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The Sun’s core is the hottest part of the Sun.

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But our star’s temperature doesn’t behave as you might expect.

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The core is roughly 27 million degrees Fahrenheit

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and 10 times more dense than gold.

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As you move outward, the layers of the Sun become cooler and less dense.

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Something unusual, however, occurs when you reach the outermost layer.

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While the surface is around 10,000 degrees Fahrenheit,

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the corona – the Sun’s outer atmosphere — is several hundred times hotter.

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That’s the opposite of what happens with a fire, when it gets cooler the farther away you get.

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Scientists call this the coronal heating problem

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and evidence for this was first uncovered during an eclipse in the 1800s.

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The corona is usually hard to see -- it’s too dim to be seen next to the Sun’s bright body.

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But it can be seen with the naked eye,

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when the moon blocks the sun during a total solar eclipse.

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To understand how this mystery was discovered,

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it helps to know how scientists started studying the chemical properties of materials on Earth.

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In the early 1800s, instruments named spectroscopes

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were invented to identify materials that emit light when heated.

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Light enters the spectroscope and is filtered through a tiny hole to isolate a single area.

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It then bounces off a special grating that disperses light into its different wavelengths.

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While sunlight contains every wavelength,

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scientists discovered that every chemical element and compound

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contains a unique pattern of wavelengths

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that allows scientists to determine the composition of light sources.

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It wasn’t long before astronomers started extracting information from the light of distant stellar objects.

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In 1869, two scientists independently decided to point a spectroscope toward the corona during a total solar eclipse.

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As the Sun’s light disappeared, the pattern of wavelengths changed.

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They saw something they had never seen before.

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A bright green line that did not relate to any element found on Earth.

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For a short while, scientists named it ‘coronium’.

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It wasn’t until 70 years later that a Swedish scientist discovered

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that these lines were the result of elements such as iron being stripped of its electrons.

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Every element has a specific number of electrons surrounding the nucleus.

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As each electron is removed, more energy is needed to remove the next one.

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The green line shows that iron has been stripped of 13 of its 26 electrons –

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– indicating that the corona needed to be millions of degrees -- counterintuitively far hotter than the Sun’s surface.

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Scientists have since proposed a variety of theories

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for what mechanisms could be adding that extra heat into the atmosphere.

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One theory suggests that small waves in the Sun’s surface pushes particles and heat into the atmosphere

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- a bit like how ocean waves push surfers.

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Another theory suggests small bomb-like explosions from the realignment of the Sun’s magnetic field create heat.

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Many scientists think it may be a mix of both.

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We’ve studied the corona from Earth during many eclipses,

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but to solve our star’s biggest mystery we have to make direct observations from the region itself.

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