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astrophysics

The oldest one (for the time being)

Oldest star ever discovered raises questions about the formation of the Universe

ESSOParanal, at an altitude of 2,600 meters: in search of witnesses of the Big BangESSO

It still shines as the oldest star in the Universe. It may be that, at any moment, another arises, even older, since the research should only end two years hence, but the discovery of this cosmic fossil of 12 to 15 billion years ago, located at a distance of 36,000 light years (one light year is equivalent to 9.5 trillion kilometers), opened up a way out in a labyrinthine quest that had been going for 20 years and mobilized an international team of researchers, with the strong participation of Silvia Rossi, an astronomer from the University of São Paulo (USP).

With its subtle lyricism, built on memories, music or poems retained in the memory, this rare celestial body has sprung up as a source of admiration, and, for scientists, of questions, particularly about the processes with which the first stars were formed. It seems that there is more than one script followed for the birth of the first celestial bodies with their own light.

Until this star was recorded for the first time, at 2 AM on November 12th, 2001, in a telescope in Australia, and confirmed ten weeks later by a more powerful apparatus, in Chile, and announced to the world three months ago, in an article in the October 31st issue of the scientific magazine Nature, it used to be thought that metals, above all iron, were indispensable for cooling down and compacting the clouds that were the precursors of the stars, made up of hydrogen and helium. But the star now held to be the oldest, the discovery of which was a result of a work coordinated by a 36 year old physicist, Norbert Christlieb, of the University of Hamburg, in Germany, exhibits a scarcity of metals never seen before: 200,000 times less than the Sun, and 20 times less than the previous oldest, known as CD -38°245, at least 2 billion years younger, which remained in its post during two years.

The supposition is therefore that, by processes as yet unknown, the oldest star was formed in an environment practically devoid of metals – as the chemical elements heavier than helium are called. Located on the halo (edge) of our galaxy, the Milky Way, it is made up of hydrogen (90%), a bit of helium (less than 10%), and any extremely small quantity of lithium (estimated at 0.0005%). It is intriguing: no one understands for sure how the lithium got there, since this element is not, as far as one knows, formed by the reactions from a fusion between helium and hydrogen atoms.

Other metals are present in an insignificant way: while in the Sun there is one atom of iron for every 31,000 of hydrogen, there must be just one atom of this chemical element for every 6.8 billion atoms of hydrogen in the star baptized as HE 0107-5240 – the initials correspond to Hamburg and to the European Southern Observatory (ESO), and the numbers to the approximate position of the star in the sky at the moment when it was found.

The purest of the stars must have taken shape a mere 1 billion years after the Big Bang, the explosion that may have started the Universe. Accordingly, crucial clues may emerge from this relic about the history of the formation of the stars and the chemical elements in the primitive Universe. Obviously, a lot of things must have happened between the Big Bang and the formation of this star”, commented Christlieb, the coordinator of the research.

According to him, the HE may have inherited the little metal that it contains from even older and larger companions, although it is now being questioned whether the first stars were really so much larger – or more massive, as astronomers would say. It is believed that the larger a star is, the faster the hydrogen and helium are consumed, in reactions that produce energy and light, and the star comes to its end more rapidly. The stars of greater mass disappear in an explosion in which metals heavier than iron are formed – launched into space, they increase the density of the interstellar clouds of hydrogen and helium, which become integrated and originate new stars.

That is why, according to the theory still in force, the older a star is, the less metal it will have. If it really has gained its meager metal from now extinct stars, HE 0107-5240 would be an example of the second generation of stars, made up of the hydrogen and helium, that were left over from the Big Bang, with an extra seasoning from their extinct companions. But it may also be a representative of the first generation: the researchers are also considering the possibility of the recently discovered star having been formed from a cloud made up only of hydrogen and helium – the metals could have been aggregated and built up as the star passed around the galactic disc, as the arms of the Milky Way are called, which is far more populated with stars than the halo.

It was Christlieb who supplied the raw material on which the group started to work: dozens of millions of celestial sources of low luminosity. It was one of the results of a survey of the sky of the Southern Hemisphere carried out in the 90s on one of the telescopes of the European Southern Observatory – located in Paranal, in the Chilean Andes. The sources of lights were distributed in groups of five in thousands of photographic plates, which recorded in the form of longer or shorter lines the intensity of the light emitted by the different chemical elements – the so-called spectrum, a kind of fingerprint, obtained by means of a prism coupled to the Paranal telescope.

