The record for the hottest temperature ever achieved on Earth is about 2 billion kelvins, which works out to roughly 3.6 billion degrees Fahrenheit. The record was set by the Z Machine at Sandia National Laboratories, which uses about 20 million amps of electricity to vaporize tungsten wires into plasma, then uses a magnetic bottle to squeeze that plasma down to extremely high pressures, which spikes the temperature. For perspective, 2 billion kelvins is about an order of magnitude hotter than the hottest nuclear explosion ever created by humans, which is impressive. It’s also definitively hotter than the temperature of the sun — which is a deceptive phrase.

The most powerful laser ever built (which is another deceptive phrase, as more powerful lasers are built at an almost yearly rate) is the Texas Petawatt Laser at the University of Texas at Austin. In a typical experiment for the laser, it can heat a slug of aluminum to 10 million kelvins, which is also hotter than the sun — except when it’s not.

Back in 2005, scientists at the University of Illinois at Urbana-Champaign used high-frequency sound waves to rapidly collapse gas bubbles, creating temperatures of 20,000 kelvins. This was also hotter than the sun — except when it’s not.

Each of these experiments, and many more like them, garnered headlines because they billed themselves as generating temperatures “hotter than the sun.” This is important work, because extremely high temperature generation is a necessary component for nuclear fusion, the same process that powers the sun. There’s some intellectual symmetry here, which is part of the reason these labs’ public relations staffers used the sun comparison, but it should be taken with a grain of salt.

To paraphrase Obi-Wan Kenobi, the truth of “hotter than the sun” varies greatly depending on your point of view. Lots of natural, commonplace phenomena are also technically hotter than the sun. Lightning regularly produces temperatures hotter than parts of the sun. The Earth’s core is technically hotter than portions of the sun. But lightning and the Earth’s core don’t stand any realistic chance of generating nuclear fusion, so what gives?

The gotcha in the “hotter than the sun” mantra is that the sun exhibits wildly different temperatures depending on where you measure the heat. Moreover, the coldest part of the sun — and the area that is most often used for comparison — isn’t really that hot, despite what common sense would lead you to believe.

WHAT IS THE COLDEST PART OF THE SUN, AND HOW HOT DOES IT GET THERE?

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Perhaps surprisingly, the coldest part of our sun is the surface of our sun. While the hottest part of our local star — its core — can reach temperatures of over 13 million K, the surface is a mere 5,700 to 6,000 K. Moreover, the all but unseen corona of our sun that extends above its surface can get as hot as a million kelvins. Thus, the so-called “surface” which sits between the core and the corona is well over a hundred times cooler than the solar regions on either side of it.

Thus, when someone says they’ve generated a temperature “hotter than the surface of the sun” it is a still-impressive feat, but they are cherry-picking the coldest part of the star as their basis of comparison. That said, 6,000 K is still insanely hot. Hotter, in fact, than the melting point of the most thermally resistant metal known to man: Tungsten. Element W is so temperature-resistant that it’s used liberally in the alloys for rockets and spacecraft (and in crazy Z Machine plasma experiments) — mostly because Tungsten’s melting point is 3695 K. However, on the surface of the sun, Tungsten would not only melt, it would boil.

Back to the solar hotness comparisons: For absolute assuredness of honesty, only accept “hotter than the sun” labels from press releases that cite temperatures in excess of 13.6 million kelvins, which works out to about 24.5 million degrees Fahrenheit. Thus, the Z Machine’s 2 billion-degree spike is a sun-topper, while the Petawatt laser and the sonic bubble-crushers are mostly not.

Incidentally, the term “surface of the sun” is itself misleading, as what we call the surface is really just the edge of the visible, luminescent part of the sun, the photosphere. As the sun is just a hyper-hot ball of gas, it extends well beyond its visible edge. As to why the corona, which is generally only visible during total solar eclipses, is hotter than the edge of the photosphere, scientists are just as surprised and stumped as you are.

That’s not just a colossally counter-intuitive astrophysical question, it’s a hyper-hot helping of heliocentric Geek Trivia.

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