Recent observations with ground-based telescopes have measured the spectra of a large number of gamma-ray sources in the TeV and PeV energy ranges. Photons of such high energies are strongly absorbed as they propagate over extragalactic distances, and most of these sources are Galactic. Combining these new results with those obtained by Fermi-LAT at lower energies it is possible to study the ensemble of the Galactic gamma-ray sources in a very large energy range extending for seven decades from 0.1 GeV to ${10}^6\mathrm{GeV}$. An interesting quantity is the cumulative spectrum obtained summing the fluxes of all resolved Galactic sources. This spectrum is not a simple power-law, but has a smooth shape that softens gradually with energy. This spectral shape can be used as a first approximation for the shape of the flux of the ensemble of the unresolved Galactic gamma-ray sources, that must be subtracted from the observations to obtain the diffuse gamma-ray flux produced in the collisions of cosmic rays with interstellar gas. The shape of cumulative Galactic sources spectrum emerges from the sum of contributions that can have very different spectral shapes, and cannot be considered as a good model for the emission of any individual source. Understanding the origin of the spectral shapes of the Galactic gamma-ray sources remains a crucial challenge for high-energy astrophysics.

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• Cosmic rays & astroparticles
• Gamma ray astronomy