شرکت در آزمون آنلاین انگلیسی 3 - انگلیسی 3 | آزمون شماره 11187

A standard must not change. Most units are standard measures; in other …(A)…, they are exact copies of official measures which are carefully kept by the government. Probably the rulers we use are not perfect copies, …(B)… if you think carefully, you will …(C)…that no one can make a …(D)… copy of anything. But they must be as accurate as we can make them. They are …(E)… by the laws of each country. No person is allowed to use a yard measure or a meter which is different from the government standard.

   Like people, stars are born, grow old, and die. Their birthplaces are huge, cold clouds of gas and dust. As the cloud gets smaller, it breaks into hundreds of pieces. Each piece finally becomes so hot that great reactions begin. When the temperature reaches 10 million degrees Celsius, the hot piece becomes a new star.

  After their birth, most young stars go to the center of a flat disc of gas and dust. Most of this material is finally burnt by the star’s heat. Before this happens, planets may form around the central star.

  Advanced observatories are able to show the heat coming from invisible stars that are forming inside such clouds. One of the most powerful of these is ESA’s Herschel space observatory, launched in May 2009. Herschel will spend at least three years studying the dusty clouds where large and small stars are born.