3月22日
地址:
http://www.unsv.com/voanews/spec ... ts/2009/01/20/0045/
文章:
SCIENCE IN THE NEWS - Milky Way Is a Lot Bigger Than Scientists Thought
VOICE ONE:
This is SCIENCE IN THE NEWS in VOA Special English. I'm Steve Ember.
VOICE TWO:
And I'm Barbara Klein. This week, we will tell you about new measurements of our own Milky Way galaxy. We will tell about a study linking diamonds to climate change nearly thirteen thousand years ago. We also will tell about efforts to grow crops with less water.
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VOICE ONE:
The biggest structures in the universe are galaxies. These huge groups of stars, gas and dust can be many thousands of light years across. They also are home to an untold number of planets.
Until recently, space scientists have thought that our galaxy was not very special. It was considered a smaller member of a group of galaxies.
Now, new measurements show that our Milky Way galaxy is bigger than scientists once thought. The measurements were reported at a meeting of the American Astronomical Society in Long Beach, California.
VOICE TWO:
Mark Reid is an astronomer with the Harvard-Smithsonian Center for Astrophysics. He led the international team that studied the Milky Way. He said the new measurements mean that our galaxy is fifty percent larger. That makes it at least equal in size to the largest galaxy in our local group, the Andromeda Galaxy. The Andromeda Galaxy is big enough and near enough to be seen by the unaided eye in the star group Andromeda on a dark night.
Mister Reid said: "No longer will we think of the Milky Way as the little sister of the Andromeda Galaxy in our Local Group family."
VOICE ONE:
The team made the discovery by measuring the speed at which our galaxy is moving through space. It found that we are traveling at about two hundred seventy kilometers a second. That is about fifteen percent faster than scientists had believed.
The difference in speed also means a difference in mass of fifty percent. Astronomers have always thought that the Andromeda Galaxy was more massive than the Milk Way because it is believed to have more stars. Now, they will have to rethink that model.
VOICE TWO:
The team measured a number of areas in the Milky Way where stars are forming. These areas, called cosmic masers, are rich in gas molecules that have the effect of strengthening radio waves. The radio waves are measured when the earth is at two points opposite one another in its orbit. The difference in the apparent position of the cosmic masers lets astronomers measure their distance from the Earth.
The team used the most powerful telescope in the world -- the National Science Foundation's Very Long Baseline Array. It is really ten telescopes that are as distant from one another as Hawaii and the United States Virgin Islands. Instead of collecting light, the twenty-five meter wide telescopes collect radio waves. Their combined observing power gives the Very Long Baseline Array the ability to see the finest details of any telescope in the world.
VOICE ONE:
The new information also adds to astronomers' understanding of our galaxy's spiral arms. Mister Reid noted that: "Our galaxy probably has four, not two, spiral arms of gas and dust that are forming stars."
Recent observations had suggested that two of the four spiral arms hold most of the older stars in the Milky Way galaxy. That information was gathered by the American space agency's Spitzer Space Telescope.
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VOICE TWO:
Many scientists believe that a huge object from space hit Earth about sixty-five million years ago. They believe the object crashed about the time dinosaurs disappeared. Some experts believe the crash led to the death of dinosaurs and other ancient creatures.
Now, a group of research scientists says it has found evidence of a similar event that happened about thirteen thousand years ago. It says an explosion or explosions by comets could explain the disappearance of many animals and human beings who lived in North America at that time.
Douglas Kennett of the University of Oregon led the team of researchers. Their findings were reported this month in Sciencemagazine.
VOICE ONE:
The researchers say the report offers evidence that one or more comets were responsible for a sudden cooling period on Earth's surface. They say this ice age lasted about thirteen centuries. It is called Younger Dryas. During this period, large North American animal species like mammoths and saber-toothed cats disappeared.
The researchers reported finding nanodiamonds in several areas across North America. These extremely small particles are believed to be linked to comets. Nanodiamonds are produced under only a very high temperature and high pressure event, such as a comet striking Earth.
VOICE TWO:
The researchers say the nanodiamonds were all found in a level of soil thought to be about thirteen thousand years old. Under this layer of nanodiamonds, they found remains of many ancient animals, including those that disappeared after Younger Dryas. The researchers also found tools used by the Clovis culture, a group of human beings who lived at that time. The tools were also found under, but not above the diamonds.
The researchers say their findings support the idea that a huge explosion or explosions caused fire and pressure powerful enough to kill off life on the Earth. They say an event of this size could have resulted in climate change. They say it would have caused huge clouds of dust to block sunlight.
VOICE ONE:
Many scientists dismiss the comet theory. They say there are other reasons why the large animals and people from that period disappeared.
Some scientists believe that the Clovis people died off because they killed too many native animals. Others say climate change caused the Clovis people and the animals to die. One argument against the comet theory is that a comet would have left a large hole in the Earth's surface. Others say the evidence is important, but that more evidence is needed to confirm the theory.
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VOICE TWO:
Scientists are working to develop crops that can reduce the amount of water used for agriculture. Almost sixty percent of the world's freshwater withdrawals from rivers, lakes and other water resources go toward irrigating fields.
Thomas "Tommy" Carter is a plant scientist in North Carolina. He works for the Agricultural Research Service in the United States Department of Agriculture. He leads Team Drought, a group of researchers at five universities. They have been using traditional breeding methods to develop soybeans that can grow well under dry conditions.
VOICE ONE:
Tommy Carter started working on drought-resistant soybeans in nineteen eighty-one. His research has taken him as far as China, where soybeans have been grown for thousands of years.
Farmers in the United States, however, have grown soybeans for only about a century. Tommy Carter says the soybeans they grow are for the most part genetically similar.
More differences could better protect crops against climate changes that can reduce production. Those changes include water shortages which could increase from rising temperatures in Earth's atmosphere.
VOICE TWO:
The Agriculture Department has a soybean germplasm collection, a collection of genetic material passed from one generation to the next.
Members of Team Drought studied more than two thousand five hundred examples from the collection. They looked at ones from Asia. They searched for germplasms that could keep plants from weakening and wilting during hot, dry summers in the United States.
Tommy Carter says they found only five. But these slow-wilting lines, he says, produce four to eight bushels more than normal soybeans under drought conditions. The yield depends on location and environment.
The team is now doing field tests. The first breeding line is expected to be released this year for use by private seed companies and public soybean breeders.
VOICE ONE:
Scientists are also working on other plants that either use less water or use it better, or both. For example, companies like Monsanto, DuPont and Syngenta are developing corn with reduced water needs. Monsanto expects to be ready in four years to market its first corn seeds genetically engineered to resist drought.
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VOICE TWO:
This SCIENCE IN THE NEWS program was written by Mario Ritter, Jerilyn Watson and Brianna Blake, who also was our producer. I'm Barbara Klein.
VOICE ONE:
And I'm Steve Ember. Join us again next week for more news about science in Special English on the Voice of America..
