Black dwarfs are not on the HR-Diagram because they are a) Theoretical (as mentioned below) and most importantly b) not luminous (See description of what the HR-Diagram is).
It is believed that there are currently no black dwarves, since the Universe is too young for that. They will likely exist in the far future, when white dwarves cool down.
White dwarfs.
A blue dwarf star would have high temperature and low luminosity in the Hertzsprung-Russell (HR) diagram. Blue dwarf stars are in the lower left corner of the diagram, characterized by their high surface temperature and faint luminosity compared to other stars of similar temperature.
Of course they are on the HR diagram. They are simply not on the main sequence.
Several regions of the HR diagram have been given names, although stars can occupy any portion. The brightest stars are called supergiants. Star clusters are rich in stars just off the main sequence called red giants. Main sequence stars are called dwarfs.
Hertzsprung-Russell (HR) diagram classifies stars based on their luminosity (brightness) and temperature. This diagram allows astronomers to categorize stars into main sequence, giants, supergiants, white dwarfs, and other classes based on their positions in the diagram. It provides insights into the life cycle and evolutionary stage of stars.
White dwarfs.
White dwarfs.
A blue dwarf star would have high temperature and low luminosity in the Hertzsprung-Russell (HR) diagram. Blue dwarf stars are in the lower left corner of the diagram, characterized by their high surface temperature and faint luminosity compared to other stars of similar temperature.
The HR diagram contains only stars - so everywhere.
Of course they are on the HR diagram. They are simply not on the main sequence.
Several regions of the HR diagram have been given names, although stars can occupy any portion. The brightest stars are called supergiants. Star clusters are rich in stars just off the main sequence called red giants. Main sequence stars are called dwarfs.
Hertzsprung-Russell (HR) diagram classifies stars based on their luminosity (brightness) and temperature. This diagram allows astronomers to categorize stars into main sequence, giants, supergiants, white dwarfs, and other classes based on their positions in the diagram. It provides insights into the life cycle and evolutionary stage of stars.
None of the above. White dwarfs and the black dwarfs they will become consist of a unique state of matter called electron degenerate matter.
their colour is one thing but a black dwarf originates from a white dwarf
One might quickly conclude that black holes don't belong on HR diagrams since the latter is designed for stars, and black holes generally aren't considered stars. Further, given that a spectral type would be necessary to plot any given entity on an HR diagram, a black hole would have to have such typification, which it does not have - given that, due to relativistic effects, it emits no significant light. If it were assigned a spectral type, it would still need to have an agreed-upon absolute magnitude, which some might argue was "infinite" for the same reason - reflecting the observer's inability to detect any (intrinsic) light. It is, however, generally accepted that black holes emit a kind of black-body radiation owing to quantum effects (Hawking radiation) for a temperature that may vary with its surface gravity. Even if a consensus could be built regarding its luminosity and its spectral type, its presence on an HR diagram may not be 'useful,' despite its significance in theories of stellar evolution to which the HR diagram so abundantly contributes.
Black dwarfs of black holes.
HR diagram