  In the search for life on other planets, the presence of photosynthetic
vegetation may be detectable from the colors of light it reflects. On the
modern Earth, this spectral reflectance is characterized by an increase in
reflectance between the red and near-infrared wavelengths, a "red edge". On
planets orbiting different stellar types, red edge analogs may occur at other
colors than red. Thus, knowing the wavelengths at which photosynthetic
organisms preferentially absorb and reflect photons is necessary to detect red
edge analogs on other planets. Using a numerical model that predicts the
absorbance spectrum of extant photosynthetic pigments on Earth from
Marosv\"olgyi and van Gorkom (2010), we calculate the absorbance spectrum for
pigments on an Earth-like planet around F through late M type stars that are
adapted for maximal energy production. In this model, cellular energy
production is maximized when pigments are tuned to absorb at the wavelength
that maximizes energy input from incident photons while minimizing thermal
emission and costs to build the photosynthetic apparatus. We find that peak
photon absorption for photosynthetic organisms around F type stars tends to be
in the blue while for G, K, and early M type stars, red or just beyond is
preferred. Around the coolest M type stars, these organisms may preferentially
absorb in the near-infrared, possibly past one micron. These predictions are
consistent with previous, qualitative estimates of pigment absorptance. Our
predicted pigment absorbance spectra depend on both the stellar type and
planetary atmospheric composition, especially atmospheric water vapor
concentrations, which alter the availability of surface photons and thus the
predicted pigment absorption. By constraining the absorbance spectra of alien,
photosynthetic organisms, future observations may be better equipped to detect
red edge analogs.
