- If you plot the brightness versus 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, we 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
- 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 - Temperature 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.