Final answer:
The Red supergiant is the brightest star type among the given choices, based on the H-R diagram comparisons and the luminosity characteristics of different spectral classes.
Step-by-step explanation:
The question asks which of the given star types is the brightest when observed from Earth. The available options are Red main sequence, Red supergiant, White dwarf, and Yellow main sequence stars. To determine the brightest star type among these, it is essential to understand their characteristics on the Hertzsprung-Russell (H-R) diagram, which classifies stars based on their luminosity, spectral type, and temperature.
According to the H-R diagram, Red supergiants such as Betelgeuse are among the most luminous stars, vastly outshining main-sequence stars and white dwarfs. Red main sequence stars, such as Spectral type-M main-sequence (E), are typically much fainter. White dwarfs are very dense and can be quite hot, but they do not match the luminosity of supergiants due to their smaller size. Yellow main sequence stars, like the Spectral type-G main-sequence (D), including our Sun, have moderate luminosity.
Considering the spectral classes (OBAFGKM), each class has main-sequence stars with varying luminosities. Class O and B stars are more luminous than class G and M stars. Nonetheless, in the context of the options given, it's clear that Red supergiants will be the brightest of the four choices provided, and hence, the Red supergiant is the brightest star type among the options.