Stellar Age Determination
Frustrating, Complex, and Absolutely Necessary
Why do we need to know the ages of stars?
All of the properties of stars change as the stars age. This includes our own Sun. Astronomers would love to have a time machine so that we could go into our Sun's past and understand its evolution, as well as the evolution of our planetary system. Since we don't have time machines (yet), we have to instead look at other stars in various stages of their evolution. For instance, if we observe a very young star, only a few million years old, and it has the same mass as our Sun, we can assume that it looks much like our Sun did at that age. The key problem with this method is that you have to be able to figure out the ages of the stars you're looking at. Unfortunately, that is much easier said than done.
What makes stellar age determination so difficult?
If you wanted to tell the age of a person just by looking at them, what would you pay attention to? You might notice if they have grey hair, or wrinkles. You might notice their height, though that doesn't always directly correlate with age. Human beings change substantially in their first 20 years of life, so it's easy to determine their ages. After that, it becomes significantly more difficult. It turns out that stars behave similarly. There are certain features you can look for to determine whether a star is old or young, and they undergo more changes in their early years.
Stars are born out of clouds of gas and dust called "molecular clouds". Once the central star begins nuclear fusion (and officially becomes a "star"), it spends the first few million years of its life scooping up the leftover gas and dust to form planets. This process is usually over in 10 million years, but can take up to 40 million years in some cases.
After that, the stars are on what's known as the "Main Sequence." In this stage of their evolution, stars are converting hydrogen fuel into helium in their cores, and will continue to do so for ~10 billion years (in the case of Sun-like stars). During this time, they change very little, so it is difficult to determine their exact age. There are a few traits you can use to determine whether a star is "old" (a few billion years old) or "young" (less than 100 million years old). This is akin to looking for grey hair or wrinkles. These techniques won't tell you the specific age. Still, in most cases, it's the best we can do.
Option 1: Use the stars' spin to determine age
When stars are first born, they're usually spinning very rapidly. This is because they have just finished collapsing from a large cloud of dust and gas. Imagine an ice skater who spins faster as she pulls her arms in. The same thing happens to a star as it forms. However, once it's done forming, it starts to slow down. Stars are like onions; they have layers. Those different layers spin at different rates. Thus, the layers of the star "rub" against each other, creating friction and heating the boundary between the layers (called the "tachocline"), causing the star's spin to slow down. This is known as "stellar spin down." Stars of different masses spin down at different rates, but all stars slow down as they age.
Here's the problem. In order to determine a star's rotation rate (how fast it's spinning), you need to watch it turn. Astronomers use starspots (dark spots on the surface of the star) to track the rotation of the star. Younger stars have shorter rotation periods, meaning it might only take a few days of observation to figure out their rotation speeds, and therefore their age. But older stars spin slower, sometimes taking more than a month to complete one rotation. Astronomers simply don't have the time to sit at a telescope for that long monitoring starspots. So gyrochronology is really only good for younger stars. In addition, all young stars spin quite fast. Meaning it's difficult to determine a star's specific age. Really, the best that we can do for now is to say that, if a star is spinning fast, it's likely young. If it's spinning slowly, it's likely old. More information is needed to actually determine a star's exact age.
Option 2: Use a star's magnetic activity to determine age
Similar to a star's "spin down", stars also experience a decrease in magnetic activity as they age. The Sun undergoes powerful solar storms and mass ejections periodically. When it does that, it can cause bright aurorae and sometimes even power outages here on Earth. These episodes are caused by the Sun's ever changing magnetic field. Other stars undergo similar eruptions as well. But these eruptions become weaker as the star ages. That is because the magnetic field slowly decays over time.
Just like gyrochronology, however, tracking magnetic activity can only really tell us if a star is old or young, not its exact age.