Move away from a campfire, and as one might expect, the temperature will quickly drop. Move away from the sun’s surface, and the temperature quickly and inexplicably climbs to around two million degrees. No one knows why.
But a coast-to-coast solar eclipse Aug. 21 will give scientists a golden opportunity to probe the roots of the mystery and, perhaps, find some answers.
“Combining ground-based eclipse results with the observations from satellites in the visible, ultraviolet, X-ray and radio parts of the spectrum will provide the most complete view of the solar atmosphere ever seen,” noted astronomer Jay Pasachoff of Williams College writes in the August issue of Scientific American magazine.
“Some might find it disconcerting that the sun, arguably the best studied of all celestial objects, is so incompletely understood. But I see the lingering questions as a wonderful excuse to share one of the greatest experiences in nature.”
Ninety-three million miles from Earth, the sun is enormous by any human standard with a diameter of 860,000 miles and the mass of 330,000 planet Earths. By volume, the sun could swallow 1.3 million Earths. The sun generates so much gravity that a 170-pound human would weigh about 4,600 pounds on the sun’s visible surface.
In the sun’s hellish core, which extends out to about a quarter of the sun’s diameter, the concentrated weight of the star creates enormous pressure and temperature — 27 million degrees Fahrenheit — enough to trigger nuclear fusion that consumes some 600 million tons of hydrogen per second.
It’s been doing that for the past five billion years, balancing the inward pull of gravity with the outward push of fusion energy in the core. And even though the sun is a relatively small, run-of-the-mill yellow star, it still has enough gas in the tank, so to speak, to burn for another five billion years.
But eventually, the core will run out of elements to fuse. At that point, gravity will take over and the core will collapse to become a compact, slowly cooling white dwarf. Along the way, the star’s outer layers will be blown away into space and Earth will be burnt to a cinder.
That’s the long-range forecast, based on theoretical analyses and decades of Earth- and space-based observations that have slowly peeled away many of the sun’s mysteries. And as it turns out, some of the most profound are not in the sun’s interior.
The core is thought to be surrounded by an opaque “radiative zone” where the outward torrent of high-energy photons generated in the core work their way up toward the surface, constantly being absorbed and randomly re-emitted during endless collisions in this dense region.
Particles are packed so tightly that it takes photons about 170,000 years to make their way across the radiative zone and into the upper convective zone. By this point, the temperature has dropped to about 3.6 million degrees, powering titanic bubbles of electrically charged plasma that roil and rise to the visible surface of the sun — the 10,000-degree photosphere.
As the name suggests, the photosphere is where light finally streams away into space in all directions, reaching Earth about eight minutes later. And this is where one might expect the temperature to steadily drop as the photons and charged particles are blasted away.
But somehow, during the very short trip from the photosphere, through a thin layer known as the chromosphere and into the sun’s outer atmosphere, or corona, the temperature suddenly shoots up to some two million degrees.