Brightness
Depends on
Apparent brightness measure by photometry
Distance Parallax d (pc) = 1/p (arcsec)
apparent luminosity = luminosity/4 pi (distance)2
total luminosity all wavelengths
Sun = 3.8 x 1026 watts often use LSun
Temperature
Wiens Law or
Spectral Type History in order by temperature
Mass by Binary Stars remember Kepler’s 3rd law
Three types [by how they are observed]
Remember spectra also tell you stellar rotation, magnetic fields, radial velocity
The Hertzsprung-Russell Diagram
Temperature (color, spectral type) vs. Luminosity
BUT
L = 4 pi r2 x sigma T4 so r is on the H-R diagram also
Just how big are they? Earth planets solar system stars big stars
Real data from Hipparcos satellite
How Long Does a Star Last?
The mass-luminosity relationship
For main sequence ONLY
L goes as M3.5 so lifetime goes as 1/M2.5
Cepheids and the instability strip
Even longer and stranger types Mira
Clusters – two types
Open (also know as galactic)
10 pc or so up to several 1000’s stars
some young (few million yrs), some old (few billion yrs)
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
How it works H-R change examples Pleiades 4 clusters globular
Why does it work?
A star’s life is preordained. (John Calvin almost had it right.)
