Final answer:
The physical constant used to calculate astronomical distances in light years is the speed of light, denoted by c, which is about 3.00 × 10⁸ m/s in a vacuum.
Step-by-step explanation:
The physical constant that is characteristic of all electromagnetic radiation and is used to calculate distances of stars and galaxies in light years is the speed of light (denoted by c). The speed of light is a fundamental physical constant and has a value of approximately 3.00 × 10⁸ m/s in a vacuum. This constant is crucial for understanding electromagnetic waves, energy of photons, and for calculating how long it takes light from distant celestial objects to reach Earth. In astrophysics, incredibly large distances such as those between stars and galaxies are commonly measured in light years, the distance that light travels in one year.