Science Focus by Colin Stuart 1/18/2026
We all know the sensation the Sun’s rays warming our faces. Its bountiful light and heat make life on our planet possible.
But our nearest star also provides us with what is literally the biggest mystery in the Solar System, spanning millions of miles. It is a puzzle so profound that, on the face of it, it challenges one of our most basic laws of physics.
Solving it may be the key to protecting our electronics and technology, not to mention our astronauts, from the worst effects of space weather: the gigantic storms of radiation and energy that occasionally burst from the Sun and collide with the planets, including Earth.
The mystery is that the Sun’s atmosphere, a gigantic gaseous envelope known as the corona, is hotter than the surface of the Sun itself, and not by a small amount. The Sun’s brilliant surface has a temperature of around 6000°C, whereas the extended corona soars to millions of degrees.
“It is very strange,” says solar physicist Dr Miho Janvier from the European Space Agency. “If you think about what we experience every day, the further you are from a source of heat, the colder it is. And so you would think the same thing would happen in terms of stars.”
But in the case of the Sun, it clearly does not. So, where does the extra energy come from that heats the corona?
The Sun’s nuclear heartBefore we get to the drama in the Sun’s outermost layer, we first need to understand what’s going on at its heart.
The Sun’s core is a natural nuclear fusion reactor of epic proportions, liberating the energy that powers the star.
Although comprising just three per cent of the Sun’s volume, the core contains more than one third of its mass. Gravity crushes it together, raising the temperature to a staggering 15 million°C (27 million°F).
At this temperature and density, atoms of hydrogen gas fuse together to become helium, releasing energy in the process. This diffuses through the rest of the Sun, heating it as it goes.
The further a layer is from the core, the cooler it is, dropping to ‘just’ 6,000°C (10,800°C) at the surface. Above this is a layer of the Sun’s atmosphere called the chromosphere, which extends to an altitude of around 3,000-5,000 kilometres (1,860-3,100 miles).
Initially, it too plays nice; the temperature drops to less than 4,000°C (7,200°F) as expected, but then something changes, and the temperature shoots up.
At the top of the chromosphere, the temperature rises to 35,000°C (63,000°F).
Moving through a transitional layer and into the tenuous corona beyond, the temperature skyrockets to more than one million degrees.
Clearly, there is an invisible energy source at play – but what?
More:
https://www.sciencefocus.com/space/solar-system-biggest-mystery
