Question

The approximate main-sequence lifetime of a star of spectral type O is 3 million years. When you observe an O star in a galaxy it is safe to conclude that ______________

A) the main-sequence of that O star ended several billion years ago. B) new stars formed in that galaxy in the recent past. C) the galaxy is flooded by infrared photons. D) there are no white dwarfs in that galaxy. E) the galaxy must be closer than 3 million light years

Which physical process in Main Sequence stars determines the boundary between low-mass stars and intermediate mass stars at 2 solar masses?

A) Stars with mass less than 2 solar masses do hydrogen fusion on the Main Sequence, while stars with mass higher than 2 solar mass start with helium fusion on the Main Sequence.

B) Stars with mass less than 2 solar masses are formed in globular clusters, while stars with mass higher than 2 solar mass are formed in open star clusters.

C) Stars with mass less than 2 solar masses have planets, while stars with mass higher than 2 solar mass do not have planets.

D) Stars with mass less than 2 solar masses mainly perform hydrogen fusion by fusing protons directly, while stars with mass higher than 2 solar mass perform hydrogen fusion in a complex reaction that involves carbon, nitrogen and oxygen.

Which physical process in Main Sequence stars determines the boundary between intermediate mass stars and high-mass stars at 8 solar masses?

A) Stars with mass more than 8 solar masses have a strong magnetic field while stars with mass less than 8 solar masses do not.

B) The energy source of Main Sequence stars with mass more than 8 solar masses is helium fusion, while the energy source of Main Sequence stars less than 8 solar masses is hydrogen fusion.

C) Stars with mass more than 8 solar masses will be able to start carbon fusion after helium in the core is depleted, while stars less than 8 solar masses do not.

D) Main Sequence stars with mass more than 8 solar masses are supported by electron degeneracy pressure, while Main Sequence stars with mass less than 8 solar masses are supported by regular gas pressure.

Answer 1

Class O stars are very young, no more than a few million years old, So they are born very few years ago in the galaxy. So B is the correct option.

Nuclear fusion generate energy for the survval of star and other chemical reaction both are the cause of the internal pressure. (c)

Star with mass less than 2 solar mass are undergoing the hydrogen fusion while other which are heavier than this can undergo the helium fusion.

Stars with mass greater than 8 solar mass can undergo the carbon fusion once its helium consumed.