How hot is the Sun?

Find out about the internal structures of our host star and their different temperatures.

By Amy Arthur

Published: Monday, 27 February 2023 at 12:00 am

The hottest part of our Sun is its core, where nuclear fusion reactions create temperatures up to 15 million degrees Celsius (ºC) or 27 million degrees Fahrenheit (ºF), according to NASA.

Here, 500,000 kilometres below the surface, immense pressure fuses hydrogen molecules together to make helium, releasing energy as heat and light in the process.

This heat radiates out from the core, through the 700,000 km of the Sun’s radius.

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A NASA diagram showing the internal workings of the Sun. Credit: NASA/Goddard

The region that encircles the core, called the radiative zone, gets its heat from the core, and is around 7 million ºC at its closest point to the centre and around 2 million ºC in its outermost point.

Surrounding the radiative zone is the convection zone, which quickly loses heat as it is carried away from the core to the surface of the Sun.

The temperature drops over this region from 2 million ºC to just 5,800ºC at the Sun’s surface.

Called the photosphere, the surface is the part of the Sun we see from Earth and it is actually the coolest region, relatively speaking.

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Coronal mass ejections captured by NASA’s Solar Dynamic Observatory in June 2013. Credit: NASA/SDO

The Sun’s corona is hotter than its surface

Interestingly, the atmosphere around the Sun, called the corona, is hotter than its surface.

Scientists don’t yet know why the corona can reach temperatures of 1 million ºC, though the leading hypothesis is that energy travels via the Sun’s magnetic field out into the atmosphere where it ‘explodes’ as a burst of heat.

We get ever closer to answering this question, among others, thanks to the NASA Parker Solar Probe passing through the corona.