An Ancient Universe: How Astronomers Know the Vast Scale of Cosmic Time

THE ANCIENT UNIVERSE

b) The Age of the Oldest Stars

The Sun and other stars shine by converting superheated hydrogen in their centers into helium in a process called thermonuclear fusion. Under the intense heat and pressure in a star's core, hydrogen nuclei fuse together and produce helium nuclei - and energy. This is the same process that occurs in the hydrogen bomb on Earth. We can determine how long a star can shine by this process as follows: we know how much energy comes from fusing each atom of hydrogen, the amount of hot hydrogen in the star's core, and how fast the star is using its energy. We can therefore calculate how long it will last before it runs out of fuel. The answer for the Sun is about 10 billion years for its total lifetime. We know from measurements of the age of the solar system - see below - that the Sun is now about 4.5 billion years old. So our star is about halfway through its life.

Other stars may have different lifetimes. Stars smaller (less massive) than the Sun have longer lives because they fuse their hydrogen fuel so much more slowly. Similarly, a sub-compact car may have a smaller gas tank than a large SUV, but it may be able to drive much longer on a full tank of gas, because it uses its fuel much more slowly.

When a star has used up the available hydrogen fuel in its center, it expands and becomes a "red giant". Once we have found such a giant star, we know that it has used up all its hydrogen. If we can estimate its initial mass, and hence its initial power, we can estimate its lifetime, and we therefore know its age. This is equivalent to saying that, if we see a car that has just run out of gas, and if we know its horsepower, fuel efficiency, and fuel capacity, we can figure out how long it had been driving since the last fill-up before it ran out of gas.

In this way, we can measure the ages of certain stars. When we apply this method to the oldest stars we can find, we obtain ages of 10 - 15 billion years.

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