Convection Cells on Red Supergiants
Red supergiants are stars nearing the end of their lifetime that are much larger than most stars in the universe. The CHARA image above (in angular units of milliarcseconds) shows the surface of the red supergiant AZ Cyg that was monitored with all 6 CHARA telescopes for 5 years. Both small and large features (which can be as large as a quarter of the stellar diameter) are revealed on the surface. The small features appear to vary on the timescale of months, while the large structures are stable for more than a year. The small spots are likely hot granules of rising gas while the dominating large spots are long-lived convection cells which represent the main means of heat and energy transport from the stellar core to the surface in cool stars like red supergiants.
Reference:
Long Term Evolution of Surface Features on the Red Supergiant AZ Cyg
Norris, R., et al., 2012, Astrophysical Journal., 919, 124