Chapter 15:          Surveying the Stars

 

Friday, February 24:
         Observatory Open House, 7:30 pm until 9:00 pm,
         roof of Physics Building

 

Introduction
         Stars come in a variety of colors
         and sizes (1) sizes (2)
         They also range in temperature, mass, and ultimate fate

 

How do we learn the properties of any given star:
         A star's temperature is related to its color by Wiens Law


         Learn the radius from power/surface area = sigma T4
                  where total surface area = 4 pi r2
                  So, need to know the power (i.e. the luminosity)

 

         Find the luminosity from the apparent brightness
                  apparent brightness = luminosity / (4 pi distance2)
                  i.e. luminosity = (4 pi distance2) x apparent brightness
                  LSun = Luminosity of Sun = 3.8 x 1026 watts

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

 

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

         Measuring the mass:
         For Binary Stars, use Kepler’s 3rd law to calculate masses:

                  period2 = 4 (pi)2 (orbital radius)3/ (G (M1 + M2))

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

 

Putting stars into categories
                  Spectral Type
                  Bigger chart with 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 determine the ages of stars?

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