(Chapter 17-19) < < prev   next > > (mass and age)
Stars are born in clouds of gas and dust, called nebulae (plural for nebula, meaning cloud).
The cloud is initially in equilibrium, with particles far enough apart that the gravitational force on each particle is so small that it has no effect.
Something, perhaps a shockwave from an exploding supernova, causes particles of gas and dust to begin to collapse. They get close enough together that the gravitational force on each particle (due to the others) increases. Dense cores form. These cores are called protostars.
During gravitational collapse, as the particles get closer together, the gravitational potential energy of the system decreases. Because energy is conserved, this energy has to go somewhere! It goes into an increase in the kinetic energy of the particles, and as a result, the temperature of the gas increases. The increase in temperature also causes an increase in pressure. The rate of collapse slows down, and it's called a pre-main-sequence star.
When the temperature gets large enough, hydrogen in the core fuses to form helium. The star is now a main-sequence star where fusion of hydrogen atoms into helium atoms is the dominant form of energy production in the star.
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