The natural explanation is a single rotating body that acts as a stable gyroscope. A variety of other evidence also is consistent with the BH picture, but the above arguments were the ones that persuaded a majority of the astronomical community to take the extreme step of adopting BHs as the probable engine for AGN activity. In the meantime, BH alternatives such as single supermassive stars and spinars were shown to be dynamically unstable and hence short-lived. Even if such objects can form, they are believed to collapse to BHs. The above picture became paradigm long before there was direct evidence for BHs.
Dynamical evidence is the subject of the present and following articles.
Meanwhile, there are new kinds of observations that point directly to BH engines. The foregoing discussion applies to the most powerful members of the AGN family, namely quasars and high-luminosity Seyfert and radio galaxies. It is less compelling for the more abundant low-luminous objects, where energy requirements are less demanding and where long jets or superluminal motions are seen less frequently or less clearly. Therefore a small but vocal competing school of thought continues to argue that stellar processes alone, particularly those that occur during bursts of star formation, can reproduce many AGN characteristics. Nonetheless, dynamical evidence suggests that BHs do lurk in some mildly active nuclei, and, as discussed in the next article, even in the majority of inactive galaxies.