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We use data from M87* central black hole shadow, as well as from the S2 starobservations, in order to extract constraints on Barrow entropy. The latter isa modified entropy arising from quantum-gravitational effects on the black holehorizon, quantified by the new parameter $\Delta$. Such a change in entropyleads to a change in temperature, as well as to the properties of the blackhole and its shadow. We investigate the photon sphere and the shadow of a blackhole with Barrow entropy, and assuming a simple model for infalling andradiating gas we estimate the corresponding intensity. Furthermore, we use theradius in order to extract the real part of the quasinormal modes, and forcompleteness we investigate the spherical accretion of matter onto the blackhole, focusing on isothermal and polytropic test fluids. We extract the allowedparameter region, and by applying a Monte-Carlo-Markov Chains analysis we findthat $ \Delta \simeq 0.0036^{+0.0792}_{-0.0145}$. Hence, our results place theupper bound $\Delta\lesssim0.0828$ at 1$\sigma$, a constraint that is lessstrong than the Big Bang Nucleosynthesis one, but significantly stronger thanthe late-time cosmological constraints.

Constraints on Barrow entropy from M87* and S2 star observations