Apparent brightness measure by photometry
apparent luminosity = luminosity/4 pi (distance)2
total luminosity all wavelengths
Sun = 3.8 x 1026 watts often use LSun
Wiens Law or
Mass by Binary Stars remember Keplerís 3rd law
Three types [by how they are observed]
Remember spectra also tell you stellar rotation, magnetic fields, radial velocity
Temperature (color, spectral type) vs. Luminosity
L = 4 pi r2 x sigma T4 so r is on the H-R diagram also
Real data from Hipparcos satellite
How Long Does a Star Last?
For main sequence ONLY
L goes as M3.5 so lifetime goes as 1/M2.5
Even longer and stranger types Mira
Clusters Ė two types
Open (also know as galactic)
10 pc or so up to several 1000ís stars
20-50 pc up to millions of stars
all old > 10 billion yrs
How do you tell the age of stars?
Use main sequence lifetime on clusters
Why does it work?
A starís life is preordained. (John Calvin almost had it right.)