Physical Cosmology

How old is the sun?
Answered by Meredith Bower and Science Channel

• ### Science Channel

1. Astrophysicists believe with near certainty that the sun, now in its middle age at 4.6 billion years (give or take a few), will die in another 5 billion years or so. At that point, our source of heat and light will have insufficient energy to continue burning like it is today. Since its infancy, the sun has not only increased in size, it has also increased in intensity, or luminosity. But being middle-aged, the sun is starting to change. The sun runs on hydrogen fuel, and each day it loses a little bit at its core. Scientists believe 0.7 percent of the sun's mass has disappeared, but it's not a bad thing. That mass is actually energy, and we know it as sunshine.

Since no one was around for the birth of the sun, how have scientists determined its age? Surely there must be some complex formula. Actually, calculating the age of the sun, once you have some measurements, such as the distance to the Earth and an understanding of the nuclear physics that power Earth's great star, only involves applying a version of Einstein's formula, E=Mc2. In other words, the sun's age (E) is determined by its total energy (0.007, corresponding to the amount of the sun's mass that has disappeared) divided by the amount of the sun's mass that can undergo nuclear reactions (M) and the speed of light (c) [source: Cornell]. Calculating the sun's remaining lifespan is a bit more complex, but astrophysicists have narrowed it to about 5 billion years, for a total lifetime of around 10 billion years.

Scientists have also determined the age of the sun using radiometric dating of meteorites found here on Earth. By studying the decay rate of uranium as it turns into lead, or the uranium-to-lead ratio, they have calculated the age of the sun. This, of course, assumes the universe is a result of the Big Bang and that everything was created instantaneously.

Looking back on when the universe was created and when humans came on the scene, then looking ahead toward the demise of the sun some 4 to 5 billion years into the future, it's safe to say we probably won't be around to witness the end of our star.

(NASA)

2. Stars have cycles of birth, childhood, middle age, old age and death, much like people. The difference, of course, is in the number of years involved. The sun is roughly 4.5 billion years old now and will keep increasing in temperature, brightness and size in the ages ahead. The sun's radius has grown by 6 percent in its lifetime, and it has enough hydrogen fuel to last it another 5 billion years, which puts it just short of middle age.

When the sun's core runs out of hydrogen fuel, it will really feel the weight of its own gravity. The core will contract, heating the upper layers just as hydrogen fusion begins to occur there. As the outer layers expand, the sun will swell into a red giant with a radius 100 times that of its current one. Eventually, the core will become hot enough to cause the helium to fuse into carbon. When the helium runs out, the core will expand and cool.

Although a black hole is the end result of a massive star that has died, the likelihood of our sun collapsing into a black hole is virtually nil. This is because the sun's core isn't massive enough to transform into a black hole should it collapse. It's believed instead that when the sun forms into a red giant, the massive star will consume Earth, along with Mercury and Venus. Millions of years later, the sun will run out of gas, becoming a white dwarf; as it continues to cool, it will turn into a black dwarf.

In theory, if the sun did collapse into a black hole, the planets would likely continue to orbit without much change because the black hole would have the same gravity the sun had. Also, the core of the black hole would continue to rotate just as the sun had rotated. Conditions on Earth, however, would change drastically if the sun became a black hole. A black hole doesn't emit light, so the Earth would be engulfed in darkness and extreme cold. The oceans would freeze over and life on Earth as we know it would come to an end. Humans could possibly save themselves by going underground and developing ways to generate heat.

Determining exactly what will happen to the sun is not much easier than determining its precise age. Calculating exact proportions of hydrogen to helium and time to nuclear fusion can only be done at the sun's core, so scientists have to rely on estimates made from the sun's spectra.

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