The Sun III 
It’s pretty easy to define the surface of Earth. Just trip over a rock, or even jump off a pier, and you won’t have any trouble telling where the atmosphere ends and the surface begins.
But many other astronomical objects have no solid surface at all. In many cases, in fact, there’s little difference between the surface and the surrounding atmosphere.
A prime example is the Sun.
Its visible surface is known as the photosphere. But that surface isn’t solid — it’s a bubbling cauldron of hot gas. And it’s surrounded by an “atmosphere” that’s only slightly less dense than the surface itself.
The surface is determined by the density of the gas. Below the surface, the gas is so dense that radiation can’t escape into space. Instead, it’s absorbed by the gas itself. Only at the surface does the density drop enough that radiation can leave the Sun and travel through space — most of it in the form of visible light.
Detailed images show that the photosphere isn’t smooth. Instead, it’s made of bubbles of gas that are typically twice as big as Texas.
And the Sun’s magnetic field creates temporary features on the surface — cool, dark sunspots, and bright, hot regions known as faculae and plages. The field also creates powerful explosions, as well as big eruptions of hot gas that speed through the solar system. These outbursts can create storms in Earth’s magnetic field, and wreak havoc with modern technology. More about that tomorrow.
Script by Damond Benningfield, Copyright 2012