Photosphere: Definition & Features

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  • 0:02 The Sun's Three…
  • 0:31 The Photosphere & Sunspots
  • 2:20 The Place Where…
  • 3:00 Granules,…
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Lesson Transcript
Instructor: Artem Cheprasov
This lesson will go over the sun's photosphere (its visible 'surface'), including where light comes from, how hot it is, and what granules and supergranules are.

The Sun's Three Atmospheric Layers

What do the Earth, sun, your skin, and cakes have in common? They all have layers!

The sun's atmosphere is made up of three layers. In order from deepest to outermost, they are the photosphere, chromosphere, and corona.

Anything below the photosphere is thought of as being part of the interior of the sun.

Since there's a lot to cover with respect to each layer, this lesson's primary focus will be elucidating the key points about the photosphere.

The Photosphere and Sunspots

The visible glowing surface of the sun is more appropriately called the photosphere, and it has a darker edge, called a 'limb.' It's darker because the temperature in the sun's photosphere decreases outward. So, when you look right at the center of the solar disk, you're looking straight into the sun, where lower (deeper), hotter, and thus, brighter layers in the photosphere are located. When you look at the sun's limb, you're looking obliquely at the photosphere, where light comes from higher (shallower), cooler, and thus, dimmer portions of the photosphere.

By the way, don't let the word 'surface' fool you in my definition of the photosphere. You can't stand on it. Neither the photosphere nor the interior of the sun is solid - it's gaseous instead.

Even if you could stand on the bright surface of the sun, keep in mind that the sun's photosphere is a terrible 5800 K or almost 10,000 degrees Fahrenheit (about 5500 C). Overall, the photosphere is a thin (<500 km deep), very low-density, and basically unblemished layer of gas. However, some sunspots appear and then disappear every now and then.

A sunspot is a relatively dark spot on the sun with a lower temperature than the other parts of the photosphere. Sunspots occur in places where the magnetic field in the photosphere is many times stronger than average. This strong magnetic field slows the upward thrust of hot gas and thus, cools those areas of the photosphere. Magnetic fields are also responsible for faculae, which are bright spots on the sun.

An even more interesting tidbit about the sun's surface is that every tiny square millimeter of it radiates out more energy than your typical household light bulb, about 60 watts.

The Place Where Sunlight Comes From

And so, it is the photosphere from which we and Earth receive all that sunlight. This is because the photosphere has a density that's just right as it allows for lots of light to be emitted. But unlike deeper layers, it's not so dense that light cannot escape from it.

What this means is, Earth doesn't receive any light from layers of gas under the photosphere. It also means that the layers above the photosphere are not dense enough to emit a lot of light compared to the photosphere.

Thus, in essence, it is the photosphere that is responsible for emitting all that sunlight you see on a bright sunny day. Make sure to thank it the next time you are outside.

Granules, Supergranules, and Convection

But don't look up at the sun in order to do that, you'll hurt your vision. Instead, if you want to examine the photosphere in a bit more detail, good photographs will be better and much safer.

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