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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.
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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 the main sequence, there are cool, bright stars in the upper right hot, dim stars in the lower left Off Main Sequence Stars
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.
