How do scientists determine the age of stars?
Normally, astronomers are unable to determine a star's age. While we can identify that certain stars are very young and others are quite old, we cannot do so for the majority of stars. However, we can determine how old a clump of stars is if there are many of them. This is feasible because it is thought that all the stars in a cluster had their births around the same time. Stars enter the adult stage of their life, known as the main sequence phase, after a relatively short period of time (in "star time," that is—we are talking thousands to millions of years here). A star's mass determines how long it stays in the main sequence phase.
The mass of the stars that are just entering the red giant phase of their lives and transitioning from this phase can be found by creating a depiction of the cluster's stars known as the HR diagram. We can estimate the age of the cluster by using computer models to anticipate how old a star of that mass must be to be at that point in its existence. Because the age it provides for the oldest star clusters in our Milky Way appears to be older than the age of the universe calculated from the most current Hubble Space Telescope data, this approach has recently come under serious investigation.
The age of a single star cannot be established on its own. Studying star clusters is the only reliable method we have for figuring out the ages of stars. Star clusters generally fall into one of two categories in our galaxy, the Milky Way. Because they resemble enormous, round globs and contain somewhere between a few thousand and a few million stars, the first type of clusters are known as globular clusters. Globular clusters appear to have formed close to the time our galaxy first formed, when the universe was still fairly young. These clusters are very old and dispersed throughout the Milky Way rather than just within it.
Since we observe them inside the galactic core, the second type of clusters were formerly known as "galactic clusters," but their stars are now more widely dispersed over the sky in open clusters, which are much looser than globular clusters. Open clusters appear in a wide variety of ages and can contain anything from a few dozen to a few thousand stars. It appears that shortly after our galaxy reached its current size, it began to produce open clusters, and it is still doing so now.
Either sort of star cluster has stars that all formed simultaneously and from the same material. Each star cluster comprises stars with a variety of masses, which is a crucial aspect of a stellar cluster that allows us to estimate its age. When a cluster is formed, it will have many stars that are roughly the same size and mass as our sun, as well as many stars that are both more massive and less massive than our sun. A star glows for around 90% of its lifespan because nuclear reactions in the star's core are converting hydrogen to helium and releasing enormous amounts of energy.
The light that emanates from the star after this energy has travelled from the star's core to its surface. The furnace in the heart of a star grows larger with increasing mass, and the star becomes brighter and hotter throughout this steady phase of its life. The stars with the highest masses are very bright and blue-hot; those with lower masses are a little fainter and white-hot; those similar to the sun are a little fainter still and yellow-hot; and the stars with the lowest masses are very faint and just red-hot. A star's brightness or temperature don't fluctuate much during this stage of its life.
The mass of a star affects how long the stable, or main sequence, phase lasts. Obviously, there is 10 times as much fuel in a star that is 10 times as massive as the sun. However, it uses that fuel around 10,000 times more quickly than the sun does. Its overall lifetime is therefore 1,000 times less than that of our sun. When a giant star's hydrogen fuel runs out in the center, which makes up around 10% of the star's total mass, the star becomes more unstable. The star is still brilliant, but it rapidly changes from being relatively small and hot to being enormous and red for a time, then it momentarily becomes smaller and bluer, then it gets even bigger and redder, and ultimately it explodes as a supernova.
Similar to this, a star that is five times as massive as the sun has an active life that lasts for around 100 times as long as the sun before it becomes unstable. The sun is currently a little under half the estimated lifespan of a star like our sun, which means we have another five billion years or so before we need to start seeking for a new home. This age can be very precisely measured from radioactive components in meteorites.
The brightness and temperature of a single star don't reveal anything to us. The star could be fairly young or fairly old, and we wouldn't be able to tell the difference because these characteristics remain largely constant throughout 90% of its existence. The fact that stars of all masses evolved in a stellar cluster around the same time gives us an edge. So all we need to do is take a look at the cluster and work out the mass and temperature of the most massive, bluest, and hottest star that is not yet in the late, unstable stage of its life. The star's brightness and mass provide information about the star's speed and the amount of fuel it had when it was born.
We may determine the star's age by dividing the amount of fuel it had at first by the rate at which it has been burning that fuel. (By analogy, we can figure out how long a hurricane lamp has been lit if we know how much kerosene it contained when we lit it, how quickly it uses up the kerosene, and if the lamp has only recently started to go out.) The age of that one star provides information on the age of the entire cluster because all the stars in the cluster are the same age.
It is quite easy to understand the fundamental physics underlying how hydrogen is changed into helium in the centers of stars and how much energy is produced in this process. Our understanding of stellar ages has largely been constrained for much of the 20th century as a result of the difficulties in determining distances to the clusters, particularly those of the oldest clusters, the globulars, which are comparatively far away. As bright as a star appears to be, its true brightness depends on how far away it is it like a spotlight a mile away or an airport beacon ten miles away? It's challenging to navigate in the night sky when there are no landmarks to use as a guide.
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