The German researcher knew that, on his own, he would take decades to screen this material. At a congress held in Canberra, Australia, in August 1998, he met two other researchers who were already working together on similar surveys: the American Timothy Beers, from Michigan State University, who since the 80’s had collected thousands of ancient stars to understand how the Milky Way was formed; and Brazilian Silvia Rossi, from USP’s Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG), who had been looking for metal-poor stars since 1995, when she started her postdoctoral studies with Beers. On the same day, they added together their discoveries and their doubts, checked their methods of work, and planned the biggest stellar prospecting exercise in the history of astrophysics.

Alliances
In two years, the three researchers, whose project was joined straight away by astronomers from Australia and the University of Munich, in Germany, carried out the first screening through computer programs that they themselves had created. They brought down to around 50,000 the number of objects to be studied – and when the limits of the computer had been surpassed, visual selection then became inevitable. In the course of two weeks, in August 2001, Silvia and Beers met up with Christlieb in Hamburg and, working 12 hours a day, analyzed one by one the records of metallic qualities.

In particular, they assessed the spectrum of the emission of light from calcium ions (an ion is an electrically charged atomic particle), a first indicator of the metallic content of a star – the weaker it is, the lower the share of metals tends to be. “In the beginning, we would examine one plate a minute, but afterwards it was much quicker”, says the researcher. They selected 8,000 objects that were candidates for being stars with extremely low metallic content, in which the line of calcium was not very intense. But they had to get back their breath and go on to the next stage, the selection carried out directly on the telescope, with a greater precision than used to make the plates, which was need to calculate the content of iron, which was now the standard indicator of the elements heavier than hydrogen and helium.

The list was split up between the members of the group, who set about observing each one of the candidate stars on the telescopes to which they had access. Six months afterwards, the leader of the Australian team, Michael Bessel, gave the group a reason for happiness, having discovered a star with impressively low metallic qualities, with the ration between the contents of iron and hydrogen equal to -5.3. But the telescope of the Mount Stromlo Observatory, where he was, had a 2.3 meter mirror – an average resolution, insufficient for the purposes of the group.

Bessel advised Christlieb, who battled for an extra time slot on the Paranal telescope, one of the largest in the world, with a mirror 8.2 meters in diameter, and, at the coordinates indicated by the Australian, there finally came confirmation of the oldest star in the Universe, with a brilliance 10,000 times less than the most tenuous of the stars observable with the naked eye. It was at this stage that there emerged the images that are most usual for the non-specialists, in which the stars appear as more or less luminous dots. Undergoing an experience of this kind is rather like living though a pregnancy without a set date for the delivery. “We were expecting to find old stars, but nothing so surprising”, comments Silvia, whose work is part of a thematic project coordinated by Beatriz Barbuy, from the IAG.

The researchers themselves, apparently little inclined to hold for long the title that HE 0107-5240 had brought them, did not quirt scanning the sky. Beers worked through four nights in December on a telescope with a 2.1 meter mirror, in Tucson, Arizona, in the United States; in February, Silvia and Beers will spend five days at the Cerro Tololo observatory, also in Chile, which has a 4-meter diameter mirror, and by December there will be at least another five rounds of observations. Up till now, the astronomers have examined only one third of the list of candidate stars, so that it is not entirely improbable that, before the year is out, sisters or cousins of the old lady of the Universe may arise – who knows, even an unquestionable specimen of the first generation of stars, still purer.

“It would be very strange if there existed only one star like this in the Universe”, Silvia observes. “We believe that we may yet find perhaps a dozen stars with as low or even lower metallic content”.The oldest star in the Universe has an uncertain origin, but a foreseeable fate: it should live for another 5 billion years or so, and die at perhaps the same time as the Sun. Already in a sort of old age, like a giant red star, HE is converting helium into carbon. It still shines intensely and its external layers are connected by gravity, forming a sort of atmosphere to which there are no limits to be seen. But it will not be like that for long.

The layers furthest out are going to come off, like peel off a fruit – this is the planetary nebulous stage, which last only some dozens of thousands of years -, and the star will lose mass and luminosity. It will then be in its death throes, like a white dwarf, difficult to be observed even by the most powerful telescopes. Next, it will be a planetoid. After that, it will burn out.

The project
Chemical Evolution and Stellar Populations of the Galaxy, Magellanic Clouds and Elliptical Galaxies, using Spectroscopy and Imaging (nº 00/10406-4); Modality Thematic project; Coordinator
Beatriz Leonor Silveira Barbuy – IAG/USP; Investment R$ 174,890.15

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