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Lecture 19

Lecture 19

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PHYS 183
Tracy Webb

PHYS 183 – The Milky Way Inside and Out – Tracy Webb Winter 2013 Lecture 19: February 18 , 2013 th  Hertzsprung-Russell diagrams describe essentially every single star in the universe o remember hottest & most luminous stars are blue, coolest & least luminous stars are red o each star occupies a very limited region in the HR diagram o most of the stars lie within a certain range so they are called main sequence stars o we can place every star on the diagram if we know the distance  this allows us to turn apparent brightness into absolute brightness  main sequence stars are limited to a certain region which tells us something about how they work  they are supported by hydrogen fusion o specifically tells us how the star produces its energy at a specific temperature o temperature sets the thermal energy of the star which sets the velocity of the gas which allows protons to overcome repulsion so more reactions (more hydrogen fusion)  sequence in HR diagrams from blue to red is a stellar mass sequence  mass is the primary property of a star o the luminosity-mass relation is L M o more massive stars are move luminous o a little increase in mass gives a huge increase in energy  when a gas cloud collapses, gravitational potential energy is converted into thermal energy so it heats up  more mass, more energy which determines temperature of the core of the star  this also affects the lifetime of the stars  l
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