Final answer:
Granules on the solar photosphere are caused by the convection process inside the Sun, where hotter gas currents rise, then spread out, cool, and sink, forming a pattern known as granulation.
Step-by-step explanation:
The granulation observed at the top of the photosphere is caused by convection processes within the Sun. Granulation is the rice-grain-like structure visible on the solar photosphere, which comes from upwelling currents of gas that are slightly hotter, and thus brighter, than their surrounding regions. These currents are produced as hot gas rises at speeds of 2 to 3 kilometers per second from below the photosphere. When this gas reaches the surface, it cools and then sinks back down into the Sun, creating darker, cooler areas between the brighter granules. Granules are typically 700 to 1000 kilometers in diameter and have a short lifespan of about 5 to 10 minutes. An even larger structure known as supergranules can be about 35,000 kilometers across and last about 24 hours. Observations of granules and the examination of Doppler shifts in the spectra of gases above them, can provide insights into these dynamic processes.