The Sun, an immense ball of superheated plasma, appears as steady, brilliant light from Earth. Upon closer inspection of the visible surface, the photosphere, this tranquility is replaced by constant, violent motion. This “seething and churning” is the direct consequence of the powerful machinery deep inside the star working to transport energy. Energy generated in the core through nuclear fusion must move outward toward space. The dynamic patterns on the Sun’s surface are the tops of vast thermal currents bringing the star’s energy to the final layer before it radiates away.
Granulation: Defining the Sun’s Bubbly Surface
The visible texture of the photosphere is a constantly shifting, grainy pattern called granulation. It is comprised of millions of individual, bright, irregularly shaped patches separated by a network of darker lines. Granulation is the clearest visual evidence of the Sun’s primary method of energy transfer in its outer layers.
These features, known as granules, are colossal in scale, typically ranging from 1,000 to 2,000 kilometers in diameter. They are incredibly short-lived, lasting between 5 and 20 minutes before dissipating or being replaced. The pattern of brighter centers and darker edges is the visible signature of plasma moving vertically through the solar atmosphere.
Convection: The Driving Force Behind Plasma Movement
The physical mechanism responsible for this constant motion is convection, the same process that causes water to boil or air to circulate. Inside the Sun, energy generated in the core initially travels outward through the radiative zone as photons. This changes in the outer 30% of the Sun’s interior, a shell of plasma known as the Convection Zone.
In this zone, the temperature drops significantly, reaching about 2 million degrees Celsius. This is cool enough for atoms like oxygen and carbon to hold onto some electrons, making the plasma highly opaque. Since radiation can no longer efficiently move energy outward, the bulk motion of the hot material takes over. This creates a thermal instability: plasma near the bottom is hotter and less dense than plasma near the surface. The hot, buoyant plasma rises, carrying heat, while cooler, denser plasma sinks to be reheated, establishing continuous circulation.
Anatomy and Lifespan of Individual Granules
Each visible granule is the top of a single, colossal convection cell, demonstrating a clear circulatory flow of plasma. Hot, buoyant plasma shoots upward in the center of the cell, appearing brighter due to its higher temperature. The plasma rises quickly, sometimes reaching speeds of about one kilometer per second.
When this hot plasma reaches the photosphere, it spreads horizontally and cools by radiating energy into space. As the plasma cools, it becomes denser and heavier, sinking back down into the Sun’s interior along the narrow, dark channels called intergranular lanes. These lanes appear dark because the sinking plasma, though only slightly cooler than the rising plasma, causes a significant drop in brightness. The circulatory movement is continuous, with cells constantly forming, fragmenting, and dissolving.
Broader Scales of Solar Convection
The Sun’s surface is churned by two much larger scales of convection beyond individual granules.
Mesogranulation
Mesogranulation is the intermediate scale, consisting of cellular structures measuring between 5,000 and 10,000 kilometers across. These features can persist for about three hours, demonstrating a more stable organization than granules.
Supergranules
The largest known convective features are supergranules, which dominate the organization of the Sun’s magnetic fields. A typical supergranule measures approximately 30,000 kilometers in diameter—more than twice the size of Earth—and lasts for a day or two. The plasma flows within supergranules are slower than those in regular granules, but they sweep and concentrate magnetic fields to the boundaries of their cells.