Question

question 4 bonus: pulsars are thought to be ob associations right at the main sequence turnoff point. rapidly rotating neutron stars. low - mass stars undergoing helium flashes every second.

Explanation:

Pulsars are known to be rapidly - rotating neutron stars that emit beams of electromagnetic radiation. These beams are observed as pulses when they sweep across the Earth. OB associations are groups of young, massive stars and have nothing to do with pulsars. Low - mass stars undergoing helium flashes do not correspond to the nature of pulsars.

Answer:

rapidly rotating neutron stars.