The curve that currently contains most stars on the HR diagram is called the "main sequence". It consists of those stars that fuse hydrogen-1, converting it into helium-4.
They are the main sequence stars.
The Hertzsprung Russell diagram plots each star as a point on the diagram, with absolute magnitude along the vertical scale and surface temperature along the horizontal scale.
The main sequence - the region across the middle of the diagram.
Absolute magnitude and surface temperature
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They are defined by their absolute magnitude and surface temperature.
It is a chart showing a scatter chart of stars according to their temperature and their luminosity (or absolute magnitude).
The most massive main sequence stars are the brightest, (O main sequence star on Hertzsprung-Russel Diagram). Main sequence stars obey a mass-luminosity relation. On the H-R diagram the vertical axis is the brightness. So the stars along the top are the brightest (intrinsically).
The HR diagram contains only stars - so everywhere.
An H-R Diagram (Hertzsprung and Russell) is used to classify stars based on their size and brightness. The main sequence stars, such as our sun, are the medium stars, and take up the middle f the diagram, while the giants and dwarves are located on either side. http://www.eso.org/public/outreach/eduoff/cas/cas2002/cas-projects/bulgaria_comaber_1/hr_local.gif <- shows what an H-R diagram looks like.
The "supergiant" stars are the brightest stars on the H-R diagram.
The HR diagram has the star's temperature along the horizontal axis and the absolute magnitude (brightness) along the vertical axis. Each star is represented by a single dot. Higher temperature is usually associated with more brightness so many stars lie on or near a line on the diagram called the Main Sequence. Red giant stars are found on the upper right hand quarter because they are relatively cool but still very bright.
The two ways are by their surface temperature (spectrum) and by their absolute magnitude (intrinsic brightness). The HR diagram has spectrum along the horizontal axis and absolute magnitude along the vertical axis. Each star occupies a point in the HR diagram.