HD 140283 is a subgiant star estimated to be 14.46 billion years old. That may raise an eyebrow or two among those who recall the universe's estimated age of 13.77 billion years. This star, commonly referred to as the Methuselah star, seems to be older than the cosmos itself.
At times, such an uncommon discovery causes us to reconsider our understanding of the cosmos and typically results in a revolution in our cosmological theories. That is not the case at the moment. To understand why we must examine how we calculate the age of a star of this kind.
The majority of stars grow in a predictable manner. A protostar gravitationally collapses, causing hydrogen to fuse in its core and transforming it into a true star. The star enters what is known as the main sequence, where it remains a stable star for billions of years (depending on its size). As it depletes its hydrogen, it transitions to a red giant or comparable stage before exploding as a supernova/white dwarf/etc., depending on its size.
Calculating the age of a main-sequence star is challenging since they undergo little change throughout this time span. There are modest alterations that may be seen. It is comparable to determining a person's age. Someone in their twenties seems to be youthful. Someone who is 80 seems to be elderly. However, someone between the ages of 30 and 50 is more difficult to evaluate, since middle-aged persons tend to keep the same appearance for decades. The same is true for main-sequence stars in their forties and fifties.
HD 140283 is not a main-sequence star, which is fortunate for astronomers. It is a subgiant star, which indicates it has descended from the main sequence and is on route to the red giant stage. This characteristic alone indicates that it is an older star since it is approaching the end of its existence. As a result, we can state categorically that it is billions of years old.
Another property of a star that may be used to calculate its age is its metallicity. In astronomy, the term "metal" refers to anything other than hydrogen and helium. Because hydrogen and helium are the two elements created in the big bang, the earliest stars (referred to as population III stars) would have been devoid of "metals." These first stars would fuse hydrogen and helium in their cores (to form metals), before exploding as supernovae. These early stars' gas and dust leftovers would then gravitationally collapse to produce new stars. These newly formed stars would contain some metals (but not a great deal), making them low-metallicity stars (population II).
With each generation of new stars developing from the leftovers of the old, one would anticipate a greater metallicity. Thus, stars with a high metallicity are younger than those with low metallicity. For example, our Sun is a high-metallicity star that is believed to be 4.57 billion years old. Due to the fact that HD 140283 is a relatively low metallicity star, it most likely originated from the leftovers of the first generation of stars. This indicates that it developed quite early in the universe's history.
As a low metallicity subgiant, Methuselah is estimated to be between 12 and 13 billion years old. To be more exact, we need to examine the mechanics of how stars generate energy in their cores, as well as the relationship between the size and brightness of a star. We have rather accurate models, but they are based on meticulous measurements of the star.
The two most critical measurements are the distance and temperature of the star. The temperature may be determined reasonably readily by examining a star's spectrum. Distance is a little more challenging. Due to the fact that HD 140283 is rather near to us, we were able to determine its parallax. We calculated its distance to be 190.1 light-years. We can calculate a star's brightness by knowing its distance and temperature, which informs us how much energy the star is presently generating.
However, here is where it becomes extremely confusing. As described in a paper published earlier this year in Astrophysical Journal Letters (which is currently behind a paywall, but a draught version is available at the reference section), the calculated age of a star is determined by not only how much energy it produces now, but also how much energy it has produced throughout its lifetime. This rate is determined by factors such as the quantity of iron and oxygen in the star (since those elements can affect the fusion rates).
The scientists determine the age of HD 140283 to be 14.46 billion years, with an error of 800 million years either way, based on detailed studies of the star's metallicity, notably iron and oxygen. This may seem to contradict the universe's recognized age (13.77 billion years), however, the uncertainty implies that the star may be as young as 13.6 billion years, which is within the universe's lifetime. It might possibly be as ancient as 15.3 billion years, which is far older than the universe, but given the abundance of evidence supporting the universe's age, there is no reason to interpret this star's oldest conceivable age as proof against the universe is 13.77 billion years old.
However, even the star's youngest age tells us something about our universe's infancy. This star is most likely a second-generation star that arose when the cosmos was less than 170 million years old. This suggests that the initial stars must have formed extremely early, had brief lifetimes, and their remains cooled rapidly in order to provide the ideal circumstances for the formation of a star-like HD 140283.
Reference(s): Peer-Reviewed Research Article