The Cosmological Distance Ladder

HR Diagram

When a plot is made of a star's surface temperature versus its luminosity for a large number of stars, it is found that almost all stars lie within well defined regions of the plot.  Such a plot is shown in the image below, (courtesy of Richard Powell). 

The horizontal axis on the HR Diagram shows a star's surface temperature.  As shown in the image above, the actual scale can be shown in any of the following terms: surface temperature, spectral class, or color index.

The vertical axis on the HR Diagram shows a stars intrinsic luminosity.  The scale is generally shown in either luminosity relative to that of the sun, (left edge in the image above), or absolute magnitude, (right edge in the image above).  It could also be given in directly in terms of flux, but that is less common.

Note that this diagram could not be created simply by plotting the observed (i.e. apparent) magnitude of a star against its surface temperature.  Such a diagram would look like a random scattering of points since the apparent magnitude is dependent on distance.  Such a diagram would look like the plot below, (courtesy of Michael Richmond).

For this reason, the HR diagram could not be created without knowing the distance to the stars being plotted, (an exception which we will exploit shortly is a plot for a cluster of stars).  It is in knowing the distances that we are able to convert apparent magnitude to absolute magnitude and hence obtain the very clear relationships shown in the HR Diagram.