Page 23 - Curriculum Visions Dynamic Book

(such as from radioactive decay of a material). vacuum A space that is entirely empty.
chromosphere The shell of gases that makes up part of the atmosphere of a star and lies between the photosphere and the corona.
convection/convection current
The circulating flow in a fluid (liquid or gas) that occurs when it is heated from below.
corona (pl. coronae) The gases surrounding a star such as the Sun. In the case of the Sun and certain other stars these gases are extremely hot.
photon A particle (quantum) of electromagnetic radiation.
photosphere A shell of the Sun that we regard as its visible surface.
plasma A collection of charged particles that behaves something like a gas. It can conduct an electric charge and be affected by magnetic fields.
radiation The transfer of energy in the form of waves (such as light and heat) or particles
The photosphere
We see nothing of the inner part of the Sun— what we observe is the slowly spinning lower part of the “atmosphere” called the photosphere.
The photosphere is about 500 km thick, almost like a skin on the surface of the star. Although it is relatively thin, we cannot see through the Sun’s photosphere as we can through the Earth’s much thicker atmosphere. That is because in the Earth’s atmosphere there are few particles to absorb (soak up) light. However, the Sun’s photosphere contains particles that absorb all of the light reaching them from the convective zone below, churns them around in their own patterns of convection within itself, and then reemits them. That means we can only see light given out by the outermost layer of particles. Everything below them is obscured. That is why we cannot see through the photosphere. You
can get a sense of this if you think about what the Earth looks like in places where cloud cover is high. When such high clouds reflect light, we cannot see through them to the surface.
The photosphere is the place where photons that began their journey at the core of the Sun eventually escape into space. No longer trapped by the dense gases of the main body of the Sun, they expand; their pressure falls, and so does their temperature. That is why the upper part
of the photosphere (what we think of as the “surface” of the Sun) is “only” between 4,000°C and 6,000°C.
Movement in the photosphere
We should think of the photosphere as behaving rather like a liquid. That is despite the fact that the photosphere is actually very “thin,” almost
a vacuum, with a density a thousand times less than that of the Earth’s atmosphere.
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