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The surface of the Sun, the part that we can see, is known as the photosphere. This is the region where light from inside the Sun can finally reach space. The temperature of the photosphere is approximately 6000 K, and glows with white light. But the Sun doesn’t end there. Directly above the photosphere is where the Sun’s atmosphere, and it stretches for several hundred thousand kilometers. Let’s take a look at the atmosphere of the Sun.
The first region in the atmosphere of the Sun is the called the temperature minimum, and it sits about 500 km above the photosphere, and has a temperature of about 4,000K. In this part of the Sun, it’s cool enough that simple molecules like water and carbon monoxide can be detected.
The next layer in the Sun’s atmosphere is known as the chromosphere. It’s only about 10,000 km thick (about the diameter of the Earth), but it quickly rises in temperature as it rises in altitude. At the top of the chromosphere, temperatures can reach 20,000 K. The chromosphere is invisible without special equipment that use narrow-band optical filters to see the region. Gigantic solar prominences can rise through the chromosphere reaching altitudes of 150,000 km.
Above the chromosphere is known as the transition region. Below this region, gravity is the dominant force that shapes the features. Above the transition region, temperatures rise quickly because helium becomes fully ionized. Once ionized, helium holds onto heat.
The next layer is the corona, and it extends outwards from the Sun for millions of kilometers into space. You can see the corona during a total solar eclipse, when the disk of the Sun is blocked by the Moon. The temperature of the corona is about 200 times hotter than the surface of the Sun. While the photosphere is only 6,000 K, the corona can reach 1-3 million degrees K. Scientists still aren’t sure why the temperature of the corona is so high.
The uppermost part of the Sun’s atmosphere is the heliosphere. This is the bubble of space filled with the solar wind that extends out about 20 astronomical units (1 AU is the distance from the Earth to the Sun). It is eventually slowed and stopped by the interstellar medium.
We have recorded an episode of Astronomy Cast just about the Sun called The Sun, Spots and All.