Final answer:
The Main Sequence on the H-R diagram is a diagonal band from the upper left to the lower right, including about 90% of all stars, where they undergo stable fusion of hydrogen into helium. The position of a star on the Main Sequence is determined by its mass, with massive stars being hot and luminous and less massive ones being cooler and dimmer. Other stages of a star's life are less populated on the H-R diagram because they represent shorter phases in a star's evolution.
Step-by-step explanation:
The group of stars that represents the Main Sequence on the Hertzsprung-Russell diagram (H-R diagram) runs diagonally from the upper left (hot, luminous stars) to the lower right (cooler, less luminous stars). These stars, which account for about 90% of all observed stars in the universe, can be found on this path because they are in a stable phase of their lifecycle where they derive energy from the fusion of protons into helium. The Main Sequence stars include a wide range of stars, with the mass determining a star's specific position on the sequence. Massive main-sequence stars are hot and luminous, while those with lower mass are cooler and less luminous.
For stars that appear brightest in the sky but are far away, they are likely to be on the upper left of the Main Sequence, indicating high luminosity and high temperature. Stars closest to us on the Main Sequence would typically be found toward the lower right, representing lower luminosity and cooler temperatures. In the H-R diagram of a star cluster, stars above and to the right of the Main Sequence could either be red giants or young stars approaching the Main Sequence. To distinguish between them, we would observe their motion and changes in luminosity over time or analyze their spectra for indicators of age.
Astronomers find that there are far fewer stars in the giant and supergiant region of the H-R diagram because these stages of a star's lifecycle are comparatively brief. Mass-luminosity relation and the star's life stages determine its path on the H-R diagram, with more massive stars evolving much more quickly through each stage, as depicted by evolutionary tracks in theoretical models.