The solar photosphere and its structures. Sunspots
The photosphere is the Sun’s visible “surface”, ≈ 300–500 km thick, at ≈ 5800 K. In photographs it has a grainy structure – granulation: granules ≈ 1000 km across are convection cells in which hot gas rises and sinks as it cools, living ≈ 10 minutes. Sunspots are regions of the photosphere with strong magnetic fields (≈ 0.1–0.4 T) that are cooler (≈ 3800–4500 K) than the surrounding 5800 K gas; they look dark by contrast, though in fact they are bright too. A spot consists of a dark centre (umbra) and a lighter surrounding penumbra, is tens of thousands of km across, and lives from days to months. The number of spots varies over about 11 years (Schwabe, 1843) and the magnetic polarity reverses every 22 years (the Hale cycle); the maximum of Cycle 25 occurred in 2024–2025. Observe the Sun only through certified solar filters (ISO 12312-2 glasses, a full-aperture filter in front of a telescope) or by projection onto a screen; ordinary dark glasses, binoculars or an unfiltered telescope will blind you.
Projection method with an adult: with the other objective capped, aim one side of a binocular or a small telescope at the Sun and cast the image from the eyepiece onto white card – NEVER look through the eyepiece! Do not leave the instrument pointed at the Sun for long, as it can overheat. Sketch the number of spots every few weeks. Looking at the Sun is forbidden; do this under adult supervision.