Chapter 19

# Chapter 19 - Physics1902 TheBirthofaStar Lecture W7.b...

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Physics 1902 The Birth of a Star Lecture W7.b Chapter 19

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Life Cycle of a Star We want to develop a model for the life cycle of a star We believe stars form in regions of space where there is a high gas/dust density Once formed, a star will spend most of its life on the ‘Main Sequence’ converting hydrogen to helium Once the hydrogen is consumed, the star becomes a red giant
First steps What conditions cause a cloud of gas/dust to collapse? We need the gravitational potential energy to exceed the kinetic energy This leads to a critical density known as the Jean’s density 3 2 1 = Gm kT M J π ρ M is the total mass m is mass of particle

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Implications of Jean’s Density From the 1/M 2 dependence of the critical density, Galaxies can start to form at very low densities. Once higher densities are reached, areas of high gas/dust concentrations can form. Once we have concentrated areas of gas/dust, stars can form
How Do Stars develop We have clouds of gas and dust If the cloud density exceeds the Jean’s density it will start to contract What is the contraction time scale? For free fall collapse, time is 2 / 1 0 ) 32 3 ( ρ π G t ff =

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Collapse process For a density of 10 -15 kg/m 3 collapse time is about 10 5 years As the cloud is collapsing local areas will exceed the Jean’s density for collapse to lighter systems. The fragmentation continues until we have stellar mass fragments A single cloud may produce hundreds or thousands of stars
Fragmentation of the Dust/Gas cloud

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Chapter 19 - Physics1902 TheBirthofaStar Lecture W7.b...

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