Wow!Stars evolve, or change, over time. It may take millions of years or it may take billions of years for a star to complete its life cycle.
|A star is a brilliantly glowing sphere of hot gas whose energy is produced by an internal nuclear fusion process. Stars are contained in galaxies. A galaxy contains not only stars, but clouds of gas and dust. These clouds are called nebulae, and it is in a nebula where stars are born. In the nebula is hydrogen gas which is pulled together by gravity and starts to spin faster. Over millions of years, more hydrogen gas is pulled into the spinning cloud. The collisions which occur between the hydrogen atoms starts to heat the gas in the cloud. Once the temperature reaches 15,000,000 degrees Celsius, nuclear fusion takes place in the center, or core, of the cloud. The tremendous heat given off by the nuclear fusion process causes the gas to glow creating a protostar. This is the first step in the evolution of a star. The glowing protostar continues to accumulate mass. The amount of mass it can accumulate is determined by the amount of matter available in the nebula. Once its mass is stabilized, the star is known as a main sequence star. The new star will continue to glow for millions or even billions of years. As it glows, hydrogen is converted into helium in the core by nuclear fusion. The core starts to become unstable and it starts to contract. The outer shell of the star, which is still mostly hydrogen, starts to expand. As it expands, it cools and starts to glow red. The star has now reached the red giant phase. It is red because it is cooler than the protostar phase and it is a giant because the outer shell has expanded outward. All stars evolve the same way up to the red giant phase. The amount of mass a star has determines which of the following life cycle paths the star will take.|
MEDIUM STARSAs a red giant, the hydrogen gas in the outer shell continues to burn as the temperature in the core continues to rise. At 200,000,000 degrees Celsius, the helium atoms fuse to form carbon atoms in the core. The last of the hydrogen gas in the outer shell is blown away to form a ring around the core. This ring is called a planetary nebula. When the last of the helium atoms in the core are fused into carbon atoms, the medium size star begins to die. Gravity causes the last of the star's matter to collapse inward and compact. This is the white dwarf stage which is extremely dense. White dwarfs shine with a white hot light but once all of their energy is gone, they die. The star has now reached the black dwarf phase.
A QuestionWhat determines just how large a star becomes?
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