E H McGregor of Dunedin asks :-
I recall reading that the sun radiates energy by converting hydrogen into helium at the rate of 600,000 tonnes a second. The sun is billions of years old so how could it use up hydrogen at that rate? Does it accumulate hydrogen from space?
Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.
The question isn't quite correct. The sun actually converts around 667.5 million tonnes of hydrogen into 663 million tonnes of helium every second. The missing 4.5 million tonnes has been converted to energy.
The tonnages involved sound enormous but, compared to the sun's mass, they are tiny. The sun is very big. It has a mass of 2 000 000 000 000 000 000 000 000 000 tonnes, or 2 times 10 to the power of 27. So 667.5 million tonnes is a tiny fraction of the sun's mass. If the sun could convert all of its hydrogen into helium then it would have enough fuel for about 100 billion years, 'burning' at the current rate.
For complicated reasons the hydrogen available for 'burning' is just that confined to a core region of the sun. That gives the sun energy to shine for about 11 billion years, more or less as we see it now. Since it has already been shining for 4.5 billion years it has another seven billion years left. At the end of that time the core will squeeze down and heat up. Then it will convert helium into heavier elements for a few hundred million years more. During that time the outer layers of the sun will expand way from the hot core, turning the sun into a red-giant star. It probably won't engulf the earth but it will turn our planet into a red-hot rock.
It is quite easy to verify the sun's energy output. At earth sunlight gives 1.4 kilowatts per square metre. From that we can calculate that the sun is producing around 380 billion billion watts of power. Then, using Einstein's famous formula that energy equals mass times the speed of light squared, one can find that the mass annihilated must be around 4.5 million tonnes per second.