Changing Sun

Our Sun has been doing the same thing for the past four-and-a-half billion years — steadily converting the hydrogen in its core to helium, releasing enormous amounts of energy in the process. And it’ll keep on doing that for billions of years more.

Over the eons, that slow, steady change produces major consequences: The Sun gets bigger and brighter as it ages. That trend has important consequences for Earth and the other planets.

As a star converts more and more hydrogen into helium, the core contracts, squeezing the core and making it hotter. That revs up the rate of nuclear reactions, which increases the amount of energy produced in the core. That energy pushes outward on the surrounding layers of gas, making the star expand. It also increases the total amount of energy the star radiates into space, making it brighter.

Models of stellar evolution say the newborn Sun was probably only about 70 percent as bright as it is now. But that creates some problems. Studies show that both Ea