Sonia Cheveresan, age 12, of Mundelein, Illinois, for her question:
How hot is the sun?
When winter winds whistle through December, we tend to wish that the sun were hotter. But cheer up, July will soon roll around again and we shall be wishing that the seething sun would cool down. Actually, these seasonal changes have nothing at all to do with the sun's temperature. This remains more or less the same all the time. It merely seems to change here on earth because our planet tilts on its axis as it orbits around the sun.
Taking the sun's temperature seems like an impossible task until you remember that burning substances tend to change color as they grow hotter. We all know that white hot is hotter than red hot. Scientists have worked out a ratio between heat and color in fine detail. They have special instruments to measure and calculate the sun's temperature from its blazing colors.
Its surface temperature is estimated to be about 6000 degrees centigrade. This is hot enough to vaporize every solid and liquid substance on our cool planet. But as stars go, the sun's fiery temperature is merely medium. Certain blue giants are 11,000 degrees centigrade, or almost twice as hot as the sun. And some of the cool red giants have surface temperatures of only 3,000 degrees centigrade.
Our sun, of course, is an enormous nuclear furnace and its nuclear activity goes on in the deep, dense gases of its fiery core. Scientists are able to compute the core's density and also to estimate how hot it must be to carry on its continuous nuclear fusion. They tell us that the temperature of the sun's fiery heart must be at least 20,000,000 degrees centigrade and maybe even hotter.
Above its seething surface, the sun has an enormous atmosphere of thinner gases. This is the corona. During a total solar eclipse, the dazzling brilliance of the surface is blotted out by the dark disk of the moon. Then the corona becomes visible, like a pale pearly halo. One would expect this thin solar atmosphere to be cool, or at least cooler than the fiery surface.
This is what everybody thought until man made space probes went aloft to take its temperature. Then it was discovered that the sun's corona is as hot as 1,000,000 degrees centigrade. One would expect the nuclear core to heat up the surface, but this does not explain why the thin gases, thousands of miles above the surface, are so much hotter. One theory suggests that the corona may be heated by noise by the thunderous activity on the surface.
The sun's surface is constantly in raging turmoil. Hence, its average temperature tends to have variations. For example, it is prone to magnetic storms called sunspots. These appear as dark blotches on the dazzling disk. They seem less bright because the storm centers are cooler than the surrounding surface sometimes more than 1,000 degrees centigrade cooler.