Photosphere

The photosphere is the outer layer of a star from which light shines out. It reaches from the star's surface to the point where the hot gas, called plasma, becomes thick enough that we can see it clearly. At this depth, about half of the light can escape without being scattered by the gas.

Stars, including our Sun, do not have solid or liquid surfaces like Earth. Because of this, the photosphere is what we think of as the "surface" when we look at a star. It is from this layer that the light we see comes.

Etymology

The word photosphere comes from ancient Greek words. "Phos" means "light" and "sphaira" means "sphere". It refers to a round surface that seems to give off light.

Sun's photosphere

The Sun's photosphere is the outer layer from which we see its bright light. It has a temperature between 4,400 and 6,600 K, which makes it look very bright to our eyes. The photosphere is not a solid surface but a thin layer about 100 to 400 kilometers thick, with a very low density.

Photospheric phenomena

Further information: Solar phenomena

In the Sun's photosphere, we see granules, which are like convection cells of plasma. Each granule is about 1,000 km (620 mi) wide, with hot plasma rising in the middle and cooler plasma sinking between them. They move very fast, at about 7 km/s (4.3 mi/s), and each granule only lasts about twenty minutes, making the Sun look like it is constantly boiling. Larger structures called supergranules, up to 30,000 km (19,000 mi) wide, last up to 24 hours and move at about 500 m/s (1,600 ft/s). They carry magnetic field bundles to the edges of the cells. Other magnetic features in the Sun's photosphere include sunspots and solar faculae, which are spread between the granules. While we can't see these details on other stars directly, we can indirectly observe similar structures, which we call starspots.