1. The Hertzsprung Russell Diagram
Order from Chaos

2. A Myriad of Stars Apparent Properties Position Motion Color Brightness
Diurnal
Proper
Color
Brightness

3. Intrinsic Properties of Stars
Mass: Usual Unit is Solar Mass
Stars Range from Solar Masses
Size: Usual Unit is Solar Radii (7(105) km)
Stars Range in Size from 2000 km (a white dwarf) to thousands of solar radii (a supergiant).
Temperature: Kelvins (Sun is 5780K)
Stars range from K
Color
Spectral Type: OBAFGKM
Brightness
Magnitudes

4. Spectral Types O Stars: Strong Helium Lines (35000K)
B Stars: Moderate Hydrogen lines and no Helium lines (20000K) -Rigel
A Stars: Strong Hydrogen lines (12000K) - Vega
F Stars: Moderate Hydrogen lines with ionized metal lines (7500K) -
G Stars: Weak Hydrogen lines with strong neutral metal lines (5500K) - The Sun
K Stars: Very strong neutral metal with some molecules (4500K) -Arcturus
M Stars: Strong molecular features (3500K) - Betelgeuse

5. Do Stars Come in All Varieties?
If all temperatures and radii were possible then a plot of intrinsic brightness versus spectral type should show No Structure -- that is, all regions of the diagram are equally populated.
However, if only certain radii (or range thereof) are possible at a particular temperature then a plot of spectral type versus luminosity should Show Structure.
The answer is possibility number 2
This as we now know directly related to the masses and evolution of stars.

6. Magnitude versus Spectra Type
The HR Diagram
Magnitude versus Spectra Type

7. The HR Diagram

8. The HR Diagram

9. CMD = Color Magnitude Diagrams
The HR Diagram
CMD = Color Magnitude Diagrams