Notes

If the sun uses 600 million tons of hydrogen atoms to make helium every second during the nuclear fusion process why is the sun still able to do this for millions of years?

Answer:OK. We went to the web and collected a few figures. Now we'll work through the
numbers and see what we come up with for a ballpark estimated lifetime for the sun.
The end result will be to flabbergast all of us at the vastness of the numbers, and
convince us again how unimaginable the numbers are.

Here's the data we got from the web:
-- Mass of the sun = 2 x 1030 kilograms
-- Fraction of the sun that is hydrogen = 3/4 (0.75, 75%).
-- Rate of consuming hydrogen = 620 million metric tons per second

Now we'll start massaging numbers:

Total mass = 2 x 1030 kg
Hydrogen mass = 0.75 of the total = 1.5 x 1030 kg

1 metric ton = 1,000 kg
Rate of consumption per second = 620 million metric tons = 620 x 109 kg

Number of seconds to consume the available hydrogen at the present rate:

(1.5 x 1030) / (620 x 109) = 2.419 x 1018 seconds

Boiling that number down, it's equal to:

2.8 x 1013 days

7.667 x 1010 years, or 76 billion years

This life expectancy is more than the prediction we often hear. But it's just some quick
scratching on the back-of-the-napkin which ... to mention just one likely inaccuracy ...
assumes that the rate of consumption will never change until the last hydrogen atom
is gone.

In any case, it may be wrong by a factor of 1,000 in either direction, and it still
guarantees that the sun is not going to flicker out at any time within our lifetime
or that of our 100-times-great grandchildren.