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Stellar Evolution Lab 5 What is a Star? • The basic difference between a star and a planet is that – a star emits light produced in its interior by nuclear 'burning‘ – a planet only shines by reflected light The Hertsprung-Russell Diagram • In 1912, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Russell independently graphed the luminosity vs. temperatures for thousands of stars and found a surprising relationship • The Hertzsprung-Russel (H-R) diagram shows the evolution of stars based on their characteristics of surface temperature, luminosity and mass H-R Diagram and Main Sequence Stars • The H-R Diagram shows that most of the stars lie along a smooth diagonal curve called the main sequence where – Hot, luminous stars appear on the upper left – Cool, dim stars in the lower right • ~90% of the stars fall on the main sequence Off Main Sequence Stars • Off the main sequence, there are – cool, bright stars in the upper right – hot, dim stars in the lower left Main Sequence Stars • A large number of stars are – chemically homogeneous – burn hydrogen to helium in their cores – emit luminosity ~ inversely proportional to the 4th power of their Temp – emit luminosity ~ directly proportional to the 4th power of their mass • These stars are called main-sequence stars Luminosity vs radius • the radius of the stars increases as proceed bottom left diagonally to top right: • Sirius B = 0.01 solar radius • Sun = 1 solar radius • Spica = 10 solar radii • Rigel = 100 solar radii • Betelgeuse = 1000 solar radii Mass vs luminosity • stars along the main sequence vary from the highest (~30 solar masses) at the top left to the lowest (~ 0.1 solar mass) at the bottom right • our Sun is an average star Classes of Stars by Luminosity The H-R diagram summarizes the types of stars in the universe: Class Description Familiar Examples • Ia Bright Supergiants Rigel, Betelgeuse • Ib Supergiants Polaris (the North star), Antares • II Bright Giants Mintaka (delta Orionis) • III Giants Arcturus, Capella • IV Sub-giants Altair, Achenrar • V Main sequence Sun, Sirius • not classified White dwarfs Sirius B, Procyon B White dwarfs stars are not classified because their stellar spectra are different from most other stars Life on the Main Sequence • Stars on the main sequence burn by fusing H into He • Large stars tend to have higher core temperatures than smaller stars, so they burn the hydrogen fuel in the core quickly and have shorter lifetimes HYDROGEN!!!!!! • Small stars burn H more slowly. • The length of time a star spends on the main sequence depends upon how quickly the hydrogen gets used up • Once the hydrogen in the core is gone the mass of the star determines what happens next • The Sun is ~ halfway through its life of 10 billion years Limits Shown on H-R • H-R shows a definite upper limit for Main Sequence stars, all stars more massive than this will have ended their hydrogen burning existence already. • Maximum mass for a Main Sequence is ~ 60 times the Solar Mass • Minimum mass of a star is ~ 0.1 of a Solar Mass, the mass required to produce nuclear reactions in the core. • The surface temperature of stars varies from 2000 to 35,000 degrees • Smaller stars are much more numerous than other types of stars. Explanation Another Version of the H-R Yet Another Version Interrelationships