What do black holes look like? We finally got the answer: they look like dark circles with a bright ring around the outside. Scientists took the world’s first photo of a black hole. The black hole is very far from Earth—about 55 million light years away. This is not the only black hole in the universe.

Black holes are mysterious in the universe. A photo can help us learn more about them. How much do you know about black holes? Try to answer the following questions.

Q: What is a black hole?

A: A black hole comes from a dying star. The star turns in to a small point. The point has much mass and strong gravity (引力) . The strong gravity of a black hole pulls things. The black hole “eats” these things. Black holes can even “eat” light. That’s why a black hole looks “black”.

Q: If black holes are “black”, how do scientists know they are there?

A: We cannot see black holes. But we can see how their strong gravity works on the things around them. A black hole’s gravity pulls dust and gas into it. And if a star is orbiting (绕轨道运行) a point in space, scientists can study the star’s orbit. They can tell if the star is orbiting a black hole.

Q: How did scientists take the picture of the black hole?

A: Scientists didn’t use a camera to take the picture. They used eight radio telescopes in different places across the world. The telescopes got information from the black hole. Scientists used computers to deal with the information. It took them two years to get the final picture.

Q: Will the sun turn into a black hole? And if it does, will it “eat” Earth?

A: The sun won’t become a black hole. According to NASA, it does not have enough mass to turn into one. When the sun dies, it will become a smaller and cooler star. Even if the sun becomes a black hole, Earth would still be safe.

For thousands of years, humans have explored (探索) the Earth. Nowadays, we are exploring space. Astronomers (天文学家) are the modern day explorers. Now, many astronomers are looking for new planets and new places for humans to live in the future. But where do astronomers start looking?

First of all, astronomers look for a star. That's because our own Earth moves around a star (the Sun).More importantly, it is the correct distance (距离) from the Sun for heat and light. So when astronomers have found the star, they look at the planets around it. In recent years, astronomers have found nearly 400 new planets with stars. However, many of these planets are either too near the star or too far away.

However, if the planet is in a good position, astronomers look for three key things: water, air and rock. Water is important because all life needs water. Humans can drink it and also grow plants with water. And plants produce air for humans to breathe and food to eat. Rock on a planet is also important. That’s because there is often water under the rock.

After many years of scrutinizing, astronomers have found a planet that is similar to the Earth. It’s Gliese 581g and it’s near a star. The astronomers think it has water and rock and the average (平均) temperature is between－31°C and－12°C. That’s cold, but not colder than Antarctica or the Arctic Circle, for example. Gliese 581g is bigger than the Earth. A year on Gliese 581g is only 37 Earth days instead of 365.But astronomers do not think these are big differences and some of them think Gliese 581g will be a new Earth. However, Gliese 581g is twenty light years from the Earth.

①On a clear night, go outside and look for the moon. Do the same thing again a week later, and the moon you see will look different. So why does it always seem to change its shape?

②Sometimes, the moon is a bright disk that lights up the night sky. Other times, it is just a white sliver. Sometimes, the moon can be seen during the day. Other times, it can not be seen. These different kinds of visibility (可见度) are called the phases of the moon (月相).

③They appear because the sun, the moon and the Earth are always moving. One thing that almost never changes is that the sun lights one side of the moon.

④A lunar eclipse (月食) is the only time the sun can’t face the moon. This appears when the sun, the moon and the Earth are in a straight line with the Earth in the middle. As the moon travels into the darkness of the Earth’s shadow (影子), it seems as though all or part of it disappears.

⑤Many scientists believe that the moon was once part of the Earth. Millions of years ago, something hit into the Earth and broke off a large part of the planet. Gravity (地球引力) kept this huge block from going away into space, and it became our moon.

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