most of what you will see is empty space.  If you look directly at a star (such as looking directly at the Sun), then it would indeed be blinding; bust most of a galaxy will be empty space, which emits no light.

If the galaxy had an atmosphere, as the Earth does, then the stars would be able to illuminate the empty space between them; but in the absence of such an atmosphere, they cannot illuminate that space, and so it remains black.

George

#### Roy P

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• Posts: 103
##### Re: A Galaxial Dilemma?
« Reply #5 on: 12/05/2006 15:34:28 »
quote:
Originally posted by another_someone

I think you missed my point.

I did indeed. And, believe me, I do understand your argument and I don't wish to appear naive. But, can I persevere with the following scenario:

Take away the Sun and replace it with a galaxy occupying approximately the same degree of sky as the Sun now occupies. Would you consider it to be, say, half as bright as a full moon?

Now, double the size of the galaxy (Bring it closer). Common sense tells me the 'object' would gain in radiance.

Roy P

#### another_someone

• Guest
##### Re: A Galaxial Dilemma?
« Reply #6 on: 12/05/2006 16:25:19 »
quote:
Originally posted by Roy P
Take away the Sun and replace it with a galaxy occupying approximately the same degree of sky as the Sun now occupies. Would you consider it to be, say, half as bright as a full moon?

Cannot say quantitatively how bright it would look (it probably could be calculated – since we do know both the size and brightness of various galaxies, so we could calculate how far away from a galaxy we would have to be in order for the galaxy to take up the same angle in the sky as the Sun, and thus what proportion of the light of the galaxy would reach us at that distance).

While it is true that a galaxy would emit the light of billions of stars, but because those stars are scattered over a very large distance, the actual distance you have to stand back from the galaxy in order that the galaxy would fill the same space as the Sun is so far away that the galaxy would still seem very dim.  Most of that space occupying the area of view will be the dark space between the stars, not the stars themselves; but the span of the disk you will see is covered by billions of stars spread very thinly over an enormously large amount of black space.

quote:

Now, double the size of the galaxy (Bring it closer). Common sense tells me the 'object' would gain in radiance.

In average brightness, I would agree with you, it should be 4 times as bright if you are half the distance away.

Let me give you a crude example.

If you have a light bulb, that is 1 inch across, and look at it closely enough that it covers half your field of view, it can be blindingly bright.

If you now have two light bulbs (of the same power), that are spaced 3 feet apart, and stand far enough back that the 3 foot span covers half of your field of view; although those light bulbs may be sending out twice as much light as the one light bulb, but because you had to stand so much further back so that you could see both of the light bulbs, the actual light you will receive at that distance is only the minutest fraction of the light you saw from one light bulb close up.

George

#### Roy P

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• Posts: 103
##### Re: A Galaxial Dilemma?
« Reply #7 on: 13/05/2006 12:55:08 »
quote:
Originally posted by another_someone

Let me give you a crude example.

That explains beautifully the concept, AS, but can you not see why I have trouble in comprehending it? I look at the picture Neil provided of a Hubble deep field photo in the 'Far Out' thread, and see the brightness of a single galaxy, then I try to imagine one of those galaxies occupying, say, half our field of view in the night sky!

Roy P

#### daveshorts

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• Physics, Experiments
##### Re: A Galaxial Dilemma?
« Reply #8 on: 13/05/2006 14:39:11 »
The photos from hubble may look bright, but remember they have been taken with a very large telescope , with very sensitive CCDs and extreemly long exposures. As things get further away they don't actually look any less bright per unit area, they just get smaller, (eventually when they get smaller than one pixel in your eye they will appear less bright).

So if a galaxy were to cover the whole of your field of view it would appear about the same brightness as andromeda does in the sky - or something similar (although probably less bright than) the milky way.

#### The Naked Scientists Forum

##### Re: A Galaxial Dilemma?
« Reply #8 on: 13/05/2006 14:39:11 »