Well, isn't that a wonderful question, friend? Those stars on the horizontal branch are different from young stars because they are in a different stage of their life. They are burning helium in their cores peacefully, shining brightly and bringing beauty to the night sky. Just like how each brushstroke adds to the beauty of a painting, each star in the sky plays its own special part in the vast universe.
On the HR diagram each star occupies a single point, depending on its surface temperature (horizontal) and its absolute magnitude (vertical).
The temperature is displayed along the horizontal axis while the vertical axis is the star's absolute magnitude. So the HR diagram is a scatter diagram relating temperature and brightness, and eah star occupies one point.
Stars in their giant and supergiant stages are located in the top right portion of the Hertzsprung-Russell diagram, also known as the "red giant branch." These stars are characterized by being large and cool compared to main sequence stars, and they represent later stages in stellar evolution.
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.
The H-R diagram constructed from the data on the brightest stars differs from the diagram constructed from data on the nearest stars because the brightest stars are typically more massive and have higher luminosities, while the nearest stars may include a mix of different masses and luminosities. This leads to a different distribution of stars in the diagram, with the brightest stars clustering in different regions compared to the nearest stars.
If you mean a Hertzsprung-Russell Diagram, which plots stars color, temperature, and absolute magnitude (see related link), then it looks like color and spectral class (temperature) are plotted on horizontal, and Absolute Magnitude/Lumenoscity are plotted on the vertical.
Stars leave the red giant branch when the temperature of the core reaches about 100 millionoK and helium fusion in the core begins. The star is now on the horizontal branch. In smaller stars - this is known as helium flash. [See related question]
The sun will enter this phase of its life as it ages and the hydrogen/helium reaction reduces. See the Wikipedia extract as follows. There are also articles on Red Giant and Asymptotic Giant Branch stars in WikipediaWhen a star exhausts the supply of hydrogen by nuclear fusion processes in its core, the core contracts and its temperature increases, causing the outer layers of the star to expand and cool. The star's luminosity increases greatly, and it becomes a red giant, following a track leading into the upper-right hand corner of the HR diagram. Eventually, once the temperature in the core has reached approximately 3x108K, helium burning begins. The onset of helium burning in the core halts the star's cooling and increase in luminosity, and the star instead moves back towards the left hand side of the HR diagram. This is the horizontal branch (for population II stars) or red clump (for population I stars). After the completion of helium burning in the core, the star again moves to the right and upwards on the diagram. Its path is almost aligned with its previous red giant track, hence the name asymptotic giant branch. Stars at this stage of stellar evolution are known as AGB stars
On the HR diagram each star occupies a single point, depending on its surface temperature (horizontal) and its absolute magnitude (vertical).
Cool and luminous stars would be located in the top right corner of a Hertzsprung-Russell diagram, in the region known as the red giant branch.
The Hertzsprung Russell diagram plots each star as a point on the diagram, with absolute magnitude along the vertical scale and surface temperature along the horizontal scale.
As one moves from left to right along the horizontal axis of the Hertzsprung-Russell diagram, the temperature of the stars increases. This is because the diagram is organized such that hotter stars, which emit more energy, are positioned on the left side, while cooler stars are found on the right. As a result, the spectral classification and color of the stars change from blue and white (hotter) on the left to red (cooler) on the right.
To read an HR diagram, look at the stars' luminosity on the vertical axis and their temperature on the horizontal axis. The main sequence represents stars in stable fusion. Interpret data by noting a star's position on the diagram to determine its temperature, size, and stage of evolution.
The temperature is displayed along the horizontal axis while the vertical axis is the star's absolute magnitude. So the HR diagram is a scatter diagram relating temperature and brightness, and eah star occupies one point.
The Hertzsprung-Russell diagram (H-R diagram) shows the relationship between absolute magnitude, luminosity, classification, and effective temperature of stars. The diagram as originally conceived displayed the spectral type (effectively the surface temperature) of stars on the horizontal axis and the absolute magnitude (their intrinsic brightness) on the vertical axis.
Stars in their giant and supergiant stages are located in the top right portion of the Hertzsprung-Russell diagram, also known as the "red giant branch." These stars are characterized by being large and cool compared to main sequence stars, and they represent later stages in stellar evolution.
The HR diagram contains only stars - so everywhere.