The dim zodiacal constellation Libra harbors just a handful of dim deep-sky objects and no bright stars. But within its boundaries lies the Methuselah Star, an ancient relic of the early universe born from the ashes of the first stars that formed after the Big Bang. It’s likely the oldest object of any kind you will ever see, and it’s an easy target in a pair of binoculars or small telescope.
The First Stars
The Methuselah Star, known less poetically as HD 140283 dates back to a time before the Milky Way itself formed in its current state. The star is one of the first offspring of the first stars that formed in the universe, which astronomers call Population III stars (for historical reasons). These stars assembled themselves about 150 million years after the Big Bang from the thin soup of hydrogen and helium gas that permeated the cosmos. Pockets of slightly denser gas in this early era drew in more gas, heating and grower denser, and eventually collapsing into the first patches of star formation. The cores of these first collapsing gas clouds contained no complex atoms or molecules to radiate away heat, so only the most massive gas clouds had enough gravity to condense and begin nuclear fusion in their cores. In rare patches here and there, visible light arrived in the universe at last.
The first stars were big, bright, hot, and fast burning, and they blew up as supernovae. None remain, at least in the local universe, so no one has ever seen one. Astronomers are, however, seeking signs of these stars in the distant universe (and therefore distant past) with the help of the James Webb Space Telescope (JWST) and other earthbound instruments.
As they blew up, these immense Population III stars forged heavier elements from hydrogen and helium in their cores and scattered traces of these elements into space. These elements, like carbon, oxygen, and iron, altered the course of star formation and universe by cooling condensing clouds and making it easier for new stars to form. An immense burst of new star formation followed as stars formed with a light peppering of heavy elements formed from the ashes of the first stars. These second-generation stars, so-called Population II stars, are nearly all gone, although a few linger in globular clusters and as individual rogue stars like HD 140283 that inhabit the halo around our galaxy. As the interstellar medium became ever more enriched with heavy elements from dying Population II stars, the latest generation of Population I stars formed along with dense and rocky planets. Our Sun, about 5 billion years old, is a Population I star.
Determining the Age of the Methuselah Star
Astronomers figure HD 140283 is one of the earliest Population II stars because it contains fewer heavy elements than most stars of its generation. Astronomers use the abundance of heavy elements such as iron and carbon as a proxy for age. The lower the iron abundance, for example, the older the star. HD 140283 has a trace abundance of carbon about 250 times smaller than our Sun, which suggests its immensely old. How old? Astronomers have been trying to answer this question for decades using computer models of stellar evolution and estimates of the star’s distance (and therefore true brightness). A couple of decades ago, crude measures of distance suggested an age of more than 14 billion years, older than the universe itself. In a 2015 study, far more accurate distance measurements from the Gaia space telescope revealed the star was 190 light years away, which lead to an age determination of 13.7 ± 0.7 billion years, a figure that lies within the presently accepted age of the universe. A new study in 2021, however, suggests the star might be as young as 12 billion years old.
How To See This Ancient Stellar Relic
With the JWST, astronomers are looking to the ends of the universe to find the first stars. So what’s one of their earliest stellar offspring doing in our part of the galaxy? The answer, it seems, is that it’s just visiting. HD 140283 moves at a high velocity more or less perpendicular to the plane of the Milky Way. It likely came from its vast halo of ancient stars and will head back into the galactic halo one day. It may have been ejected by a globular cluster long ago or originated in a small primeval dwarf galaxy gobbled up by the Milky Way billions of years ago.
The Methuselah Star gets high marks for being astronomically rare and fascinating, but it’s not particularly visually striking. It looks like any other yellow-white star. This elder statesman of the galaxy about 6.5o east-southeast of Beta Librae (Zubeneschamali) in a field devoid of any bright stars. The star shines at 7th magnitude and lies within reach of any pair of binoculars or small telescope. It’s about 19o north of the plane of the Milky Way and just west of the more star-rich region of Scorpius.
Despite its modest appearance, the Methuselah Star is worth a look. It was born when the universe was perhaps less than 5% of its current age, and it’s witnessed the formation of the Milky Way, the assembly of galaxy clusters, and the lives and deaths of countless trillions of stars over its long lifetime.
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