Scientists think that there has been life on Earth for millions of years. However, we haven’t found life on other planets yet. What not?

The Earth is a planet and it goes around the Sun. Seven other planets also go round the Sun. None of them has an environment with air, so people and things cannot grow. The Sun and its planets are called the solar system. The solar system（太阳系）is a small part of our galaxy.

The stars we see at night are the suns in other solar systems. There are more than 200 billion stars in our galaxy, called the Milky Way and our Sun is only one of them.

But scientists have discovered many other galaxies in universe. They are a long way away and their light has travelled for many years to reach us. It is hard to understand how large the universe is.

Scientists have sent lots of spacecraft to look at other planets in our solar system, and some spacecraft have gone beyond the solar system. However, no one has discovered any life in space yet.

But why has no one from other planets sent us a message? Have they tried to send information to us? With so many stars in the universe, are we alone, or is there life out there in space? We don’t know…yet.

How many things can you see in the night sky? A lot! On a clear night you can see the moon, some planets, and thousands of sparking（闪烁的）stars.

You can see even more with a telescope. You might see that many stars look larger than others. You might see that some stars that look white are really red or blue. With bigger and bigger telescopes you can see more and more objects in the sky. And you can see those objects in more and more detail.

But scientists believe there are some things in the sky that we will never see. We won’t see them with the biggest telescope（望远镜）in the world on the clearest night of the year. That’s because they’re invisible. They’re the mysterious dead stars called black holes.

You might find it hard to imagine that stars die. After all, our sun is a star. Year after year we see it up in the sky, burning brightly and giving us heat and light. The Sun certainly doesn’t seem to be getting old or weak. But stars do burn out and die after billions of years.

As a star’s gases（气体）burn, they give off light and heat. But when the gases run out, the star stops burning and begins to die.

So next time you look up at the night sky, remember: there’s more in the sky than we can see!