# Light years and stars not adding up?

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If the observable universe is 13.8 billion light years across doesn't that mean that the majority of the 200 billion stars in our galaxy have to be extremely close? I'm sure there's some amazingly obvious explanation for this

Along with that our galaxy is only 100,000 light years across so wouldn't that mean like a billion stars within a light year?

Edited by windshields
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well, let me first remind you of a mistake that many people make. we can see out to 13.8 billion light year's distance. that means it is twice that as it is a radial measurement.

pi*r^2=A to find the area of the galaxy roughly would be 7850000000 yr^2.

that's plenty of space even enough to fit all those brown dwarves we cannot see.

fact of the matter is that stars are much closer together towards the center than at the outer reaches.

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If the observable universe is 13.8 billion light years across doesn't that mean that the majority of the 200 billion stars in our galaxy have to be extremely close? I'm sure there's some amazingly obvious explanation for this

Along with that our galaxy is only 100,000 light years across so wouldn't that mean like a billion stars within a light year?

The average thickness of the galactic disk is 1000 ly, with a 50,000 light year radius this gives a volume of 7854 billions of cubic light years. For 200 billion stars, that works out to one star per 39.27 cubic light years or 1 star per a cube 3.4 light years to a side.

The stars are more tightly packed in the core of the galaxy and less in the spiral arms, so there is plenty of room for stars in the Earth's vicinity without crowding.

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well, let me first remind you of a mistake that many people make. we can see out to 13.8 billion light year's distance.

Another mistake is that owing to expansion, we can see further than that. It's been ~13.8 BY since the big bang, but the observable universe is around 46 billion light years in radius

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

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Didn't think about that really, sorry for my idiocy.

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Another mistake is that owing to expansion, we can see further than that. It's been ~13.8 BY since the big bang, but the observable universe is around 46 billion light years in radius

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

If the observable Universe is 46 billion light-years in radius, shouldn't it soon be giving us a red background to the sky at night?

I mean, all the trillions and trillions of stars in the Universe are constantly getting further away. They're increasing in velocity. Relative to us. They're getting increasingly red-shifted.

Will that red-shift become apparent in our night sky. When will we see our sky turning from black to red?

Edited by Dekan
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If the observable Universe is 46 billion light-years in radius, shouldn't it soon be giving us a red background to the sky at night?

I mean, all the trillions and trillions of stars in the Universe are constantly getting further away. They're increasing in velocity. Relative to us. They're getting increasingly red-shifted.

Will that red-shift become apparent in our night sky. When will we see our sky turning from black to red?

We do have a background - it is redshifted beyond the red to the Microwave; the Cosmic Microwave Background Radiation. CMBR.

The stars we can see with the naked eye are close to us in the milky way and are not moving away from us. The local cluster is gravitationally bound and will not get redshifted - some is even blue-shifted slightly as it is moving towards us. The Milky Way, Andromeda etc are just a tiny patch of the Universe - they are massive enough and close enough to each other that they will stay in proximity to each other. It is only gaps on the scale of chasms between superclusters that matter in this regard - thus your distant ancestors will see stars like Sirius get brighter for the next 50 or so millennia.

"The universe is big, really big, you won't believe..."

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If the observable Universe is 46 billion light-years in radius, shouldn't it soon be giving us a red background to the sky at night?

I mean, all the trillions and trillions of stars in the Universe are constantly getting further away. They're increasing in velocity. Relative to us. They're getting increasingly red-shifted.

Will that red-shift become apparent in our night sky. When will we see our sky turning from black to red?

Black means essentially no light. Why should light suddenly appear where it didn't exist before?

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Black means essentially no light. Why should light suddenly appear where it didn't exist before?

It shouldn't suddenly appear from nothing. That's true. But existing light ought to get more red. The redness, being caused by the expansion of the Universe.

As the Universe expands, everything in it gets further away, so its light gets redshifted. From our viewpoint.

OK, I know there are local exceptions, for example blueshifts in gravitationally-bound neighbouring galaxies like M31.

But very remote galaxies, at the edge of the observable Universe, should all look red. I mean, can we see any blue stars in remote galaxies?

Edited by Dekan
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Red shift does not mean that everything is red. It means that all frequencies are shifted towards the red end of the spectrum. Ultraviolet will shift down into the blue end,so we still see blue.

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It shouldn't suddenly appear from nothing. That's true. But existing light ought to get more red. The redness, being caused by the expansion of the Universe.

As the Universe expands, everything in it gets further away, so its light gets redshifted. From our viewpoint.

OK, I know there are local exceptions, for example blueshifts in gravitationally-bound neighbouring galaxies like M31.

But very remote galaxies, at the edge of the observable Universe, should all look red. I mean, can we see any blue stars in remote galaxies?

http://en.wikipedia.org/wiki/Redshift#Highest_redshifts

The entry for the current "largest redshift" galaxy

http://en.wikipedia.org/wiki/UDFy-38135539

"The infrared light that we now observe from the galaxy was emitted as ultraviolet radiation"

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Thanks Swansont and ACG52 for your posts.

After studying these, and the links, I realise that my suggestion that the sky should be red, was probably wrong.

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Geez, I know my vision is really bad, especially at night, but how many galaxies, far enough away to have a noticeable redshift, can you see with the naked eye, Dekan ?

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Well, that's the point Mig. I've seen the M31 galaxy with my naked eye. But haven't seen any others, without using artificial optical aid, such as binoculars or a telescope.

These optical aids prove, that our natural human eyes are too small, to see the sky properly. Our eyes have a maximum diameter of only 7 or 8 millimeters, in the light- gathering pupil, or lens.

With a lens so small, it's surprising we can see so well as we do. The eye is greatly inferior to, for example, a pair of 7 X 50 binoculars. These have lenses which gather 50 times as much light!

But they don't show the night sky as anything other than black. Therefore I'm persuaded that Swansont, ACG and you are correct. The Universe does look black. At least as far as our present human eyes are concerned.

However, suppose we evolved bigger eyes. With a lens, perhaps 100 miles in diameter. Such a gargantuan eye, could gather even weak streams of photons from the remote regions of the Universe.

Then, wouldn't we see space, not as black, but gently glowing, in a true red colour?

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However, suppose we evolved bigger eyes. With a lens, perhaps 100 miles in diameter. Such a gargantuan eye, could gather even weak streams of photons from the remote regions of the Universe.

Then, wouldn't we see space, not as black, but gently glowing, in a true red colour?

Why? What is the source of this light that you expect to be red?

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