Spectra and Motion – Doppler Effect Q. 68 Shifted Spectrum Q. 69 Doubled Spectrum

Summary – What Spectra Tell Us Temperature From the peak of the spectrum Composition From wavelengths and strength of dark lines Motion From the Doppler shift Multiplicity From the number of sets of spectral lines Orbit and masses From the changing Doppler shift Pressure and rotation From width of lines

Luminosity, Temperature, and Radius The spectrum of a star is pretty much a black body distribution How bright each point on the surface is depends only on temperature Multiply by the area to get the Luminosity F =T4 L = AF = 4R2T4 Q. 70: Luminsity, Temperature, and Radius

Intrinsic Properties of Stars To describe stars, we want to talk about intrinsic properties Luminosity Composition Temperature Composition is almost always the same Mass is difficult to measure Radius can be deduced from Luminosity and Temperature Radius Mass Temperature and Luminosity

The Hertzsprung-Russell Diagram A plot of temperature vs. luminosity Hot on left, cold on right Luminous at top, dim at bottom Stars fall into categories: The Main Sequence contains about 90% of the bright stars The Giants are rare but very bright The Supergiants are very rare but extremely bright The White Dwarfs are not uncommon but very dim

Main Sequence Stars Main Sequence stars have different sizes, masses, and luminosities But spectral class determines everything else This diagram shows correct relative sizes and approximate colors of stars But not correct relative luminosities

Luminosity from Spectral Class Suppose you have a G2 star. What is its luminosity? 90% of all stars are main sequence G2: B5: K5: For main sequence stars, the spectral type tells you the luminosity Together with brightness, this tells you the distance Spectroscopic parallax

Spectroscopic Parallax Another distance method Has nothing to do with parallax Works only on main sequence stars How it works: Observe the star – determine it’s brightness B Measure its spectral type from spectrum Deduce its luminosity from the Hertzsprung-Russell Diagram Find its distance from: L = 4d2B

Stellar Masses T M T M T M O5 60 F0 1.6 K5 .74 B0 18 F5 1.3 M0 .51 Only some stars can have their masses measured They need to be in binary systems The masses of main sequence stars depends pretty much only on their spectral type T M T M T M O5 60 F0 1.6 K5 .74 B0 18 F5 1.3 M0 .51 B5 5.9 G0 1.05 M5 .21 A0 2.9 G5 .92 M8 .06 A5 2.0 K0 .85

The Main Sequence The mass of a main sequence star affects everything Temperature More massive is hotter Luminosity More massive is much more luminous Radius More massive is bigger

The Life History of Stars – Young Stars The Importance of Mass The entire history of a star depends on its mass and almost nothing else The more mass a star has, the faster it does everything The stages of a star differ based on what is happening in the core of the star The properties of a star vary wildly as it passes through different stages Qualitatively, stars have similar histories, with one big split: Low mass stars (< 8 MSun) have quiet deaths High mass stars (> 8 MSun) go out with a bang