ASTR 101 Lecture 13 Outline of Last Lecture I. LuminosityII. Brightness and LuminosityIII. Magnitude scaleIV. Stellar temperatureOutline of Current Lecture I. Hertzsprung-Russell diagramII. Star ClassificationIII. Mass and LifetimeIV. Main Sequence StarsCurrent LectureWhat is Hertzsprung-Russell diagram?- Applies to all stars. There are no exceptions- One side is temperature, the other is luminosity (need temperature and radius of star to determine luminosity)- Most stars fall somewhere on the main sequence of the H-R diagramA star's full classification includes spectral type (line identities) and luminosity class (line shapes,related to the size of the star):I. SupergiantII. Bright giantIII. GiantIV. SubgiantV. Main sequenceExamples: Sun - G2 V, Sirius - A1 V, Proxima Centauri - M5.5 V, Betelgeuse - M2 IH-R Diagram depicts:- Temperature- Color- Spectral type- Luminosity- RadiusMain-sequence stars are fusing hydrogen into helium in their cores, like the Sun.- Measurements of main sequence stars show that the hot, blue stars are much more massice than cool, red stars- The mass of a normal, hydrogen-fusing star determines its luminosity and spectral type- The core temperature of a higher-mass star needs to be higher in order to balance gravity.- A higher core temp boosts the fusion rate, leading to greater luminosityMass and Lifetime- Sun's life expectancy: 10 billion years- Life expectancy of a 10Msun star:-10 times as much fuel, uses it 104 times faster- 10 million years - 10 billion years x 10/104Main Sequence star summary- High mass: high luminosity,, short lived, large radius, blue- Low mass: low luminosity, long lived, small radius,
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