photosphere
/ FOH-toh-sfeer /
The photosphere is the visible surface of the Sun — the bright golden disk you see (never directly, always safely) at sunset. It is the layer from which almost all of the Sun's light escapes, and it is what we mean when we talk about the Sun's 'surface', even though the Sun has no solid surface at all.
The word means 'sphere of light'. The Sun is gas all the way through, growing gradually thinner with height, so there is no hard edge. The photosphere is simply the depth at which the gas finally becomes transparent enough that light can fly straight out to space instead of being absorbed and re-emitted. It is a remarkably thin shell — only a few hundred kilometres deep, like the skin on an apple compared to the whole fruit — yet it is where the Sun's light, and the dark absorption lines that reveal what the Sun is made of, are imprinted. Its temperature is about 5,500 degrees Celsius, which sets the Sun's yellow-white colour.
The photosphere matters because it is the boundary between the Sun we can see and the Sun we must infer. Nearly everything astronomers measure about the Sun's light, colour, and composition is a property of this layer. Its mottled granulation, dark sunspots, and gradual darkening toward the edge are all surface features we can watch change from day to day.
The dark Fraunhofer absorption lines used to read the Sun's chemical makeup — hydrogen, sodium, iron and more — are stamped onto the light as it leaves the photosphere.
The photosphere is where the Sun writes its barcode of spectral lines onto the sunlight.
Never look at the photosphere through any telescope, binoculars, or camera without a proper certified solar filter — concentrated sunlight causes instant, permanent eye damage.