convective energy transport
/ kun-VEK-tiv /
Watch a pot of thick soup on the stove and you will see it churn: hot blobs of soup rise from the bottom, spill their heat at the top, cool, and sink back down, over and over. That rolling overturn is convection, and it is the second way a star moves energy outward. Instead of light slowly leaking through the gas, whole parcels of hot gas physically rise, dump their heat near the surface, and sink back to be reheated. Convective energy transport is heat carried by this bulk churning of stellar gas.
A region of a star starts to convect when radiation can no longer keep up — when the gas is so opaque, or the energy flowing through is so concentrated, that the temperature would have to fall too steeply with height for radiation alone to carry it. Past that tipping point a small blob of gas, nudged upward, finds itself hotter and lighter than its new surroundings and keeps rising, like a hot-air balloon; cooler blobs sink. The result is a boiling layer that overturns and ferries energy far more efficiently than the slow photon stumble of radiation.
Convection shows up in different places depending on a star's mass. The Sun's outer third is a convective zone — its bubbling top makes the mottled 'granulation' we can photograph on the solar surface. Low-mass red dwarfs may be convective all the way through, while massive stars convect in their cores instead. Where a star convects matters enormously: it stirs and mixes the gas, dredges fusion products up toward the surface, and helps decide how long a star can keep burning. Modeling convection accurately is one of the hardest open problems in stellar physics.
The Sun's surface looks pebbled with bright cells about 1,000 km across, each one a column of hot gas welling up from below and cooling at the top — granulation, the visible roof of the Sun's convection zone, the same churning you see in a heated pot, scaled up to planet size.
Solar granulation is the top of the Sun's convection, a boiling soup the size of continents.
Whether a layer convects or radiates depends on mass and opacity, not on which is 'better' — both are just ways of moving heat. And convection is notoriously hard to model; the simple recipe used in most stellar models is a known weak spot.