H-R diagram and stellar classification

One of the most useful tools for identifying star types in astronomy is the H-R Diagram. This idea, independently conceived in 1910 by Ejnar Hertzsprung and Henry Russell, is a graphical representation of intrinsic brightness as a function of temperature. It is largely based on this diagram that stars are classified.

There are a few variations of the H-R diagram:

• Absolute visual magnitude (Mv) vs. Spectral Type
• Absolute visual magnitude vs. Temperature
• Luminosity of star (sometimes given as relative to Sun’s luminosity) vs. Spectral Type
• Absolute visual magnitude vs. Color Index (B - V)


Other variations exist as well. The purpose and effect of each diagram is the same, however. Points plotted fall in limited regions on the graph, rather than in a wide distribution.

The Spectral Types are (in order of decreasing temperature):
O, B, A, F, G, K, M

Further, each of these can be subdivided into 10 categories, 0-9, though most of our stars today will be in the 0-5 range. A graph will resemble the graph noted below.

There is a broad roughly diagonal band running from upper left to lower right. This is referred to as the Main Sequence. Most stars spend the bulk of their lives along the Main Sequence