How Hot is the Sun?

by Fraser Cain on July 22, 2013

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The Sun is hot, really hot. How hot hot really is, depends on which part you’re talking about:

The sun has a core, a middle, a surface, and an atmosphere.

Starting from the inside out…

Cutaway to the Interior of the Sun. Credit: NASA

Cutaway to the Interior of the Sun. Credit: NASA

There’s the core, where the pressure and temperature are so great that atoms of hydrogen are fused into helium. Every second, 600 million tons of material go through this conversion, releasing vast amounts of gamma radiation. This is the hottest natural place in the Solar System, reaching temperatures of 15 million degrees Celsius. Photons generated at the core of the Sun are emitted and absorbed countless times over thousands of years on their journey to reach the surface.

Outside the core is the radiative zone. Here, temperatures dip down to where fusion reactions can no longer occur, ranging from 7 million down to 2 million degrees Celsius.

Next on our journey outwards from the centre of the Sun, is the convective zone, where bubbles of plasma carry the heat to the surface like a giant lava lamp. Temperatures at the bottom of the convective zone are 2 million degrees.

Finally, the surface, the part of the star that we can see. This is where the temperature is a relatively cool 5,500 degrees Celsius.

Here’s the strange part, as you move further away from the Sun into its atmosphere, the temperature rises again. Above the surface is the chromosphere, where temperatures rise back up to 20,000 degrees Celsius.

Solar CoronaThen there is the corona, the Sun’s outer atmosphere. The corona as a wispy halo around the Sun, visible during eclipses, that stretches millions of kilometres out into space. In the corona, the gases from the Sun are superheated to more than a million degrees – some parts of can even rise to 10 million degrees Celsius.

How can the atmosphere of the Sun get hotter than regions inside it? Astronomers aren’t really sure, but there are two competing theories. It’s possible that waves of energy are released from the surface of the Sun, sending their energy high into the solar atmosphere. Or perhaps the Sun’s magnetic field releases energy into the corona as currents collapse and reconnect.

There are space missions in the works right now to help answer this baffling mystery, so we might have an answer soon.

Stars can get much hotter or colder than our Sun. From the coldest, dimmest red dwarf stars to the hottest blue giants; it’s an amazing Universe out there.

References:
Solar Probe Plus Mission
Solar Orbiter Mission

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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