Chapter 15 in the 4th edition of Bennett et al.
Surveying the Stars

 

Introduction
         Sizes of the Sun, Sirius, Pollus, and Arcturus figure

         How do we find the Temperature, Distance, Apparent Brightness,
         Luminosity, Magnitude, and Mass of a star?

 

         Temperature

                  Star's color is associated with temperature (Wiens Law)


         Measure distance using the parallax angle,   d (pc) = 1/p (arcsec)

         Learn the size from power/surface area = sigma T4
                  So, need to know the power (i.e. the luminosity)

         Find the luminosity from the apparent brightness

         apparent brightness = luminosity/4p (distance)2

         LSun = Luminosity of Sun = 3.8 x 1026 (counting over all wavelengths) watts

         Describing brightness in terms of "magnitudes"
                  Original starting point = Vega (star map)

         Measuring the mass:
         For Binary Stars, use Kepler’s 3rd law
         Three types of binary stars -- based on how they are observed:
                  Visual Binaries:    Mizar A & B,   Sirius A & B ,   simulation movie

                  Eclipsing Binaries:    drawing    movie    explanation (simulation)

                  Spectroscopic Binaries:    drawing    movie
                 Note spectra also tell you about stellar rotation, magnetic fields

                  Note: some stars that look close together aren't binaries (ex: Mizar and Alcor)

                  Use Kepler's 3rd law ( p2 = 4 (pi)2 a3/ (M1 + M2) ) to find masses from p and a

 

Putting into categories

                  Spectral Type
                  Bigger chartwith labelled, distinct absorption lines
                  Examples    Rigel = B star, Sirius = A star, Procyon = F star, Betelgeuse = M star
                  History: Henry Draper catalog work done by women at Harvard Observatory
                          Annie Jump Cannon reorganized sequence of spectral classes -> OBAFGKM
                          Cecilia Payne-Gaposchkin saw that the OBAFGKM relates to temperature

 

The Hertzsprung-Russell Diagram

         Temperature (color, spectral type) vs. Luminosity
         Radius affects the H-R diagram also (L = 4 (pi) r2 s T4)

         Real data from Hipparcos satellite
         Luminosity Classes

 

         Main Sequence Stars
                  mass-luminosity relationship
                  L goes as M3.5   so lifetime goes as 1/M2.5 (lifetime program)

         The instability strip: Cepheid variables and RR Lyrae stars
                  (delta Cepheid's location on sky) example Cepheid periodicity
                  Cepheid Period-Luminosity relationship

         Giants

         Subgiants

         White Dwarfs

        

Two types of Star Clusters
         "Open Clusters" (also know as "Galactic Clusters")
                 diameter ~ 30 lightyears
                 number of stars is up to several 1000
                 young (few million years), some medium (~100 million yrs), some old (few billion yrs)
         "Globular Clusters" (example: M80)
                 diameter ~ 100 lightyears
                 millions of stars
                 all old (> 10 billion yrs)

 

How do you tell the age of stars?

         Use main sequence lifetime on clusters
                 How it works
                 examples:  Pleiades 4 clusters globular cluster M4