Track on the HR Diagram

We may track the dying AGB star on the HR diagram if we extend it to very high temperatures. The figure below illustrates schematically the typical evolution after the AGB region, through planetary nebula emission and finally to a white dwarf. This animation illustrates the same evolution.

These temperatures are very large compared with that for normal main sequence stars. The following image illustrates the central star of a forming planetary nebula that may be even hotter.

This image shows a newly formed white dwarf embedded in a planetary nebula that may be the hottest star known. Click on the "Show Labels" button to annotate the image. The surface of the white dwarf appears to have a temperature of about 200,000 K, which is 30 times the surface temperature of the Sun.

Even though the temperature is so high, this is a dying star. It has used all its nuclear fuel and can no longer produce energy by thermonuclear fusion. Technically, it is not even a star, since we normally require a star to be producing most of its energy from fusion reactions (see the right panel).