A Star That Doesn’t Look Its Age
By Judith E Braffman-Miller
Stars are a lot like people–they are born, they live, they grow old, and then they die. The ancient birth of the first stars to blast our Universe with their raging fires and brilliant light is swathed in mesmerizing mystery. The first stars are thought to have ignited a “mere” 100 million years after the Big Bang birth of the Universe almost 14 billion years ago, and the oldest of all known stars is almost as old as the Universe itself. But some stars, like some people, fake their ages–and 49 Lib is just such a deceitful star. In January 2017, a team of astronomers announced their discovery that 49 Lib, once generally considered to be a stellar teenager, is really very old–and it was born at about the same time that our ancient 13.21 billion-year-old Milky Way Galaxy was born.
Dr. Rolf Chini, the Chair of the Astrophysics Faculty of Physics and Astronomy at Ruhr-University, Bochum (RUB), Bochum, Germany, has been observing approximately 400 stars in our own Sun’s neighborhood. Dr. Chini has, for many years, been on the hunt for stars that share some of our own Star’s properties. It was during this stellar hunt, that Dr. Chini and his team discovered that 49 Lib is much older than it looks. A relatively bright star, sparkling brilliantly in the southern sky, 49 Lib is twelve billion years old rather than a mere 2.3 billion–as originally thought. So, why did astronomers have such a difficult time catching this act of stellar deceit?
Apparently, astronomers were baffled by the conflicting data they had collected concerning the nature of this mysterious star–and, as a result, had estimated it to be considerably younger than it really is. When trying to determine 49 Lib’s age anew, the team of astronomers at RUB managed to resolve the various inconsistencies that have baffled astronomers for many years about this very unusual star. Dr. Klaus Fuhrmann and Dr. Chini published their new results in the January 2017 issue of The Astrophysical Journal under the title Bright times for an ancient star.
When we look up in wonder at our sky at night, we see that it has been beautifully and brilliantly blasted by the distant light of a myriad of stars. But where did the stars come from, and when did they first appear on this enormous expanse of blackness–setting on fabulous fire, with their wonderful light, the most distant and secretive corners of the observable Universe?
The birth of the first stars to inhabit the Universe is certainly one of the most intriguing of scientific mysteries. The oldest stars are believed to have ignited as early as 100 million years after the Universe was born about 13.8 billion years ago–and many astronomers now think that the first generation of stars to inhabit the Cosmos were not like the stars that we are familiar with today. This is because they were born directly from pristine primordial gases manufactured in the Big Bang itself. The ancient gases were mostly hydrogen and helium, and these two lightest of all atomic elements are thought to have been pulled together by the force of their own gravity to create ever tighter and tighter blobs. The cores of the first protostars to form in the Universe caught fire within the mysterious and secretive cold, dark hearts of these extremely dense blobs of pristine hydrogen and helium–which then ultimately collapsed under the pull of their own relentless and merciless gravity. Many astronomers think that the first stars were gigantic when compared to the stars inhabiting the Universe today. This is because they did not form in the same way, or from the same mixture of elements, that the stars that we are familiar with formed. The very first stars are designated Population III stars, and they were likely blazing megastars. Our Sun is a brightly shining and beautiful member of the youngest stellar generation–the Population I stars. In between the first and the most recent generations of stars are the stellar “sandwich generation”–the appropriately named Population II stars.
Extremely massive and very ancient Population III stars were probably dazzling in their brilliance, and their existence is thought to have been responsible for triggering a sea-change in the Cosmos–dramatically altering it from what it was to what it is today! These very massive, glaring ancient stars changed our Universe by heating things up–and, in this way, ionized the primordial gases.
In astronomy, the metallicity of a star refers to the fraction of the mass of a star–or any other class of cosmological object–that is not hydrogen or helium. Most of the atomic matter in the Universe is in the form of hydrogen and helium–the lightest of all elements–and both were born in the Big Bang itself. Astronomers and physical cosmologists use the term metal as a convenient reference to “all (atomic) elements except hydrogen and helium” listed in the familiar Periodic Table.
All of the atomic elements heavier than hydrogen and helium were created in the nuclear-fusing, searing-hot hearts of the Universe’s treasure trove of trillions of sparkling stars–or, alternatively, in the explosive supernovae heralding the demise of the more massive stars. Therefore, the term metal for an astronomer does not carry the same meaning that it does for a chemist. The reason for this is that in the searing-hot, extremely high temperatures and strong pressure environment within a star, atoms are unable to to conduct chemical reactions. As a result, atoms within the hot heart of a star effectively have no chemical properties–including that of being a metal in the usual use of the term. For example, stars that contain relatively high abundances of nitrogen, oxygen, carbon and neon are considered to be “metal-rich” in the terminology of astronomers–even though they are not considered to be metals by chemists.
It is generally thought that the primordial Universe was barren of metals–according to the terminology used by astronomers–which were later produced within the hot furnaces of the first stars by way of the process of stellar nucleosynthesis.
Determining a star’s metallicity provides astronomers with a valuable tool that they can use to determine a star’s true age. When the Universe first came into being, its normal supply of “ordinary” atomic matter was almost entirely hydrogen. Stars that are elderly (Populations II and III) reveal much lower metallicities than their younger counterparts (Population I). Nucleosynthesis refers to the process by which heavier atomic elements are formed from lighter ones, as a result of nuclear fusion (the fusing of atomic nuclei).
