Why is the sun so hot?

What fuels the sun? Image: NASA

What fuels the sun? Image: NASA

As you roast – or grill – on a beach this summer, you may be thinking the sun is hotter than usual. But of course, the sun is just as hot on a chilly winter evening; it doesn’t change. What changes is the heat received per square yard of ground, which depends on cloudiness and how high the sun is in the sky, which depends on your latitude, the season and the time of day.

When the sun is high at noon these summer days, the ground gets hot, and that heats up the air, and you swelter. The hot ground radiates heat back up, and on a clear day the heat would go into space, if carbon dioxide and water vapor in the atmosphere did not absorb some of it. Then the oceans get a bit warmer, making more water vapor. It’s complicated, but Earth does not have a thermostat like your air-conditioned room, and when it gets hot, “positive feedback” can make it hotter still. Seriously.

Back to my question: what makes the sun hot? In the 19th century, scientists had a puzzle. Geological and fossil evidence told them that Earth is at least hundreds of millions of years old. If the sun’s heat came from burning coal, gasoline or anything known, it would have burnt out long ago. There must be some other much more powerful fuel.

The first clue came when Marie Curie – girls note: a woman who won two Nobel Prizes – discovered the element radium. It feels warm, it is radioactive, and nuclear energy is released when a heavy element breaks into pieces. That is nuclear fission, which led to atom bombs and nuclear reactors.

Not long after Curie’s discovery, Albert Einstein’s theory of relativity said that mass is a form of concentrated energy. Two nuclei of deuterium, or heavy hydrogen, can fuse together and make a nucleus of helium. A little mass is transformed into a lot of energy. Remember that E = mc2, and c2 is huge.

This explained how the sun could keep shining for billions of years, and so Earth could be that old. It all fits together.

Having (unfortunately) made a hydrogen bomb releasing fusion energy, we are trying to put the genie back in the bottle and make a controlled fusion reactor. It is very difficult. Why bother? We already have one, steady, free and at the safe distance of about 93 million miles.