How Old Is the Universe?

If you want to know how old someone is, you could probably just ask them. But what if they could not, or would not, tell you? You could probably figure out a couple of different ways to estimate their age. For example, you could use the average age of their classmates as an estimate. Or, if you knew they had a younger brother who was 10 and an older sister who was 13 you could conclude that their age was between 10 and 13 years old. Similarly, astronomers can use a number of methods to estimate the age of the universe.

One method of figuring out the minimum age of the universe is to calculate the ages of some of the oldest stars in the universe. To understand the merits of this method, you need to know something about the history of the universe. Data suggests that the universe was born with an event known as the Big Bang; in a single moment all the heat, matter, and energy that makes up the universe exploded into being. Since that moment, the universe has continued to evolve—the temperatures have cooled, and some of the matter has redistributed and formed stars, planets, and other celestial bodies. This means that even the oldest stars were formed after the universe was first created. So, if you figure out the age of the oldest stars, you know the minimum age of the universe. Just like you know, from the example above, that the person's age is, at minimum, 10 years old since their brother, who is younger, is already 10 years old.

Astronomers have discovered that some of the oldest stars are often found in globular clusters. Globular clusters are tightly bound groups of stars. In fact, as you can see by looking at the M80 globular cluster in Figure 1, there are so many stars in one place that in the center of the cluster it can be hard to distinguish one star from another! The stars within a globular cluster are all approximately the same age because they all developed at the same time from the same gas cloud. This is similar to a litter of puppies— because they all developed at the same time from the same mother, they’re the same approximate age. But just like the puppies in a litter can be very different from one another in looks and behavior, so can the stars in a globular cluster. In fact, a globular cluster can have many different types of stars. To understand more about star types, you first need to know a little bit about the evolution of stars.