Variability is one of the most conspicuous properties of AGN. The starburst model postulates that the variability observed in radio-quiet sources is produced by the supernova (SN) and compact supernova remnant (cSNR) activity resulting from the evolution of a metal-rich massive stellar cluster, product of a starburst in the nucleus of an early-type galaxy. In this context, the optical light curves of AGN are reproduced by a random sequence of SN events. The parameters that describe a given light curve are the overall rate of explosions (νSN), the energy released in each cSNR (∊51), and the density of the circumstellar medium in which the remnants evolve (n7). In the case of low-luminosity AGN (MB ≳ −22 mag) these three parameters are well constrained by the observations: νSN by the minimum of luminosity and/or by the number of peaks of the light curves; ∊51 by the amplitude and duration of typical oscillations in the light curve and/or by the equivalent width of recombination lines, such as Hβ; and n7 by the decay rate of well-isolated peaks (Aretxaga & Terlevich 1993). The physics involved in the parameters above provides two independent constraints on the distance to a low-luminosity AGN.

Firts distance indicator: SN rate versus stellar luminosity

The B-band luminosity arising from a coeval cluster at its SN II explosion phase, 10 to 60 Myr, is mainly due to Main Sequence stars and cSNR.