Star of the Month
HD 140283
AKA: the Methuselah Star.
Position: 15 hrs 43 min 03.1 sec -10 degrees 56 min 01 sec
Due south at 01:09 (BST) on 16 May 2021
HD 140283
Image: Simbad (http://simbad.u-strasbg.fr/simbad/)
Can a star be older than the Universe? On one level this is obviously an absurd question, any star by definition cannot be older than the Big Bang. Yet the age of an otherwise unremarkable star in Libra has been estimated to be 14.5 billion years, an whole three-quarters of a billion years older than the Universe! It has been known since 1951 that HD 140283 is metal-poor compared with stars like our Sun (so-called Population I stars). By metal-poor astronomers don’t just mean lacking metals like calcium or iron, but any elements other than hydrogen or helium (although metallicity is measured by the ration of iron to hydrogen). But HD 140283 does have some metal content, it is just very low. This means that it cannot be one of the earliest stars in the Universe as they only contained hydrogen and helium (with perhaps a just a smidgen of lithium and beryllium), they are the Population III stars. These first stars were huge in size and hence disappeared in massive explosions a few million years later spewing the first metals into interstellar space. Although HD 140283 is a Population II star its age was put at 14.5 billion years after its parallax was carefully measured by the Hubble Space Telescope in 2013. However it is very difficult to calculate the age of stars like HD 140283 and its age has recently been reduced to 14.3 billion years but this still well beyond the calculated age of the Universe. However the doubts of the age of this star (and our models of stellar evolution) are greater than our doubts about our modelling of the Universe’s age, but clearly at some point its age will have to be revised downwards again or the age of the Universe revised upwards. One recent estimate of the age of HD 140283 put it at “only” 12.2 to 13.7 billion years but this has been challenged.
At magnitude 7.2, this elderly star is not visible to the naked eye but it is easily seen in telescopes or binoculars. It lies roughly halfway between two wonderful double stars Zubeneschamali (Beta Librae) and Xi Scorpii in a fairly barren field. With a B-V of about 0.5 it will look yellowish, in keeping with its spectral type of F9. It lies in the galactic halo about 202 light years away from us.