Constraints on Barrow Entropy from M87* and S2 Star Observations

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