North Hawaii News Articles from CFHT

How old is that star? How do we know?

If you have been reading these articles, or other articles about astronomy, you will have seen statements like, "that star is about 10 billion years old", or "that is a young star, only 5 million years old". Such incredible ages might inspire awe at the scale and grandure of the Universe and the amazing brevity of human existence in comparison. After all, the entire history of human civilization (being generous) spans only 10,000 years - one part in five hundred of the 'young' star and only one millionth of the age of the old star!

These ages are so beyond our experience, you might ask yourself, 'how do astronomers know these ages?' How can we measure the age of a star that lives thousands or millions of times longer than all of human history? This is especially amazing, given that we can't touch the stars or collect samples to study in detail.

As in all fields of science, our understanding of the ages of stars has developed through the interplay of theory and experiment. Astronomy is unique in that we can never perform direct experiments since it is impossible to affect the stars. Even if we could, the experiment would certainly be rejected by the EPA -- just imagine the environmental impact of an experiment on the nearest star, our Sun! Instead, astronomers must be content to observe stars and learn what they can just by watching.

The first big breakthrough in understanding star ages came in understanding how the Sun is powered. In the 19th century, the best suggestion was that the Sun was heated by the pressure generated by its own weight. This process can only supply so much energy, leading to a lifetime for the Sun of only 6 million years. By the beginning of the 20th century, geologists had shown that the Earth was much older than that, more like 5 billion years. Astronomers had to find a better answer, and one eventually came from new discoveries in physics. When physicists discovered the process of hydrogen fusion reactions, which power hydrogen bombs, astronomers realized this was probably the source of power for the Sun. Since that breakthrough in the late 1930s, there has been steadily more and more confirming evidence, including the chemical composition of the Sun, the details of solar oscillations, the consistency of the age of the Earth and the Sun, the detection of neutrinos produced in the hydrogen fusion reactions, and so forth.

The Sun presents a relatively easy case, however. With the Sun, we have the constraint of the Earth, which we can touch. We can measure the age of the Earth in many ways to confirm the age of the Sun. In order to measure the age of other stars, we need to understand in great detail how the stars develop as they age. For this, astronomers turn to computer models.

With a computer model, you can build a star and watch its billion-year lifetime in just a few hours or days. You have the luxury of generating stars of any mass or chemical composition you like, or changing whatever other variable you think might be important. You can see what happens to the star throughout its lifetime, not just on the outside, but also the details on the inside. But computer models are built by humans, and are only as accurate as our understanding of the pieces used to assemble them. We come back to the same question: How can we check the accuracy of the models in our lifetime?

The answer comes from clusters of stars. Throughout our galaxy, there are many clusters of stars that formed at the same time and place. Since these cluster contain many stars of just about the same age, comparing clusters of different ages tells how different stars change as they age. What we have learned is that the most important quality of a star governing its aging process is its mass: the heavier the star, the quicker it ages and the quicker it will die.

We can compare the properties, such as the brightness and temperature, of stars in a cluster to the predictions from our computer models. The comparison tells us that our models work, and it also tells us the ages of the stars in the cluster. Such a comparison can also tell us when our models are incomplete or inaccurate. For example, for many years there were small discrepancies between the models and measurements for certain predicted quantities for old star clusters. In the 1970s, when the measurements were still fairly inaccurate, the models seemed to be good enough. As the measurements improved, it became clear that there was an important discrepancy. Our models were close, but our measurements were now accurate enough that the error was noticeable. Astronomers spent years discussing possible reasons for the errors: details of the structure of the stars, the exact chemical composition of the stars, the effect of the rotation of the stars. All of these aspects had some impact on the predictions, but even together they could not completely fix the problems.

It turns out that the computer models depend on how different chemical elements absorb light. Every element absorbs light in different ways, and the older models used a rough approximation for these details. Over the course of many years, several groups of physicists worked very hard to measure all of the details of this light absorption in all possible chemical elements. The new measurements (of millions of absorption 'lines') were not hugely different from the approximations that had been used, but they were different enough to fix the discrepancies. And like many areas of science, the solution to one problem was also the solution to several problems: the new measurements cleared up discrepancies with several peculiar types of stars.

Our understanding of how stars age has improved dramatically throughout the 20th century, from a vague hypothesis which had obvious flaws, to a detailed set of models that successfully predict a huge variety of observable properties of stars ranging from our Sun to ancient, distant stellar clusters. Hundreds of astronomers and physicists around the world over many years have provided bits and pieces of the solution. So, when you read about a star that is 100 million years old, you can be impressed at more than just the amazing age. You can now appreciate the hard work and cooperative effort and that has gone into producing that number.

Eugene Magnier