  Variable-intensity astronomical sources are the result of complex and often
extreme physical processes. Abrupt changes in source intensity are typically
accompanied by equally sudden spectral shifts, i.e., sudden changes in the
wavelength distribution of the emission. This article develops a method for
modeling photon counts collected from observation of such sources. We embed
change points into a marked Poisson process, where photon wavelengths are
regarded as marks and both the Poisson intensity parameter and the distribution
of the marks are allowed to change. To the best of our knowledge this is the
first effort to embed change points into a marked Poisson process. Between the
change points, the spectrum is modeled non-parametrically using a mixture of a
smooth radial basis expansion and a number of local deviations from the smooth
term representing spectral emission lines. Because the model is over
parameterized we employ an $\ell_1$ penalty. The tuning parameter in the
penalty and the number of change points are determined via the minimum
description length principle. Our method is validated via a series of
simulation studies and its practical utility is illustrated in the analysis of
the ultra-fast rotating yellow giant star known as FK Com.
