Is the Sun losing weight?

Sunlight has to come from somewhere, and the somewhere is the Sun's own mass. Einstein's equation says a little mass makes an enormous amount of energy, and the Sun is the working demonstration. Every second about 600 million tons of hydrogen is fused into about 596 million tons of helium. The missing four million tons is what warms your face.

Four million tons a second sounds like a crisis, and over a day it comes to around 350 billion tons. But the Sun weighs about two billion billion billion tons. Across its entire ten-billion-year life it will convert well under a tenth of one percent of itself into light. It is a very large candle, burning very slowly.

There is one more dieting trick: the solar wind, a thin stream of particles blowing off the Sun's surface, carries away roughly another million and a half tons a second. Add it up and the Sun is lighter by about five or six million tons every time a clock ticks. You would never notice. It would take longer than the age of the universe to make a visible dent.