Are the stars spread out evenly on the Hertzsprung- Russell diagram?

Answer 1

The Hertzsprung--Russell diagram is a scatter graph based on their luminosity verses their spectral class.

Because certain stars are more common than others a certain "band" does exist, but they are not spread out evenly.

See related link for a pictorial diagram

Answer 2

Every star, except black holes (if you consider them stars) & neutron stars fit onto the Hertzsprung-Russel diagram.

The Hertzsprung-Russell diagram is a scatter graph showing the relation between luminosity and the surface temperature of stars.

(See Related links - Hertzsprung-Russel diagram)

The important sections are the main sequence bang, which are all the stars which are in the process of fusing hydrogen, the giants/supergiants and hypergiants that are fusing helium and the white dwarf section which have stopped fusion and are literally dead stars. As stars age they move around the diagram. 1st from left to right as they increase in diameter and the surface temperature cools as they become red giants, then the move from top right to bottom left as they exhaust all their fuel and become white dwarfs.

Since stars age at different rates dependent on size we can use the diagram to guess at the age of star clusters and irregular galaxies since most of the stars are of the same age. Heavier stars tend to be more luminous and hotter than smaller stars. They also age more quickly than smaller stars.

(See Related Links - Cluster ageing)

In the diagram we can see 2 clusters M67 and NCG 188. In both cases a lot of the more massive stars have already left the main sequence. M67 shows more massive stars in the main sequence suggesting that it is younger in age than NCG 188.