# The Sun's Photosphere: Definition & Temperature

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

This lesson will explain what we mean by the photosphere of the Sun, and describe it's properties, including temperature and pressure. A short quiz will follow.

## What is a Photosphere?

When you look at the Sun, what do you see? You see a plain, circular disk. But in fact, you're looking through miles and miles of the outer layers of the Sun.

There's a point where you can't see any deeper, a point where the Sun's surface becomes truly opaque. This imaginary surface is called the Sun's photosphere, and is the part of the Sun that we see with our eyes.

When people talk about the 'diameter of the Sun', they're usually talking about the diameter of the photosphere.

## Where is the Sun's Photosphere?

The Sun's outer layer is called the corona and is the source of solar flares. It is hard to define because the pressure is so low -- it's practically like still being in space. Underneath this is the 2000-km-deep chromosphere, and beneath that is the photosphere. The exact place the photosphere begins is difficult to measure, because the Sun doesn't have a clear surface -- the Sun's particles just get less and less dense gradually until you're in space.

The photosphere is believed to have a thickness of approximately 400 kilometers and is where the transparency of the Sun changes from 100% down to 0%. At a depth of 200 km into the photosphere we are still able to see 89% of the light that is emitted. At a depth of 300 km we see only 64%. And by 400 km we only receive 4% of the light being emitted.

## Properties of the Sun's Photosphere

The outer part of the Sun's photosphere has a temperature of 4465 K, and this is usually the quoted figure when people talk about the 'surface temperature' of the Sun. As you move through the 400 kilometers, this temperature gradually increases until it reaches 7610 K at the innermost part of the photosphere.

The pressure of the gases and plasma in the Sun also gradually increases, starting at 0.0068 bar and increasing to 0.16 bar, which is a 24-fold increase. This is still much lower than the pressure on Earth, which is defined as 1 bar.

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