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Measuring the Stars How big are stars? How far away are they? How bright are they? How hot? How old, and how long do they live? What is their chemical composition? How are they moving? Are they isolated or in clusters? By answering these questions, we not only learn about stars, but about the structure and evolution of galaxies they live in, and the Universe. Relationship of Luminosity, Size, and Temperature Burner and Boiling Spaghetti Analogy Relationship of Luminosity, Size, and Temperature Luminosity α radius2 x temperature4 Building the Hertzsprung-Russell (H-R) Diagram Which is larger, S or T? => A.) S B.) T C.) same size D.) can’t tell Building the Hertzsprung-Russell (H-R) Diagram Which is larger, S or X? => A.) S B.) X C.) same size D.) can’t tell Building the Hertzsprung-Russell (H-R) Diagram Which is larger, X or Y? => A.) X B.) Y C.) same size D.) can’t tell Building the Hertzsprung-Russell (H-R) Diagram Which is larger, Y or T? => A.) Y B.) T C.) same size D.) can’t tell Building the Hertzsprung-Russell (H-R) Diagram Why is it hard to predict the size of Star S compared with star W w/o calculating? The Hertzsprung-Russell (H-R) Diagram H-R Diagram of Well-known Stars H-R Diagram of Nearby Stars Note lines of constant radius! H-R Diagram of Well-known Stars The Hertzsprung-Russell (H-R) Diagram Red Supergiants Red Giants Increasing Mass, Radius on Main Sequence Sun Main Sequence White Dwarfs Star Classifications Super Giant => From 100 to 1000 times larger than the Sun Giant => From 10 to 100 times larger than the sun Dwarf => any star of size comparable or smaller than the Sun White Dwarf => about the size of the Earth Building the Hertzsprung-Russell (H-R) Diagram Which stars have the same temperature? How can you tell? Are stars of the same temperature always of the same type? Building the Hertzsprung-Russell (H-R) Diagram Which star(s) on this diagram (A – G) match this description? This star is very bright (high luminosity) and very hot (high temperature) Building the Hertzsprung-Russell (H-R) Diagram Which star(s) on this diagram (A – G) match this description? This star is very dim and very cool Building the Hertzsprung-Russell (H-R) Diagram Which star(s) on this diagram (A – G) match this description? This star is very dim and very hot Building the Hertzsprung-Russell (H-R) Diagram Which star(s) on this diagram (A – G) match this description? This star is very bright and very cool How does a star's Luminosity depend on its Mass? L M 3 (Main Sequence stars only!) How Long do Stars Live (as Main Sequence Stars)? A star on Main Sequence has fusion of H to He in its core. How fast depends on mass of H available and rate of fusion. Mass of H in core depends on mass of star. Fusion rate is related to luminosity (fusion reactions make the radiation energy). So, lifetime α Because luminosity lifetime mass of core fusion rate mass of star luminosity (mass) 3, mass or 3 (mass) 1 (mass) 2 So if the Sun's lifetime is 10 billion years, a 30 MSun star's lifetime is only 10 million years. Such massive stars live only "briefly". Clicker Question: The HR diagram is a plot of stellar A: mass vs diameter. B: luminosity vs temperature C: mass vs luminosity D: temperature vs diameter Clicker Question: What would be the lifetime of a star one tenth as massive as our sun? A: 1 billion years = 109 years B: 10 billion years = 1010 years C: 100 billion years = 1011 years D: 1 trillion years = 1012 years Star Clusters Two kinds: 1) Open Clusters -Example: The Pleiades -10's to 100's of stars -Few pc across -Loose grouping of stars -Tend to be young (10's to 100's of millions of years, not billions, but there are exceptions) 2) Globular Clusters - few x 10 5 or 10 6 stars - size about 50 pc - very tightly packed, roughly spherical shape - billions of years old Clusters are crucial for stellar evolution studies because: 1) All stars in a cluster formed at about same time (so all have same age) 2) All stars are at about the same distance 3) All stars have same chemical composition