Question

A main-sequence star can be no smaller than 0.08 solar mass. The reason is that

A) thermonuclear reactions begin so suddenly in stars of less than 0.08 solar mass that the star is disrupted by an explosion.
B) protostars of less than 0.08 solar mass cannot form.
C) protostars of less than 0.08 solar mass are not massive enough to contract.
D) the temperature in a contracting protostar of less than 0.08 solar mass does not become high enough for nuclear reactions to start.

Answer 1

A main-sequence star must have a minimum mass to achieve sufficient core temperatures for the nuclear fusion of hydrogen into helium. Below this mass, such as that of brown dwarfs, objects can't sustain these nuclear reactions and are not classified as true stars.

Step-by-step explanation:

The reason a main-sequence star cannot be smaller than about 0.08 solar masses is because an object of mass less than that threshold—such as a brown dwarf—does not achieve the high central temperatures required for nuclear fusion of hydrogen into helium, the process that powers main-sequence stars.

Stars on the main sequence sustain nuclear reactions at a sufficient rate to halt gravitational contraction, and the critical mass for such a star is roughly 0.075 to 0.08 times the mass of the Sun. A star's mass is crucial for determining whether it can ignite nuclear fusion and thereby shine steadily on the main sequence. Unlike a true main-sequence star, a brown dwarf might only support limited nuclear reactions involving deuterium but will not initiate sustained fusion of protons into helium.

Main-sequence stars like our Sun generate energy through a process called the proton-proton chain reaction, where hydrogen nuclei fuse to form helium. This process requires extremely high temperatures and pressures found only in the cores of stars that meet or exceed the critical mass threshold.