Are there enough stars to "cover" the sky?
January 6, 2015 4:16 PM   Subscribe

Today's new image of the Pillars of Creation formation show many stars in the background that I've not seen before. That made me wonder: are there enough stars in the universe such that the sky could be covered entirely by starlight?

Here's the image in question:

I'm asking this question here precisely because I don't even know how to word it in such a way that google would return a helpful response.

Maybe this will make sense: that photograph is not actual 3D is a 2D grid of pixels. Increasing the power and the resolution of the camera has consistently resulted in photographs with "more" stars. Maybe normally that 2D grid is 5% covered by white pixels (stars) on a 95% black background (the "void of space"). So maybe this new photograph is 10% white pixels on black, for instance.

Are there enough stars situated relative to one another in the vastness of space that a powerful enough camera could capture the light from them ALL within a single photograph and thereby result in a photograph that is 100% white pixels--just wall-to-wall stars that might exist millions of light-years away from one another along a given Z-axis, but that on an XY grid are immediately adjacent?
posted by jefficator to Science & Nature (14 answers total) 5 users marked this as a favorite
Best answer: This is known as Olber's Paradox: "...the simple observation that the night sky is dark allows us to say something very profound about our universe: it cannot be infinitely big, infinitely old and static all at the same time!"
posted by Johnny Wallflower at 4:22 PM on January 6, 2015 [5 favorites]

Best answer: This is called Olber's paradox. The Wiki article does a pretty good job of explaining it, surprisingly enough!
posted by un petit cadeau at 4:22 PM on January 6, 2015

Best answer: I don't think you've set up the question in a way that excludes taking a photo filled entirely with one star / the Sun?

(And, tangentially, you might be interested in reading about Olber's Paradox.)
posted by solitary dancer at 4:22 PM on January 6, 2015 [1 favorite]

Best answer: (On not-preview: This isn't directly answered by a discussion of Olber's paradox, I don't think. You're not asking why the entire sky isn't bright, but only if there is any small patch that is. And as I suggest, there is, but it's not the anwer you probably want.)
posted by solitary dancer at 4:26 PM on January 6, 2015

Olber's paradox kind of dances around the question you're asking, but it is a related question that influences how to think about this. Depending on the optics, the section of sky you point at and the length of the exposure, you can absolutely get a completely white image--but it's not because of wall-to-wall stars on 3D arc of sky you're looking at, it's because of limitations in resolution of your detectors and optics. There are not an infinite number of stars in the observable universe precisely because the universe had a beginning and continues to expand, and our ability to see the most distant objects in space is limited by how far light can travel since the beginning of the universe. So, if you draw a line from you to any random point in space at that 13.8 billion light year edge of the observable universe, it is far more likely that you'll hit inky blackness where no star shines than that that ray will intersect a star or other visible celestial object. Thus, even an absolutely perfect camera with infinite light-gathering capability and infinite resolution and no optical distortions of the light getting to it (which is impossible; even gravity itself will bend and distort light) will observe a starfield with points of light on the black background of space.
posted by Aleyn at 4:42 PM on January 6, 2015

Also, to clarify, you could also get a white image if you restrict your field of view to a specifically constrained section of space where there are enough overlapping stars (or even a single star) to fill the frame, but I would assume that is the less interesting scenario in your question. You can't just point at a random bit of sky and expect that outcome.
posted by Aleyn at 4:52 PM on January 6, 2015

Response by poster: I remember once asking metafilter how I could find the most efficient route among multiple destinations, only to learn that I was asking about the Traveling Salesman Problem.

I love when the answer to a question is "yeah someone already thought of that, and there's no answer..."
posted by jefficator at 5:00 PM on January 6, 2015 [7 favorites]

Oh, if you want to think about the question in cosmological terms, and looking back to the edges of the universe, we DO see light in every direction, entirely filling our images -- the cosmic microwave background, and before that, the universe was opaque. But I think the question is asking about starlight, in which case we only need to talk about looking back as far as to when stars first formed, much later.
posted by solitary dancer at 5:00 PM on January 6, 2015 [1 favorite]

Aleyn's answer is about right; the speed of light places a fundamental limitation on how big the observable universe is, so even if there were a star in that precise direction, if it's too far away there's no way the light from it could have reached us. Because the universe is expanding that distance is further than the 13.8 billion years the light could have been traveling; the Wikipedia article puts it at about 46 billion light years.

You might also find the Cosmic Microwave Background of interest. That's the light we can see from the first 300,000 years or so after the Big Bang, from the time before there were atoms. Sort of a warm lumpy glow. The CMB is a topic of intense research right now, lots of fun science is being done with new observational data.

Astronomy Cast had a good episode on the observable universe: EP. 295: THE OBSERVABLE UNIVERSE.
posted by Nelson at 5:02 PM on January 6, 2015 [2 favorites]

Link. "Located nearly 7,000 light years away in the distant M16 part of the Eagle Nebula, the new image is even more breathtaking than the one captured by Hubble in 1995." Those are "false color" images that show a range of the electromagnetic spectrum but not what you see. That image is a small slice. 7,000 light years is tiny from what we know of the fifteen or so billion years that we can see.

Olber's paradox states that a static, infinitely old universe with an infinite number of stars distributed in an infinitely large space would be bright rather than dark. We know the universe isn't static. All we know is what we can "see" in the electromagnetic spectrum detectable by instruments.

The stars are moving away the farther they are as well so the further away the star the greater the red-shift. And of course light is bent by gravity.

It's not a camera thing it's a speed of light thing. You can't see through time. The only time you can see is the time from when the light was emitted divided by your distance from the source.

And a funny thing about cosmic backround radiation - it's what you see on your analog TV when you don't have it tuned to a channel. For example.
posted by vapidave at 5:09 PM on January 6, 2015 [1 favorite]

Matter isn't distributed evenly, astronomers use the Hubble deep fields to see older stars/galaxies than before. and still there are voids in the picture. They could be of such a distance they are from before there was light, which no camera powerful enough could resolve.
posted by TheAdamist at 5:39 PM on January 6, 2015

Just a clarification on the CMB: microwaves are what you get when you start with something you'd normally think of as light and then redshift it for 14 billion years.
posted by flabdablet at 8:36 PM on January 6, 2015

This seems less a question about the science of light (as great as the responses have been) but about a very theoretical view of the sky.

Would another way to ask this be: Assuming every star in the universe that would potentially be in my field of view was given an arbitrary (but consistent relative to me) brightness, would the density of the stars be such that I would see a sky consisting entirely of said arbitrary brightness?
posted by jalexei at 10:27 AM on January 7, 2015

I love when the answer to a question is "yeah someone already thought of that, and there's no answer...

It's not that there's no answer. The answer is no, because at some point, you hit the CMB, which is the beginning of the universe.

However, one could consider that 'white' isn't a really well defined concept, and think about 'light' instead, or even go a bit broader and think about 'electro-magnetic radiation', and the answer is different-- There is EM coming from every point in space, not just from stars, but from the big-bang. We just can't see light at that wavelength.
posted by empath at 1:24 PM on January 7, 2015 [1 favorite]

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