Which statement is true about FU Orionis stars?

FU Orionis stars are a type of young stellar object that experience significant outbursts, which are characterized by substantial increases in brightness. This behavior is closely linked to the presence of instabilities within their circumstellar accretion disks. These disks are not highly stable; rather, they can undergo gravitational instabilities that lead to increased accretion of material onto the star.

As material from the disk falls onto the star, this process can trigger dramatic changes in the star’s luminosity, leading to the characteristic outbursts associated with FU Orionis stars. These instabilities are part of the dynamic nature of star formation and the early life cycles of stars, making option B the correct choice.

Conversely, the other statements fail to capture the nature of FU Orionis stars accurately. For instance, an accretion disk that is highly stable would not result in the significant brightness changes observed in these stars. Furthermore, FU Orionis stars do exhibit mass ejection as they evolve, and their lifetimes, while considerable compared to human timescales, are not necessarily longer than those of other similar stars.