It’s too hot (or makin’ a dragon wanna retire part 2).

The photosphere of the sun (the part of the sun that is usually considered to be its surface) has a temperature of about 6,000 °C.  For comparison purposes, animal fat burns at 800 – 900 °C which means you wouldn’t last long on the surface of the sun. Wood and gasoline burn at about 1,000 °C. A propane torch burns at 1,200 to 1,700 °C while an oxyacetylene torch can reach temperatures of 3,300 °C.  So, yeah, the surface of the sun is hot but you already knew that.

What happens as you move closer and closer to something hot? The temperature gets hotter. Duh. As you move further away, the temperature decreases. Another duh. Except that’s not the way it works on the sun.

The sun’s corona is the outer layer of it’s atmosphere. It extends millions of miles into space and it can only be seen during a full lunar eclipse like the one shown in the gif on the right. The bright ring around the blacked-out disc of the sun is the corona.

Here’s where things get interesting. The corona can’t be seen unless there’s a lunar eclipse because the photosphere is about a million times brighter. Remember that the temperature of the photosphere is about 6,000 °C.  The temperature of the corona, which is about 2,000 kilometers away from the photosphere, is about a million °C. The corona is much, much dimmer and much, much hotter than the surface of the sun.

What’s going on? The dimmer part is easy.  The corona is much less dense than the photosphere so there is much less material to emit light.  The hotter part presents a problem.  It’s called the coronal heating problem and, as it’s put on a Caltech website, it’s like trying to explain a flame coming out of an ice cube. Until recently a plasma wave theory and a magnetic field theory have been the main contenders to explain why the corona is so much hotter than the photosphere but neither appears adequate to the task.

Now there’s another theory to consider.

The image above is one of the first pictures of the sun taken by NASA’s Nuclear Spectroscopic Telescope Array or NuSTAR. NuSTAR, which launched in 2012, is an orbiting telescope that is sensitive to high-energy X-rays.  The picture shows an eruption of those X-rays above sunspots.

It has been hypothesized that the extreme heat of the corona might be explained by nanoflares which are small eruptions of charged particles and high-energy radiation.  The problem with testing this hypothesis is that there has been no way to detect whether or not these nanoflares exist.  Until now.  If nanoflares exist, NuSTAR should be able to detect them because the nanoflares are expected to emit high-energy X-rays.

An interesting side story here is that NuSTAR was designed and developed to gather information from black holes and other phenomena that are far removed from our galaxy, let alone our solar system.  The people who run the NuSTAR program had to be convinced that a tool made to look at what was very far away could also produce valuable results if used to look at something that was nearby. Sometimes thinking outside the box involves looking differently at something that is inside the box.

Here’s another picture of the sun which was taken on January 1, 2015 by the Atmospheric Imaging Assembly on NASA’s orbiting Solar Dynamics Observatory.

The large black area at the bottom is a hole in the corona where the magnetic field allows particles to escape into space instead of being trapped and returned to an area near the surface where they produce the golden glow seen over the rest of the image.  This has nothing to do with trying to solve the problem of why the corona is so much hotter than the photosphere but the picture is too cool to pass by.