The Photosphere - the "Surface" of the Sun

The Photosphere - the "Surface" of the Sun

Photosphere - Sunspots and Granulation

A full-disk view of the Sun, showing sunspots on the photosphere (top) and a close-up of solar granulation (bottom).
Credit: NASA/SDO and the AIA, EVE, and HMI science teams (left) and Royal Swedish Academy of Sciences - Oddbjorn Engvold, Jun Elin Wiik, Luc Rouppe van der Voort (right).

The photosphere is the visible "surface" of the Sun. The Sun is a giant ball of plasma (electrified gas), so it doesn't have a distinct, solid surface like Earth. Sunlight that is created by nuclear fusion in the Sun's core (center) gradually works it's way outward, colliding over and over with atoms in the Sun's interior. After a million-year journey, the sunlight finally reaches a level where the plasma is less dense and photons stop running into atoms and can finally escape into space. This level is what we see as the glowing "surface" of the Sun - the photosphere.
 
The temperature of the photosphere is around 5,500° C (about 9,900° F). The photosphere is much cooler than the Sun's core, which has a temperature well above 10 million degrees. Surprisingly, the photosphere is also much cooler than the Sun's atmosphere above it, which has regions with temperatures ranging up to a couple of million degrees. There is some variation in temperature from place to place in the photosphere - for example, sunspots are relatively cool regions with temperatures as low as 3,000 to 4,500° C (5,400 to 8,100° F).
 
The area of the Sun's interior immediately below the photosphere is called the convection (or convective) zone. The lower section of the Sun's atmosphere, the chromosphere, lies above the photosphere.
 
Sunspots, indicators of disturbed magnetic fields, are the most common features seen in the photosphere. Sunspots are generally surrounded by lighter areas called faculae, those these are sometimes harder to see than sunspots. Close-up views of the photosphere also show patterns of light areas surrounded by darker borders, called "granulation." Similar to the patterns you can see at the top of a pot of boiling water or oatmeal, granulation is caused by heat rising upward to the photosphere from the hotter solar interior. Where the hot, rising blobs of plasma reach the "surface," we see bright areas. The darker boundaries of the granulation "cells" are places where the plasma has cooled and is sinking back down into the Sun's interior.
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