Final answer:
The proposed calculation using the Schwarzschild radius and comparing sizes between the event horizon and photon sphere does not work. The size of a photon cannot be measured because photons are massless particles. Therefore, it is not possible to calculate the number of moles of photons that would fill the photon sphere around Sagittarius A*.
Step-by-step explanation:
The calculation you proposed to find the size of the photon sphere around Sagittarius A* using the Schwarzschild radius of the event horizon is not directly applicable. The size of the photon sphere is determined by the distance from the black hole where photons can maintain a stable orbit. It is not simply a matter of comparing sizes between the event horizon and photon sphere.
The size of a photon cannot be accurately measured because photons are massless particles that do not have a defined size. They only possess energy and momentum. Therefore, it is not possible to calculate the number of moles of photons that would fill the volume of the photon sphere.