Back To CourseBasics of Astronomy
28 chapters | 325 lessons
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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 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.
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.
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.
If you look at photographs of the photosphere, you'll notice it has a sort of mottled appearance to it. This is due to granulation, a cell-like pattern on the photosphere caused by granules. Granules are regions on the photosphere that have a dark-edge, just check them out on your screen. It sort of reminds me of the surface of a basketball with all of its little pebbles. These granules can be quite large, thousands of kilometers across, but only last for about 15 minutes before they fade away and are replaced by another granule.
While the pattern of granulation may be pretty, what does it really indicate? Studies have shown that the centers of the granules are hotter than their dark edges and that their centers rise and the edges sink. This is evidence of convection currents underneath the photosphere.
Convection is a process where circulation of a fluid occurs when hot fluid rises and cool fluid sinks. Remember, gas is technically a fluid, hence the use of the word fluid in the definition. Convection is the same process that helps bubbles rise to the top in boiling water. They rise to the top because they are the hotter parts of the water rising towards the cooler areas of the water.
Larger gas currents underneath the photosphere help produce supergranules, collections of hundreds of granules, and they are indicative of even larger convective processes. The false color Doppler image you see on your screen of supergranules indicating large-scale convection is one where blue indicates areas of rising gas and red indicates areas of sinking gas. You normally can't see supergranules in ordinary images because unlike granules, there's little contrast between their edges and centers.
Let's not forget what these convective processes are representative of, it's convection, a process where circulation of a fluid occurs when hot fluid rises and cool fluid sinks. Convection helps explain the pattern on the surface of the sun called granulation, a cell-like pattern on the photosphere caused by granules. Collections of hundreds of granules are called supergranules.
These things form on the photosphere, the visible glowing surface of the sun. Recall that it's the photosphere from which sunlight comes from.
The photosphere is a pretty unblemished surface, except for the occasional sunspot. A sunspot is a relatively dark spot on the sun with a lower temperature than the other parts of the photosphere. Recall that magnetic fields are also responsible for faculae, which are bright spots on the sun.
When you are finished with this lesson, you should be able to:
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Back To CourseBasics of Astronomy
28 chapters | 325 lessons