The Oldest Star In The Universe

The oldest star ever found in the universe is nicknamed Methuselah, after the oldest figure in the bible, and its age has been estimated at greater than 12-billion years. Although Methuselah is known to be ancient, its exact age remains somewhat of a mystery, and some estimates of its age are greater than the age of the universe. Obviously a star cannot be older than the universe itself, and so Methuselah has become somewhat of a cosmic contradiction. 

The First Generation of Stars

History of the universe
Illustration showing the history and evolution of the universe. Image credit: NASA

Although determining the age of a star can be rather difficult, there is a fairly simple way to determine if a star is among the first generation of stars to form in the universe. After the Big Bang, all visible matter in the universe came in the form of hydrogen, helium, and small amounts of lithium. Elements heavier than lithium were either extremely rare or nonexistent. Since heavy elements are created within massive stars, virtually no heavy elements existed before the first stars could form, live, and eventually die. Thus the oldest stars in the universe will have a ratio of hydrogen to helium that is similar to the amount of hydrogen and helium present in the Big Bang. In the case of Methusalah, it is composed almost entirely of hydrogen and helium, with virtually no heavier elements. This suggests that Methusalah is likely among the first generation of stars to form after the Big Bang.

Age of Methusalah

Image of the star Methusalah. Image credit: Digital Sky Survey 

The universe is widely accepted as being 13.8-billion years old. This number primarily comes from two observations: the rate at which the universe expands, and dating the cosmic microwave background radiation. The universe being 13.8-billon years old is the most accurate estimate of its age, yet the existence of Methuselah put this number to the test. In 2000, estimates of Methuselah’s age placed it around 16-billion years old, which is noticeably older than the age of the universe. One of these numbers must be wrong, and either Methusalah is younger or the universe is older. Later observations of Methusalah and estimates of its age placed it around 14.46-billion years old. Although that is smaller than the earlier estimate of 16-billion, it is still older than the age of the universe. Interestingly, the uncertainty in this new estimate is 800-million years, and so the lower estimate of the star’s age places it around 13.6-billion years old, which is compatible with when some of the first stars formed in the universe. Then in 2021, the age of Methusalah was calculated as being 12-billion years old, which makes it 1.8-billion years younger than the universe. Perhaps Methusalah is not as much of a contradiction as it once was.