地理
Except our sun ,the lights we see in the sky are all stars light-years from Earth.It's impossible to know how many stars exist ,but astronomers think that in our Milky Way alone ,there are about 300 billion .
All normal stars' birth takes place inside H2-based gas and dust clouds.Over the course of thousands of years ,gravity causes the clouds to shrink until they collapse under their own weight .It is the stars' first stage of birth .As the clouds get smaller and smaller , they turn around faster and faster ,increasing pressure creates rising temperatures inside the gas and dust clouds .And during this time ,the stars enter what is known as the relatively brief T-stage that usually lasts millions of years .
When the temperature inside the clouds climbs to about 15 million degrees Celsius , nuclear fusion starts and they enter the next of life ,known as main sequence . Stars spend 90 percent of their lives in their main sequence phase.Most of the stars ,including the sun ,are main-sequence stars .They exist in a state of nuclear fusion .This process sends out a huge amount of energy ,keeping the star hot and shining brightly .This stage may take billions of years .For example ,Earth's sun is now around 4.6 billion years old ,and it will remain in its main sequence stage for several billion more years .
But there are different ways a star's life can end ,depending on how massive the star is . Less massive stars just burn out .But massive stars die with a Big Bang .While the massive stars may appear to get bigger and bigger on the outside ,the matter inside will eventually become so dense that they explode .Some of the remains may become neutron stars or even black holes if they are massive enough.
1. 结合图表
2. 一些知识理解记忆，看一次后基本解决。常识性问题不能错。
我国三大高产土壤
1.东北黑土
2.西川盆地紫色土
3.南方水稻土