Question

What fraction of the stars in the night sky are main-sequence stars?

a. roughly half of them, about 55%

b. relatively few of them, about 20%

c. almost none of them, less than 1%

d. almost all of them, about 90%

Answer 1

About 90% of stars in the night sky are main-sequence stars, as stars spend most of their lifecycle in this phase, despite the brightest stars sometimes being evolved beyond the main-sequence phase.

Step-by-step explanation:

The fraction of stars in the night sky that are main-sequence stars is about 90%. This is due to the fact that stars spend the majority of their lifetimes in the main-sequence phase, where they derive energy from the fusion of hydrogen into helium. Although the brightest stars we see may often be giants or supergiants, which are past the main-sequence phase, these stars are exceptions due to their high luminosity and less common existence. Surveys indicate that about 90% of true stars in our part of space are main-sequence stars, while only about 10% are white dwarfs, and fewer than 1% are giants or supergiants.

One might wonder why the brightest stars are not predominantly main-sequence stars. This is because the most luminous stars, which have evolved beyond the main-sequence, are over-represented in observations of the brightest stars. However, when considering the overall stellar population, particularly with more comprehensive surveys that look beyond just the brightest stars, we find that the vast majority are on the main sequence. This is supported by the mass-luminosity relation, which applies to 90% of stars, confirming their main-sequence status.