23

The HR diagram;
Luminosity is plotted with the brighter ones higher, and the spectral type is plotted hottest to coldest:left to right. The stars on the diagonal band, represents a main sequence; these have hydrogen cores. There are some stars within the wider, sparser verticalish band, from up and to the right towards the colder and brighter: this is the giant sequencce, which consists of red giant stars. The few stars at the top of the diagram are supergiants; blue on the left side and red on the right. (more or less.) Then there are the stars located below the diagonal band, at the bottom center, are white dwarfs.

Main sequence stars are plot according to brightness and tempreture but these dependon mass. Hence the diagonal shows a trend of high to low mass stars. (high as in higher mass than the sun.) YSO aren't usually plotted and neutron stars and black holes are too dim to plot with normal stars.

___________

Why is one star brighter than another?
Because Brightness is determined due to tempreture and surface area: the hotter something burns the brighter light it omits, and the more surface area, the more space there is to burn.

==============

White Dwarfs; which are near the bottom due to theur small size, hence they're not the brightest of stars, and due to the fact they only fade and not die, they slide down, and to the right, on the diagram as they grow fainter and colder. They aren't on the right side, because they grow too faint they fall below the range of the diagram.

Supergiants; which are at the top end of the diagram as the are huge in mass, up to around 1000 times larger than the sun, hence they are naturally bright also. The supergiants are generally the same height: indicating that the blue supergiants on the left, are smaller than the red one, on the right. Because the red supergiants, which are colder, must be larger to produce the same total amount of light as the blue which are brighter.

Main Sequence; which are on the diagonal band, from upper left to lower right, due to the fact they burn hydrogen in their cores. The position they are on the diagram depends on their size, as that is the main differential. Although the ones on the left are hotter and therefore brighter. The main sequence stars on the right are the dim, cold, red dwarfs.