Final answer:
The Hertzsprung-Russell diagram is a plot that classifies stars based on luminosity and surface temperature, with most stars lying on the main sequence. Luminosity is on the y-axis and surface temperature on the x-axis.
Step-by-step explanation:
The Hertzsprung-Russell diagram, also known as the H-R diagram, is a vital tool in astronomy used to classify stars based on two key properties: luminosity and surface temperature. On this diagram, the luminosity of stars is plotted on the vertical (y) axis, which generally measures the intrinsic brightness of a star. The surface temperature, which is often represented by spectral class, is plotted on the horizontal (x) axis, with temperature increasing to the left. Many stars are positioned along the main sequence, which is a diagonal path characterizing stars that fuse hydrogen into helium in their cores. The position of a star on this main sequence correlates with its mass, with higher mass stars being more luminous and hotter. The diagram is further populated by white dwarfs, giants, and supergiants, although these represent smaller portions of the stellar population compared to main sequence stars.