"Astronomers usually cannot tell the age of an individual star. There are certain stars that we know are very young, and others that are very old, but for most stars we cannot tell. When we have a large group of stars, however, we can tell its age. This is possible because all of the stars in a cluster are presumed to have begun their life at approximately the same time. After a relatively brief time (in 'star time,' that is--we are talking thousands to millions of years here) stars reach the adult phase of their life, which we call the main sequence phase. The length of time a star spends in the main sequence phase depends on its mass.
"Constructing a plot, called the HR diagram, of the stars in the cluster, scientists can determine the mass of the stars that are just ending this phase and moving on to the next phase of their life, the red giant phase. Computer models allow us to predict how old a star of that mass must be to be at that juncture of its life, and hence to estimate the age of the cluster. Recently, this procedure has come under close scrutiny because that age it gives for the oldest star clusters in our Milky Way seems to be older than the age of the universe derived from the most recent Hubble Space Telescope data."
Peter B. Stetson, senior research officer at the Dominion Astrophysical Observatory in Victoria, British Columbia, provides a more detailed reply:
"It is impossible to determine the age of a single star all by itself. The only real means we have to determine stellar ages is through the study of star clusters. In our galaxy, the Milky Way, there are two basic types of star cluster. Clusters of the first type are called 'globular clusters' because they appear as huge, round globs containing anywhere from a few thousand to a few million stars. Globular clusters are very old, and they are scattered around (not just within) the Milky Way; these clusters seem to have originated near the time our galaxy started to form, when the universe was quite young. Clusters of the second type used to be called 'galactic clusters' because we see them inside the body of our galaxy, but now it is more common to refer to them as 'open clusters' because they are much looser and their stars more spread out on the sky than are those in globular clusters. Open clusters can contain anywhere from a few dozen to a few thousand stars, and they come in a wide range of ages. Apparently our galaxy started making open clusters soon after it settled down to its present size and continues making them even today.
"The stars in either type of star cluster were all formed at the same time and out of the same material. The essential feature of a star cluster that lets us estimate its age is that each cluster contains stars with a range of masses. When a cluster is born, it will contain many stars of about the same size and mass as our sun, but there will also be numerous stars more massive than our sun and many other stars less massive than our sun. For about 90 percent of its lifetime, a star shines because nuclear reactions are converting hydrogen to helium in the star's center, releasing vast amounts of energy. This energy works its way from the center of the star to the surface and escapes the star in the form of light. The more massive a star is, the bigger the furnace in the center, and the brighter and the hotter the star is in this stable stage of its life. The most massive stars are very bright and blue-hot; a less massive star is somewhat fainter and white-hot; a star like our sun is a bit fainter still and is yellow-hot; and the least massive stars are very faint and merely red-hot. During this period of its life, a star hardly changes either in brightness or in temperature.
"The duration of the stable, or 'main sequence,' phase depends on a star's mass. A star 10 times as massive as the sun contains, clearly, 10 times as much fuel. It consumes that fuel roughly 10,000 times faster than the sun, however. As a result, it has a total lifetime 1,000 times shorter than that of our sun. When the hydrogen fuel in the center of a massive star is exhausted--'the center' representing about 10 percent of the star's total mass--it becomes increasingly unstable. The star remains bright, but it quickly switches from being comparatively small and hot to being huge and red for a while, then it briefly becomes smaller and bluer, then even larger and even redder, and finally explodes as a supernova, spewing its nuclear ashes as well as its unburned fuel back into space. Similarly, a star five times more massive than the sun has a lifetime roughly 100 times shorter than the sun before it becomes unstable and ends its active life. A star like our sun is calculated to have a total stable life-span of around 10 billion years; the sun is now a bit less than half that age (this age is very accurately determined from radioactive elements in meteorites), so we have another five billion years or so before we have to start looking for a new home.
"In the case of a single star, its brightness and temperature don't tell us much. Because these properties stay fairly constant for 90 percent of its lifetime, the star could be fairly young or fairly old, and we wouldn't be able to tell the difference. In a star cluster, we have the advantage that stars of all masses formed at about the same time. So all we have to do is look at the cluster and determine how hot and how massive is the hottest, bluest, most massive star that has not yet entered the late, unstable period of its life. The star's mass tells us how much fuel the star had when it was born, and the star's brightness tells us how fast it is burning that fuel. We know that the star is just about to start becoming unstable--after all, the stars that are more massive have already started to become unstable. We also know that its fuel is just about exhausted. The ratio of how much fuel the star had in the beginning to how fast it has been burning that fuel tells us how long the star has been alive. (By analogy, if we know how much kerosene our hurricane lamp contained when we lit it and how fast it consumes the kerosene, and if the lamp is just now starting to go out, then we can deduce how long it has been lit.) Because all the stars in the cluster are the same age, the age of that one star tells us the age of the entire cluster.
"The basic physics of how hydrogen is converted to helium in the centers of stars and the amount of energy generated by this process is comparatively simple and well understood. For much of the 20th century, the main limitation to our knowledge of stellar ages has been due to the difficulty of measuring the distances to the clusters--especially the distances to the oldest clusters, the globulars, which are comparatively far away. (We know how bright a star looks, but to know how bright it really is, you have to know how far away it is: is it like a headlight a mile away or an airport beacon 10 miles away? In the dark of the nighttime sky with no reference points, it's pretty hard to tell.) Technical advances, such as the introduction of charge-coupled devices to replace photographic plates for the measuring of stellar distances and brightnesses, are making our observations more secure.
"Distance measurements have improved to the point at which other details needed to determine the ages of star clusters--such as the fine details of how a star converts nuclear energy to visible light--can no longer be ignored. How exactly does the energy get from the center of the star, where it is generated, to the surface, where it becomes the light that we see? How important is convection as a means of transporting energy, and how efficient is the convection? The answer to these questions has some effect on the inferred relationship between mass and surface temperature. Just how much oxygen is in the stars, along with the hydrogen and helium? The relative amount of oxygen present has a modest effect on the efficiency of the central furnace, affecting the relation between mass and brightness and, hence, age.
"Taken together, the uncertainty in the observations and the uncertainty in the relevant theoretical physics probably lead to an uncertainty of 10 percent to 20 percent in our estimate of the absolute ages of the globular clusters. According to our best available estimates, stars having about 90 percent of the sun's mass are just now starting to die in the globulars. These stars are most probably around 15 billion years old, but they could conceivably be as young as 12 billion years or as old as 18 billion years. It is very unlikely that most of them could be either younger or older than this range. This estimate is already accurate enough to place some very interesting limits on the age and life history of the universe."