11.2 Classifying Stars Our Goals for Learning How do we classify stars? Why is a star’s mass its most important property? What is a Hertzsprung–Russell diagram?

Temperatures and sizes of stars

Most of the brightest stars are reddish in color. There are also many very faint red stars (including the closest star to the Sun) which are too faint to see with the eye.

Red stars all have the same temperature, but they have a huge range in luminosity because they have a huge range in size. L = Luminosity, T = Temperature: L=Area × T 4 L=4*pi*R 2 × T 4 for a star of radius R (to within a constant; see page 319) Biggest red stars: 1000 R sun Smallest red stars: 0.1 R Sun

1) Hotter objects emit more light per area at all wavelengths 2) Hotter objects tend to emit light at shorter wavelengths and higher frequencies Laws of Thermal Radiation

1) Hotter objects emit more light per area at all wavelengths 2) Hotter objects tend to emit light at shorter wavelengths and higher frequencies Activity 10, pages 31-34

How massive are stars?

Isaac Newton We measure mass using gravity, applying Newton's version of Kepler's Third Law (see below). Direct mass measurements are possible only for stars in binary star systems: p = period a = semi-major axis p 2 = a 3 4π 2 G (M 1 + M 2 ) Newton’s Version of Kepler’s 3 rd Law

Visual Binary We can directly observe the orbital motions of the stars...but we only observe the angular separation between the stars, not the physical separation. To find the physical separation, we need to know the distance.

Need 2 out of 3 of the following observables to measure mass: 1)Orbital Period (p) 2)Orbital Separation (a or r=radius) 3)Orbital Velocity (v) For circular orbits, v = 2  r / p r M v

Spectroscopic Binary We determine the orbit by measuring Doppler shifts

Most massive stars: 150 M Sun Least massive stars: 0.08 M Sun (80 M Jupiter )