1. Star Classes
Los Cumbres Observatory

2. Red Star, Blue Star Compared to a red star, a blue star always has
A. more mass.
B. larger radius.
C. hotter temperature.
D. all of the above.

3. Luminosity vs. Temperature
Astronomers plot the luminosity of a star compared to its temperature.
Absolute magnitude for luminosity
Spectral type/class for temperature
Sun

4. Hertzsprung-Russell Diagram
The chart of luminosity vs. temperature is called the Hertzsprung-Russell diagram.
This is the H-R diagram for hundreds of nearby stars.

5. Main Sequence Most stars are on a line called the main sequence.
Hot stars brighter
The size is related to temperature and luminosity.
Hot = large radius
Medium = medium radius
Cool = small radius
Sirius
1 solar radius

6. Giants
Stars that are brighter than the main sequence are giants or supergiants.
Large size
More luminosity
Betelgeuse is a red supergiant.
Radius 1000 R
Rigel
supergiants
Betelgeuse
giants
Aldebaran
Capella

7. Dwarfs Stars that are dimmer than the main sequence are dwarfs.
Small size
Less luminosity
Red dwarfs are on the main sequence.
White dwarfs are hot and small.
white dwarfs

8. Spectroscopic Parallax
Parallax measurements of close stars determine the luminosity for placement on the H-R diagram.
Distant stars similar to close ones
Determine surface temp and spectral class of star.
Place the star on the HR diagram where it should go.
Read off the absolute luminosity from the diagram.
Measure apparent luminosity and calculate distance.