Survey of the Universe Tom Burbine

Quiz #4 Next Wednesday Covers up to today Cumulative You can bring in one 8 ½ by 11 inch piece of paper with anything written on it

Cecilia Payne-Gaposchkin ( ) Payne argued that the great variation in stellar absorption lines was due to differing amounts of ionization (due to differing temperatures), not different abundances of elements

Cecilia Payne-Gaposchkin ( ) She proposed that most stars were made up of Hydrogen and Helium Her 1925 PhD Harvard thesis on these topics was voted best Astronomy thesis of the 20 th century

Hertzsprung-Russell Diagram Ejnar Hertzsprung and Henry Norris Russell independently discovered this diagram’s usefulness Both plotted spectral type (temperature) versus stellar luminosity Saw trends in the plots Did not plot randomly

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Remember Temperature on x-axis (vertical) does from higher to lower temperature O – hottest M - coldest

Most stars fall along the main sequence Stars at the top above the main sequence are called Supergiants Stars between the Supergiants and main sequence are called Giants Stars below the Main Sequence are called White Dwarfs Hertzsprung-Russell Diagram

wd white dwarfs

giant – a star with a radius between 10 and 100 times that of the Sun dwarf – any star with a radius comparable to, or smaller than, that of the Sun

Classifications Sun is a G2 V Betelgeuse is a M2 I

Radius Smallest stars on the main sequence fall on the bottom right Largest stars on main sequence fall on the top left At the same size, hotter stars are more luminous than cooler ones At the same temperature, larger stars are more luminous than smaller ones

Main Sequence Stars Fuse Hydrogen into Helium for energy On main sequence, mass tends to decrease with decreasing temperature

What does this tell us The star’s mass is directionally proportional to how luminous it is L = M 3.5 More massive, the star must have a higher nuclear burning rate to maintain gravitational equilibrium So more energy is produced

Things to remember 90% of classified stars are on main sequence Main sequence stars are “young” stars If a star is leaving the main sequence, it is at the end of its lifespan of burning hydrogen into helium

Remember Largest stars on main sequence are O stars Largest stars that can exist are supergiants

Classifications Sun is a G2 V Betelgeuse is a M2 I Vega is a A0 V Sirius is a A1 V Arcturus is a K3 III

Stellar Evolution Changes that occur while stars age The mass of a star is the main factor deciding its fate because mass determines the gravitational force and the amount of fuel

Any Questions?