Saturday, April 17, 2010

That savage of fire

Some of you will have seen this photograph already and know what it is. If not, try to guess. You can click the photo to enlarge.

Okay, here's a hint. The white bar at lower left represents 5000 kilometers (3000 miles). That is to say, a couple of the seedlike granules in the photograph would span the United States.

Let me make it more vivid. Here is the same pic on which I've superimposed an Earth to scale.

By now you have probably guessed. We are looking at a high-resolution image of the surface of the Sun, a recent APOD (Astronomy Picture of the Day).

If you could put the pic in motion, you'd see that each of the granules is a bubbling upwelling of hot plasma from below, bringing energy from the Sun's core to the surface, like the churning surface of boiling water in a pan on the stove.

The temperature of the Sun's surface is about 6000 degrees Centigrade. At the core the temperature is more like 10 million degrees C, hot enough for nuclei of hydrogen (protons) to fuse together to form helium nuclei, turning mass into energy in the process.

Every second the Sun converts roughly 700 million tons of hydrogen into helium. And the helium weighs less than the original hydrogen. Five million tons less. Matter has disappeared. Matter has been turned into pure energy. The famous Einstein equation -- energy equals mass times the speed of light squared. Every second the Sun turns five million tons of its own substance into radiant energy.

In summer, about a millionth of an ounce of the Sun's depleted mass falls each second onto my college campus. In winter less than half as much. A fraction of a millionth of an ounce per second is all it takes to ignite the felicities of spring.

The Sun never misses so little of its mass; it will go on burning for more billions of years. But for us, that smidgen of star stuff turned into sunshine is the difference between the snows of March and the green explosion of May.

(For photo credit, see APOD link above.)