* The H-R diagram plots luminosity against surface temperature. These properties are based on the emission of light from stars. Neutron stars are incredibly dense and don't emit significant amounts of light in the visible spectrum.
* Neutron stars are remnants of massive stars. They are not actively fusing hydrogen like main sequence stars or evolving like red giants. Their primary energy source is the slow cooling of their incredibly dense matter.
So, where do neutron stars fit in?
While not on the H-R diagram, neutron stars occupy a distinct area on a different type of diagram called the "pulsar period-luminosity diagram." This diagram plots the rotation period of a neutron star (pulsar) against its luminosity.
In summary:
* H-R diagram: For actively fusing stars.
* Neutron stars: Not on the H-R diagram due to their lack of significant visible light emission. They are plotted on a specific diagram for pulsars.
