Sun is not only one star that burns in the universe, if you see up in the night sky there are millions of stars.
Our own Milky Way galaxy contains over 200 billion stars, and the entire universe probably contains over 100 billion galaxies. You might suppose that that many stars would light up the night like daytime!
Why doesn’t their combined light add up and make our night sky bright?
Why is the night sky dark?
German physicist Heinrich Wilhelm Olbers put the same puzzle this way in 1823
If the universe is infinite in size, and stars (or galaxies) are distributed throughout this infinite universe, then we are certain to eventually see a star in any direction we look. As a result, the night sky should be aglow.
Why isn’t it?
This question makes Astronomers to realize that the universe is not infinite.
A finite universe—that is, a universe of limited size—even one with trillions and trillions of stars, just wouldn’t have enough stars to light up all of space.
Although the idea of a finite universe explains why Earth’s sky is dark at night?
The under lying reason is the universe is still expanding, the distant stars and galaxies are moving farther away all the time.
If alien astronomers are looking to our planet Earth from a million light years away, they might just see That reflected light from Earth has been visible since vegetation first began.
Although we know that nothing travels faster than light, it still takes time for light to cross any distance.
The light that leaves that galaxy today will have much farther to travel to our eyes than the light that left it a million years ago or even one year ago, because the distance between that galaxy and us constantly increases. That means the amount of light energy reaching us from distant stars dwindles all the time. And the farther away the star, the less bright it will look to us.
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