The Properties of Stars

• The Hertzsprung-Russell Diagram (H-R Diagram).

• If you plot the brightness verses color or Spectral type of stars you get a ``scatter plot'' - these quantities are not correlated.

  

• But, if we use the sample of stars for which we have distances and can calculate luminosities a plot of Luminosity vs Spectral Type (or color or Temperature) is called an H-R Diagram and there are some definite sequences.

  

• Most stars fall along a sequence that was (and is) called the Main Sequence. For this sequence, there is a correlation in the sense that Hotter stars are also More Luminous.

• The H-R diagram has played a crucial role in developing our understanding of stellar structure and evolution and you will see this diagram over and over again.

  For example, a major focus of astronomy during the 1940's and 50's was constructing Stellar Models that would accurately predict the Luminosity - Temp relation seen in the H-R Diagram. We will get to the results of these investigations in about a week.

• For now, we will note some interesting things about the differences between the nearby stars and the brightest stars and use the H-R diagram to demonstrate one more property of stars.