ASTR 101 Lecture 13 Outline of Last Lecture I Luminosity II Brightness and Luminosity III Magnitude scale IV Stellar temperature Outline of Current Lecture I Hertzsprung Russell diagram II Star Classification III Mass and Lifetime IV Main Sequence Stars Current Lecture What 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 diagram A star s full classification includes spectral type line identities and luminosity class line shapes related to the size of the star I Supergiant II Bright giant III Giant IV Subgiant V Main sequence Examples Sun G2 V Sirius A1 V Proxima Centauri M5 5 V Betelgeuse M2 I H R Diagram depicts Temperature Color Spectral type Luminosity Radius Main 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 luminosity Mass 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 10 104 Main Sequence star summary High mass high luminosity short lived large radius blue Low mass low luminosity long lived small radius red
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