The Sun is roughly middle-aged; it has not changed dramatically for more than four billion
[a] years, and will remain fairly stable for more than another five billion years. It currently
fuses about 600 million tons of
hydrogen into
helium every second,
converting 4 million tons of matter into energy every second as a result. This energy, which can take between 10,000 and 170,000 years to escape from its core, is the source of the Sun's light and heat. In about 5 billion years, when
hydrogen fusion in its core has diminished to the point at which the Sun is no longer in
hydrostatic equilibrium, the core of the Sun will experience a marked increase in density and temperature while its outer layers expand to eventually become a
red giant.