Our Solar System is old. Our best estimates from radiometric dating of meteorites suggest the Sun and its planets (including Earth) were forming some 4.6 billion years ago. The Universe itself appears to be some 13.8 billion years old based on its observed rate of expansion and the properties of the Cosmic Microwave Background. That means, perhaps surprisingly, that the Earth has been here for a third of the lifetime the Universe has existed. That is an impressive fraction to be sure, but let us look at a star which is much, much older. A star so old, it seemed to have been born before the Universe itself!
A seventh magnitude star in the constellation Libra, HD 140283 has always been seen as unusual. For more than a century astronomers have been ware of its fast motion across the sky. It is speeding through our galactic neighbourhood at a staggering 800 000 miles per hour (1 280 000 km/h). This rapid motion shows us that the star is just passing through our region of space. Currently about 190 light years from the Sun, it is endlessly circling the Galactic Core in an orbit, vast in size and duration, which carries it down through the plane of our galaxy to the halo of ancient stars that encircle the Milky Way. Halo stars are old, dating back to the formation of the Milky Way some 12.6 billion years ago. This suggests HD 140283 is an elderly star but when astronomers used their tested techniques to calculate its age they got a shocking result. HD 140283 appeared to be 16 billion years old, more than two billion years older than the rest of the cosmos (measured to be 13.78 ± 0.037 billion years old)!
A star older than the Universe is an absurdity, enough to give cosmologists a few grey hairs, how could this be true? The star’s age was assessed by two independent techniques. Knowing a star’s intrinsic brightness is essential for estimating its age, to know its intrinsic brightness you must know its distance from Earth. Unfortunately the distance to HD 140283 was not known as precisely as astronomers would have liked. Alternatively the chemical composition of stars should change at a known rate enabling their ages to be calculated by observing the relative proportions of the their constituent elements. However astronomers are refining their models of exactly how stars work all the time. The estimated age of 16 billion years had an uncertainty of two billion years from the inaccuracy of its measured distance alone, if any of the predicted details of the star’s interior processes were off this uncertainty could be larger still. Astronomers revisited the matter of HD 140283’s age.
The most recent estimates using both improved theories of stellar structure and more accurate distance data from the Hubble Space Telescope give a more reasonable estimate of 14.5 billion years (with an uncertainty of plus or minus 800 million years). This makes HD 140283 the oldest known star with a well-determined age. It has been called the “Methuselah Star”, and although it looks quite normal at first, looking closely we see what a strange star it is. Its diameter is almost half as a large again as our own Sun, but its surface temperature is roughly the same as the Sun’s. This combination of size and temperature means HD 140283 is almost four times as bright as our Sun. None of these statistics are unusual, but the star’s composition has been known to be anomalous since the 1950s. Compared to other relatively nearby stars HD 140283 is “metal-poor”, in astronomical jargon this means it is lacking in elements heavier than hydrogen and helium. This may not seem particularly odd but is a vital clue in determining the star’s age.
As far as we know only hydrogen and helium formed in the seething chaos of the Big Bang. Every other naturally occurring element came later, formed by nuclear fusion in the giant atom crushing machines that are the cores of stars or even in the titanic stellar detonations called supernovae. The process in which stars turn light elements like hydrogen into heavier elements is called nucleosynthesis. For billions of years, new stars have formed from hydrogen and helium salted with traces of heavier elements forged inside previous generations of stars. A star rich in hydrogen and helium but deficient in all the heavier elements must be a very old star, dating back to an early phase of the Universe’s life before heavy elements were common. HD 140283 fits this description perfectly having only 1/250th of the heavier element content of our Sun and other stars in our part of the Galaxy.
Metal-poor stars like our subject are said to belong to Population II, today’s stars with a richer mix of elements are Population I, while Population III are theoretical stars composed of hydrogen and helium only. Population III represents the very first generation of gigantic stars dating from only a few hundred million years after the Big Bang. No Population III stars are believed to exist in the present era, as such large stars would live short fiery lives a few million years long before exploding as supernovae. Any later generation of stars had to be built from the hot gases of these ancient detonations but could not form until the debris had cooled down enough. Theorists expected there to have been a distinct gap between these two generations of stars. HD 140283 ‘s great age suggests that the gap between Population III and II stars was shorter than anyone had though. HD 140283 and its stellar generation may have arose only millions of years after the explosive deaths of their predecessors.
It is tempting to speculate if HD 140283 could have a family of planets, even one bearing life. Imagine beings that have evolved over most of the age of the Universe , what heights of achievement could they have attained! (If this kind of speculation fascinates you, you really need to read Stephen Baxter’s Xeelee books!) Sadly life is extremely implausible around HD 140283, any planets it may have will be gas giants, great globes of hydrogen and helium like Jupiter and Saturn but lacking the organic compounds which colour their atmospheres. An ancient planet of this sort has been discovered, in the form of a 13 billion year old gas giant in an odd binary system in the globular cluster M4. Gas giant planets around HD 140283 are perfectly plausible but worlds like Earth or Mars are made of heavy stuff, iron, oxygen, silicon and so on. When HD 140283 was born heavy elements were so scarce it is hard to imagine it could accumulated enough from the primordial nebula where it was born to possess any rocky planets.
As the Universe has grown old around it HD 140283 has seen many changes. Born in a primeval dwarf galaxy, the star blazed bright in a Universe smaller and denser by far than it is now. After a billion years or more the developing Milky Way’s gravitational grip seized the dwarf galaxy. Eventually, over 12 billion years ago, the dwarf galaxy was absorbed into the growing Milky Way. The star’s current elongated orbit is evidence of this long ago cannibalism. This remarkable “living fossil” star is a direct link to a lost and mysterious epoch in the history of the Universe.
(Article – 300th since Astronotes changed to blog format – by Colin Johnston, Science Communicator)