If sun gets its heat from burning, could only last 10,000 years or so. Best That 19th century astronomers could guess was that sun was slowly contracting. We now believe sun gets its heat from nuclear reactions. The Geologists were right after all. For My Team Absolute Ages Use Radioactive Decay to Date Rocks.
Radioactive atoms give off particles and change into different types of atoms.
Radioactive atoms decay at a certain rate. They have half lives . After each half life, half of the atoms will have decayed. An hour later you flip the pennies that came up tails and put the heads aside Each hour, you repeat the process. You have a large bowl full of jelly beans.