Life Cycle Of A Star
Übersicht
Look up on a clear night and every point of light you can see is temporary. Stars are not permanent fixtures that were always there and always will be. Each one is partway through a story that has a beginning, a long quiet middle and an ending, and the ending was settled before the story started, by a single number: how much matter fell together to make that star in the first place.
This lesson follows the story twice, because there are two versions of it and the specification wants both. A star of roughly the Sun's mass ages one way and finishes as a cold dark cinder no bigger than the Earth. A star many times heavier races through the same stages far faster, dies in an explosion bright enough to outshine its whole galaxy, and leaves behind an object so dense that either its protons and electrons have been crushed together or nothing at all, not even light, can climb away from it. Along the way you will find out what keeps a star steady for billions of years in between, why only the very centre of it does any work, and why the calcium in your teeth and the iron in your blood could not have existed until an earlier star had already lived and died.
Ziele
- Stars form when enough dust and gas (mainly hydrogen and helium) from space are pulled together by gravitational attraction. Smaller masses may form and be attracted by a larger mass to become planets, or even stars.
- During the ‘main sequence’ period of its life cycle, energy is released by the fusion of hydrogen nuclei to make helium nuclei in the core and a star is stable because the forces within it are balanced. The term ‘radiation pressure’ will not be required.
- The core (centre) of a star is where the temperature and density are greatest and where most nuclear fusion takes place.
- The more massive a star, the hotter its core and the heavier the nuclei it can create by fusion.
- Stars change over time; they have a life cycle. This life cycle is determined by the mass of the star.
- A main sequence star uses nuclear reactions to produce light and heat. When it runs out of hydrogen, what happens next in its life cycle depends upon its mass.
- A larger star will swell to become a red supergiant, in which helium nuclei fuse to form carbon, followed by further fusion that produces heavier nuclei such as nitrogen and oxygen. It expands, cools and turns red. The outer layers then blast away as a supernova is formed. The core collapses and depending upon mass, it forms either a neutron star or a black hole.
- A smaller star, similar to our Sun, follows a different sequence, expanding to become a red giant. It then sheds out layers of gas, exposing the core as a white dwarf and finally cools to become a black dwarf. Students should be familiar with charts that show the life cycles of stars.
- Fusion processes in stars are the source of energy and produce all of the naturally occurring elements. These elements may be distributed throughout the universe by the explosion of a massive star (supernova) at the end of its life. Students should be able to explain how stars are able to maintain their energy output for millions of years, why the early universe contained only hydrogen but now contains a large variety of different elements and that elements heavier than iron are formed in a supernova. j. Protostar Main sequence star Stars about Stars much the same size bigger than as the Sun the Sun Red Red giant supergiant White Supernova dwarf Black Neutron Black dwarf star hole
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- What are stars formed from? A. Dust and gas pulled together by gravitational attraction B. Light and heat released by nearby stars C. Iron and uranium left behind by an earlier supernova D. Neutrons that have escaped from a neutron star Answer: A
- In which part of a main sequence star does most nuclear fusion take place, and why? A. The surface, because it is in contact with space B. The core, because the temperature and density there are greatest C. The outer layers, because most of the star's mass is there D. Evenly throughout the star, because it is made of the same material all through Answer: B
- A star has about the same mass as the Sun. Which sequence correctly describes the stages of its life cycle after the main sequence? A. Red supergiant, supernova, neutron star B. Red giant, supernova, black hole C. Red giant, white dwarf, black dwarf D. Protostar, red supergiant, black dwarf Answer: C
- Where are elements heavier than iron formed? A. By fusion in the core of a main sequence star B. By fusion in the core of a red giant C. In a supernova D. In the cloud of dust and gas before a star forms Answer: C
- Why does a main sequence star stay the same size for thousands of millions of years? A. Because it has stopped releasing energy B. Because the forces within it are balanced C. Because there is no gravity acting inside a star D. Because its outer layers have already been shed Answer: B
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Fusion
While basking in the warmth of the summer sun, a student reads of the latest breakthrough in achieving sustained thermonuclear power and vaguely recalls hearing about the cold fusion controversy. The three are connected. The Sun’s energy is produced by nuclear fusion (see Figure 32.13 ). Thermonucle
"College Physics: Fusion" by OpenStax, OpenStax, Rice University, licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Source: https://openstax.org/books/college-physics/pages/32-5-fusion. Access for free at https://openstax.org/books/college-physics/pages/1-introduction
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