Organizing the Family of Stars:

Organizing the Family of Stars: We know: Stars have different temperatures, different luminosities, and different sizes. To bring some order into that zoo of different types of stars: organize them in a diagram of Slide Absolute mag. or Luminosity Luminosity versus Temperature (or spectral type) Hertzsprung-Russell Diagram Temperature Spectral type: O B A F G K M

Spectral Classification of Stars Hertzsprung-Russell Diagram This is the Hertzprung- Russell (HR) diagram, which is a stellar classification system developed by Ejnar Hertzprung and Henry Norris Russel in Denmark around 1910. Ejnar Hertzsprung Henry Norris Russell The HR diagram relates the magnitudes and colors of stars as a function of their temperature and luminosity.

Henry Norris Russell s first diagram

Hertzsprung-Russell Diagram 1911 1913 Betelgeuse Absolute magnitude Rigel Sirius B Slide Color index, or spectral class

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Slide Stars in the vicinity of the Sun

Slide 90% of the stars are on the Main Sequence!

Check whether all stars are of the same radius: Total radiated power (luminosity) L = σt 4 4πR 2 J/s Slide

Slide No, they are not of the same radius

Enormous spread in radii Rigel Betelgeuse 10,000 times the sun s radius Polaris 100 times the sun s radius Sun As large as the sun Slide

The mass-luminosity relation for 192 stars in double-lined spectroscopic binary systems. L ~ M 3.5 much stronger than inferred from L ~ R 2 ~ M 2/3 Slide

Specific segments of the main sequence are occupied by stars of a specific mass Majority of stars are here Slide

Slide However, this M 3.5 dependence does not go forever: Cutoff at masses > 100 M Ÿ and < 0.08 M Ÿ

Brighter 30,000 K 10,000 K 7500 K 6000 K 5000 K 4000 K 3000 K Luminosity Text HR diagram where data points show measurements from 22,000 real stars from the Hipparcos satellite. (Lines are Theoretical, expected luminosities and temperatures of stars) Temperature: Hotter Color Index: B-V

Brighter HR diagram Absolute Magnitude (M) (Luminosity) Spectral Type Hotter Temperature (Color, B-V)

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Is there any spectral signature of giants? The width of spectral lines! Slide

Spectral Lines of Giants Pressure and density in the atmospheres of giants are lower than in main sequence stars. => Absorption lines in spectra of giants and supergiants are narrower than in main sequence stars => From the line widths, we can estimate the size and therefore, the luminosity of a star. Slide Distance estimate (spectroscopic parallax)

Luminosity Classes Ia Bright Supergiants Ia Ib II III IV V Ib Supergiants II Bright Giants III Giants IV Subgiants V Main-Sequence Stars Slide

Luminosity classes Ia bright supergiant Ib Supergiant II bright giant III giant IV subgiant V main-sequence star Slide

Example Luminosity Classes Our Sun: G2 star on the Main Sequence: G2V Polaris: G2 star with Supergiant luminosity: G2Ib Slide

Puzzles of H-R diagram Why > 90% of stars are on the main sequence? Reason for massluminosity dependence and mass cutoff Same stars at different stages of life or just different stars? Slide

How can we learn about the life of stars?? Our life span is ~ 80 years Human civilization exists ~ 5000 years Our Sun exists at least 4.6 billion years! Slide

Star Clusters School Classes for Stars They consist of stars of the same age! Slide Open clusters 100 s of stars Globular clusters 100,000 of stars

Slide Pleiades

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Age of the cluster from turnoff point Turnoff point: stars of that mass are going to die and move away from the main sequence Slide

Stars spent most of their lives on the Main Sequence At the end of its life the star moves away from the Main Sequence More massive and more luminous stars die faster Hypothesis: Stars on the Main Sequence live due to nuclear fusion of hydrogen! A. Eddington (1920), G. Gamow (1928), H. Bethe (1939) Slide Stars stay on the main sequence until all hydrogen in the core is consumed Then something should happen