The visible surface of the Sun, the photosphere, has a textured look, much like the surface of boiling liquid. This pattern is called solar granulation. The grainy texture is produced by millions of individual, bright cells covering the entire solar surface, except where sunspots occur. This phenomenon is a visible manifestation of massive energy transport processes acting within the Sun’s interior. The constant motion reveals the Sun’s surface is a turbulent boundary between the interior and space.
The Scale and Lifespan of Solar Granules
Solar granules are immense. A typical granule measures around 1,500 kilometers in diameter, with sizes ranging from 100 kilometers to 3,000 kilometers. Approximately four million granules cover the Sun’s photosphere.
The lifespan of these structures is short, lasting only between 8 and 20 minutes before dissipating. Granules are constantly being born from fragmentation or through the merging of smaller cells, maintaining the ever-changing pattern.
The Driving Mechanism: Convection
Solar granulation is caused by convection, a massive heat transfer process occurring in the outermost layer of the Sun’s interior, the convection zone. Energy generated in the core is transported outward through this zone via the bulk motion of hot plasma.
In the convection zone, buoyant, hot plasma rises toward the surface because it is less dense than the surrounding cooler plasma. This upward flow carries energy to the photosphere. When the plasma reaches the cooler photosphere, it radiates heat into space, cools, and becomes denser. This denser, cooler plasma then sinks back down into the convection zone, completing the convection cell loop.
The convective flow is driven by cooling at the photosphere, pushing the plasma downward at the edges of the cells. Vertical motions of the plasma can reach around 2 kilometers per second. This continuous cycling generates the visible, grainy texture.
The Physical Structure of a Granule
The distinct appearance of each solar granule results directly from the convective flow. Every granule is the top of an individual convection cell, appearing brighter in the center than at its edges. The bright center is where the hotter, rising plasma first breaks through the photosphere.
This hotter plasma emits more light, making the center significantly brighter than its surroundings. The brightness difference corresponds to a temperature variation of approximately 400 Kelvin between the center and the edges. This luminosity difference is governed by the Stefan-Boltzmann law.
As the hot plasma reaches the surface, it spreads outward horizontally, cools rapidly, and begins its descent back into the Sun’s interior. This sinking, cooler plasma collects in the narrow channels surrounding each bright cell, called the intergranular lanes. The darker appearance of these lanes is due to the lower temperature of the descending plasma.