All three of the stellar generations show decreasing metal content with increasing age. The youngest stars, Population I stars–like our own Sun–show the greatest metal content. The three populations of stars were named in a somewhat confusing way because they were designated according to the order in which they were discovered–which is just the reverse of the order in which they were born. Therefore, the most ancient stars to blast the Universe with light, the Population III stars, were barren of metals. In contrast, the stars that show the highest metal content–the Population I stars–are the stellar babies of the Universe.
Population II stars–the stellar “sandwich generation”–are very ancient, but not as ancient as the primordial Population III stars. Stars of this intermediate “sandwich generation” contain the metals manufactured–for the very first time in Cosmic history–by massive Population III stars. When the Population III stars perished explosively, in the brilliant fireworks of supernovae blasts, they sent screaming into space the first batches of freshly-fused heavy metals, which were then incorporated into later generations of stars (Populations I and II).
Even though the oldest stars carry smaller quantities of heavy elements than younger stars, the discovery that all stars contain at least some scanty amount of metals is a bewitching mystery. The favored explanation for this strange observation is that Population III stars must have once inhabited the Universe when it was very young–even though not even one lone Population III star has ever been detected. This particular line of logic proposes that in order for the ancient Population II stars to contain the metals that they do, their metals must have been created in the nuclear-fusing furnaces of an earlier generation of stars, that left no other trace of their former existence in the primeval Cosmos.
Population II stars have low metallicities, and are the most ancient stars observed by astronomers. However, even so-called “metal-rich” Population I stars–like our Sun–carry only relatively small amounts of any element heavier than helium. All stars, regardless of their generation, are mostly composed of hydrogen.
A Star That Doesn’t Look Its Age
Referring to the puzzling star, 49 Lib–that fakes its age–Dr. Chini noted in a January 16, 2017 RUB Press Release that “It had previously been assumed that the star was only half as old as our Sun. However, our data have shown that it had been formed at the time that our Galaxy was born.”
The reason for the error? The object being observed is not one star, but two! In other words, the bewildering and bewitching 49 Lib is really a dual stellar system, as was demonstrated by a different group of astronomers in 2016. Dr. Chini’s team went on to show how the mechanism used by the stellar companion of 49 Lib enabled the bothersome star to fake its age.
The stellar companion of 49 Lib is a star that has almost, but not quite, reached the end of that long stellar road, and is as good as invisible. The very faint and almost-dead companion star has contributed a portion of its material to 49 Lib–which is the reason why the elderly star looks much younger than it really is.
Astronomers calculate the age of the stars that they are observing according to their chemical composition. The older populations of stars, born in the primeval Universe, contain little in the way of heavy metals. In contrast, the bouncing baby stars of our own Sun’s Population I generation, contain more metals because they have formed from the remnants of ancient generations of stars that populated the early Universe.
As stars come to the end of the stellar road, they swell in size, becoming enormous. In fact, dying stars become so huge that their own gravity can no longer keep their matter together. The stellar matter escapes as gas. If there happens to be another star nearby, its gravity might attract–and finally snare–the expelled matter that was shed by the dying older star. This is how the lucky 49 Lib acquired its supply of the heavy metals that made it appear so deceptively young.
Astronomers calculate the age of stars based on their spectra. They separate the light that is being emitted by the star into its individual components, and then decode the wavelength at which the star sends forth most of its travelling light. The composition of a star’s chemical elements determines its spectrum.
Based on their new collection of data, the RUB astronomers did more than determine the true age of this very deceptive star. “We are able to track this dual system’s entire evolution,” commented Dr. Chini in the January 16, 2017 RUB Press Release. The astronomers now know, for example, that the masses of this strange stellar duo have evolved over time.
In the beginning, the two stars had mass properties similar to those of our own Sun. When 49 Lib snared some of the matter of its unfortunate, dying stellar companion, it gained weight equivalent to about 0.55 solar-masses. The more massive the star, the shorter its life. Massive stars live fast and die young–in marked contrast to their smaller stellar kin who take their time burning their supply of nuclear-fusing fuel and, as a result, happily and peacefully live considerably longer. In fact, small red dwarf stars–the smallest and most abundant class of stars in our Milky Way Galaxy–live so long that no red dwarf star has had enough time to die since the Big Bang birth of the Universe. Tiny, relatively cool, red dwarf stars can live for trillions of years–and our Universe is less than 14 billion years old.
49 Lib’s weight gain has, therefore, shortened its life. “It will soon become a red giant and then collapse into a white dwarf,” Dr. Chini added, describing 49 Lib’s final fate. Small stars, when they run out of their necessary supply of hydrogen fuel, first become bloated red giants–that eventually toss their shimmering, varicolored gaseous outer layers out into the space between stars. The remnant core of the small star-that-was becomes the very dense stellar ghost termed a white dwarf. The expelled outer gaseous layers of the small erstwhile star become the new white dwarf’s beautiful shroud–a so-called “butterfly of the Universe” called a planetary nebula.
When 49 Lib finally morphs into a red giant, it will no longer be able to keep a grip on its matter, and it will experience the same fatal process that its companion star suffered earlier. Part of 49 Lib’s matter will be snared by its companion star’s intense gravitational attraction.
Dr. Chini explained that “If that partner cannot rid itself of the matter in small eruptions, it will fully explode as a supernova.”
This article is dedicated to the memory of the journalist Wayne Barrett.
Judith E. Braffman-Miller is a writer and astronomer whose articles have been published since 1981 in various journals, magazines, and newspapers. Although she has written on a variety of topics she particularly loves writing about astronomy, because it gives her the opportunity to communicate to others the many wonders of her field. Her first book, “Wisps, Ashes, and Smoke,” will be published soon.