How much time do we have before the sun burns out?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

15 September 2024

“How can scientists possibly know when the sun will die?”. This simple question came from a primary school child recently at session where I was invited to give a science talk. But looking at the faces of the teachers, it was clear that they too wanted to know but didn’t have the courage to ask. Thank God for children; they are fearless!

There are four things that are required in order to work out the answer: firs, the mass of the sun, second, its composition, third, the process by which it generates energy and fourth, the rate at which it emits energy. Thankfully, all these can be determined without having to go to the sun!

Using Newton’s laws of motion and gravity, we can calculate the mass of the earth – it’s about 6,000 billion-billion metric tonnes. With this value and knowledge of how the planet moves round the sun (once every 365.25 days), the mass of the sun can be calculated. It comes to about 2 billion-billion-billion tonnes.

In other words, the sun is about 300,000 times the mass of the earth. This value should not be confused with the approximately 1.4 million times for the volume. Mass and volume are two totally different and separate quantities. We see is that, being a hard, rocky ball, the earth is 5 times as dense as the sun which is a soft, gaseous ball.

Next, we need to find out the chemical composition of the sun. For this, we study the nature of the radiation that it emits. Every element and every compound has a unique distribution of frequencies of energy it radiates – the so-called ‘emission spectrum’. From this, it turns out that the sun is composed mainly of Hydrogen (73 per cent) and Helium (25pc).

The emission spectrum also reveals that the sun makes energy through a fusion nuclear process where hydrogen is converted into helium. All that remains now is to know how much energy it emits every second. That will tell us the rate of consumption of hydrogen and, therefore, how long it will last.

The rate at which the energy reaches the earth per unit area (square metre) can be easily measured (it’s about 1,000 watts). Next, we multiply this by the surface area of a sphere that encloses the entire orbit of the earth. The result is about 400 trillion-trillion-trillion megawatts. At that rate, it will take about 5 billion years to exhaust the hydrogen-fuel. But it won’t just go off like a stove running out of paraffin!