Answer to Question 1
Most clouds do not appear to be gravitationally unstable and will not contract to form stars on their own. However, a stable cloud colliding with a shock wave (the astronomical equivalent of a sonic boom) can be compressed and disrupted into fragments. Theoretical calculations show that some of these fragments can become dense enough to collapse under the influence of their own gravity and form stars.Supernova explosions produce shock waves that compress the interstellar medium, and recent observations show young stars forming at the edges of such shock waves. Another source of shock waves may be the birth of very hot stars. A massive star is so luminous and hot that it emits vast amounts of ultraviolet photons. When such a star is born, the sudden blast of light, especially ultraviolet radiation, can ionize and drive away nearby gas, forming a shock wave that could compress nearby clouds and trigger further star formation. Even the collision of two interstellar clouds can produce a shock wave and trigger star formation.
Answer to Question 2
Among the four laws of stellar structure, the first two laws of stellar structure have something in common-they are both what astronomers and physicists call conservation laws. Conservation laws say that certain things cannot be created out of nothing or vanish into nothing.The law of conservation of mass is a basic law of nature that can be applied to the structure of stars. It says that the total mass of a star must equal the sum of the masses of its shells.The law of conservation of energy is another basic law of nature. It says that the amount of energy flowing out of the top of a layer in the star must be equal to the amount of energy coming in at the bottom, plus whatever energy is generated within the layer. That means that the energy leaving the surface of the star, its luminosity, must equal the sum of the energies generated in all the layers inside the star.
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