The Universe
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3. Why is the night sky dark?

The reason that this question is so important is because its answer can tell us about the distribution of stars and galaxies in the Universe.
Figure 10.25 Night sky

While we know the solution today for Olber's Paradox, it took until well into the 20th century for us to truly understand the nature of the Universe well enough to explain the answer to this question.

In the rest of this lesson, you will find out how we came to understand the Universe and prove to ourselves the reason the night sky is dark.

There are 5 main reasons:

1. The Universe is not infinitely large, that is, it ends at some point.

2. Since light takes time to reach us, we can see only those objects that are near enough to us that their light has reached us. For example, we see the Sun as it was eight minutes ago and the Andromeda galaxy as it was two million years ago, and so on. This means that the light from some more distant stars has not yet reached us.

3. Stars have been shining for only a while. The Universe that we can see is not infinitely old, but only about 14 thousand million years old .

4. Stars do not shine forever. They typically die after several thousand million years.

5. There are not enough stars in our Universe. If there were 10 million times more stars, there would be 10 million times more starlight! But our current starlight is too weak.

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