填空题 .Different Types of Stars
    When viewed from the ground, the thousands of stars in the night sky have relatively similar appearances, yet in reality, there are a wide variety of stars, each with its own distinct characteristics. There are three main types of stars: main sequence stars, giant stars, and white dwarf stars. They are actually stages in the lives of stars since, as they age, they change in size, luminosity, and temperature until the only things that eventually remain are their inner cores.
    All stars begin as main sequence stars. They are called that because they fall on the mid-range of the Hertzsprung-Russell diagram of star classification, which categorizes stars based upon their size, luminosity, and temperature. Main sequence stars, of which the Earth's sun is one, constitute the vast majority of stars in the universe. When stars form from stellar gas and dust, the process of fusion eventually begins, whereby the stars convert hydrogen into helium, which creates light and heat and also provides stability as the stars' internal energy pushes out and gravity pushes in to form their spherical shapes. Main sequence stars are roughly the same size of the sun but can have up to six times its luminosity, and their surface temperatures average around 3,500 to 7,500 degrees Kelvin. Most main sequence stars are neither very large nor hot though. Instead, they are red dwarf stars, which are smaller and much cooler than the sun and are not even visible to the naked eye from the Earth.
    Giant stars are the first stage dying main sequence stars experience. Main sequence stars lack an infinite amount of hydrogen, so they eventually exhaust their supply and begin dying. The largest of these stars burn through their hydrogen supply faster than smaller ones because of their greater internal pressure and temperature. As gravity contracts these stars, their last remaining inner shell of hydrogen ignites and causes their rapid expansion, pushing them to giant size. Most dying main sequence stars become red giant stars, like the star Betelgeuse, although some become blue giant stars. Giant stars can be gargantuan in scale, with some being more than 1,000 times the size of the sun, but most never attain sizes that big. Their temperatures vary from around 7,500 degrees Kelvin to approximately 30,000 degrees Kelvin.
    After some time, the last remnants of energy in giant stars are nearly depleted, so there is nothing holding their remaining matter together. At that point, some stars explode into supernovas, yet many fail to do so as their outer layers simply dissipate into space and form planetary nebulae while gravity collapses their inner layers and leaves a dense core of material that astronomers call a white dwarf. While white dwarves are not stars but are merely their remains, they are still bright and observable as they cool. Most white dwarves are the size of the Earth but possess much higher densities and masses. Their temperatures fluctuate as they cool, but they begin at a high of roughly 100,000 degrees Kelvin when they form, making them among the hottest stars in the universe. As their remaining heat scatters into space, they slowly cool.
    *luminosity: brightness
    *Kelvin: a unit of temperature
    *supernova: a star that violently explodes near the end of its life21.  Vocabulary
    ______=to make up; to comprise