WORLD OF FIGURES: How many years remain before sun runs out of fuel?

Alex Nduva is a worried man. He writes: “According to scientists the sun is described as a star and consists of burning gases, mostly hydrogen. It is said to have been in existence for the last millions of years. My question to you is how many more years are these burning gases expected to last? I ask this because I normally spend a lot of money in replenishing my cooking gas every other day as it gets exhausted.”

First of all, as a scientist, I must come out clearly and say that we have been accused of saying very many things that we never said.

No scientist has ever said that the sun consists of burning gases! Those who say that are not scientists.

Burning is a chemical reaction where an element or a compound mixes with oxygen. There is no oxygen in the sun and so nothing can burn there!

So what happens in the sun? It is a nuclear process: the fusing of hydrogen atoms to form helium. One of the by-products of this process is a large amount of heat and light. But only a very small portion of that energy gets to earth.

We can easily measure the amount of energy arriving on every square metre area on earth. The answer is about 1,000 joules per second, that is, 1,000watts or 1 kilowatt (1kW).

Now; we know that we are about 150 million kilometres from the sun. So, we can visualise its energy radiating evenly on the surface of a sphere with a radius of 150million km.

The area of that surface can be calculated (in square metres) and then multiplied by the 1,000W per square metre. The result will be the total amount of energy emitted by the sun every second.

The answer comes to 280 trillion-trillion watts! That is, the number 280 followed by 24 zeros. Take a short break and let that sink…

Using Newton’s laws of motion in conjunction with his law of gravity, any form 4 physics student should be able to calculate the mass of the sun (Those who can’t should ask for a school fees refund!).

 The answer comes to about 2 trillion-trillion-million kilograms. That is, the number 2 followed by 30 zeroes. Let that one sink as well…

Scientists know the amount of energy generated when one kilogram of hydrogen fuses to form helium. Knowing that figure as well as the total energy emitted by the sun per second, it is easy to calculate how many kilograms of hydrogen are consumed in the sun every second – you simply divide the two numbers.

Finally, scientists also know the total mass of hydrogen remaining in the sun. Therefore, they can work out how long it will last before it is exhausted. Another simple division of the total amount remaining by the consumption rate. The answer comes to about 4.5 billion years. In other words, there is no need for Alex to be worried!

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