granulation

Granulation is the fine, bubbling pattern that covers the entire visible surface of the Sun, looking a bit like a pan of simmering rice porridge or the surface of a boiling liquid. Each little bright cell, called a granule, is the top of a rising column of hot gas.

Granulation is convection made visible. Hot plasma rises from below, reaches the photosphere, and spreads out across the bright centre of each granule; as it cools it sinks back down along the narrow, darker lanes between granules. A typical granule is about 1,000 km across — roughly the size of a small country — and it lives for only about 8 to 20 minutes before being replaced. At any moment the Sun's surface is paved with a few million of these constantly renewing cells, the whole pattern shifting like a boiling sea.

Granulation matters because it is direct, watchable evidence of the convective zone churning just beneath the surface. By tracking how the granules move and how their light shifts, helioseismologists and spectroscopists can measure the speed of the rising and falling gas and test their models of the Sun's outer layers.