Why isn't the light of the stars bright enough to lit our sky?

Question

The light of millions of stars reaches the Earth and allows us to see them, but, why isn't it enough to provide light like the sun does? I thought it may have something to do with the distance but I'm not fully satisfied with that theory.

Answer 1

This is called "Olber's Paradox" and it was noticed centuries ago. The current explanation is that it is caused by the expansion of the Universe. There are better explanations than I can give on the net. Try these -

http://zebu.uoregon.edu/~imamura/123/lecture-5/olbers.html

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

Answer 2

What fraction of the sky is coveredy by stars?

Stars beyond about a billion lightyears away don't count, because their light is redshifted away. So if you only count stars that are less than a billion lightyears away, what fraction of the sky is covered? In other words, if you had a randomly-chosen ray that extended from the Earth to infinity, what are the chances that it would hit the surface of a star?

The answer is: very small. Less than a ten millionth (10^-7) of the sky is covered by stars, because stars are very far away and so their disks subtend less than a thousanth of a second of arc.

The Sun, on the other hand, is the exception: the Sun covers about 10^-5 of the sky.

Answer 3

maybe its because those distances are so difficult to comprehend. Take your buddies out with flashlights and see what distance does.

Even the sun from Mars looks small compared with what we see from Earth. The sun is only 8 light minutes from Earth. The nearest star is about 4.5 light years away. Some of the lights in the sky we see are actually galaxies millionjs of light years away.

Answer 4

NASA doesn't give an explanation, "official" or otherwise. But any photographer can tell you why: exposure. The pictures taken on the Moon are of objects being lit by the Sun. And so they are very bright. When we set the camera to expose those objects properly, the stars are simply far too dim to show up on film, atmosphere or not. Using ISO 160 Ektachrome E-3 film, typical daylight exposures are f/8 and f/11 at 1/250 second. Using the same film, it takes 15 seconds at f/5.6 for stars to begin to be visible in the picture. It's no mystery: just basic photography. EDIT 1: I have tested this personally, using a Hasselblad MK70 70mm camera and ISO 160 Ektrachrome film. These figures I'm giving are tested fact, not supposition. At night, nothing around you is being lit by the sun. The general brightness of the scene is rather dim, even in a brightly lit city. This is why you can see stars with the naked eye at night. However, photographic film -- especially Ektachrome reversal film -- has a much narrower exposure latitude. Air does not attenuate much in the visible spectrum, otherwise you couldn't see through it. Photometry confirms that stars are SEVERAL HUNDRED times dimmer than sunlit objects. That huge difference cannot be accounted for by atmospheric attenuation. If you're going to dispute the facts, please have something more substantial than your think-so. EDIT 2: Air does not stop much visible light. If it did, you couldn't see very well from one point to another on Earth. Look at the pictures taken from space of Earth's surface (absent clounds). See how clear and bright the images are? That's from light reflected off the surface, coming up through Earth's air. The absence of air will not suddenly make stars appear HUNDREDS of times brighter than as seen from the ground. Air does not block that much light. The notion that the stars "must" be so much brighter in space comes from author Bill Kaysing, who tried to make this argument in his hoax book. Kaysing was an English major. He had no training in astronomy, photography, or photometry (the science of measuring radiant energy such as light). None. He's just making up all those expectations. You don't see stars in video taken in space for the same reason you don't see it in regular still photography. Cameras have to be adjusted for the apparent brightness of the objects they're photographing. Video taken in space is almost always of objects lit by the sun -- spacecraft, space stations, the Earth, etc. Cameras cannot simultaneously photograph sunlight objects and starlight.

Answer 5

You see, Imagine our universe is a room and stars as the bulbs.and the size of the bulbs does not varies from an atom. If there are 10 to the power of 16 stars in our galaxy and still can't light up our whole galaxy, the earth which is present in our galaxy can't get light from those stars. Hence the stars are not capable of lighting up our sky. Observe in the night, the stars brightness is not as much as our moon's.

Answer 6

if the universe was in a steady state, in other words if everything was simply staying still out there, and had been doing so for countless years, then by now the entire sky would be full of light.

this is called the heat death of the universe.

since the universe is expanding though the light from distant galaxies takes far too long to get here, and by the time it does its redshifted right out of the visible spectrum.

edgar allan poe (yes the raven man) actually wrote an essay about it.

i hope this clears it up.

Answer 7

ya u thought it right, it has indeed to do with the distance, sun is the closest star which has enough energy to radiate to earth, other stars however are much more light years (distance travelled by ray of light at 300000km/sec in 1 yr.), away from earth as well as sun. hence we can only see these stars as bright spots.

Answer 8

AKA the Olbers Paradox.

If the universe is infinite in age and in size, then every line of sight should hit a star. And the night sky should NOT be black, but it is. Why?

Because the universe is finite and size and age and is expanding; there is a horizon 15 billion-ish light-years away (the edge of the observable universe) that we can not see past.

Answer 9

Imagine a pulse of light from our sun. It would come off like a sphere. The further from the sun the light gets the larger the sphere. That means that the light is being spread out more and more over the area of that sphere. Now if that sphere is hundreds of light years away then that light is so spread out that we are only recieving the tiniest fration of it.

Answer 10

They are much farther away than our sun. Being that far away make their light very faint by the time it reaches Earth.