Milky Way formed stars from inside out

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<p><strong>By Peter Moon  |  Agência FAPESP</strong> – The first of the hundreds of billions of stars in the Milky Way – estimates range from 100 billion to 400 billion – may have begun shining 13 billion years ago, even before the galaxy had completely formed.</p>
<p>This possibility is raised by a groundbreaking chronographic map of the oldest stars in the Milky Way. The main conclusion of the study is that the galaxy started to form from the inside out: the first stars appear to have emerged at the center, with more and more popping up closer to the outer rim, or galactic halo, explained astrophysicists Rafael Santucci and Vinícius Placco, who took part in the international study and co-authored an <b><a href="http://iopscience.iop.org/article/10.1088/2041-8205/813/1/L16" target="_blank">article</a></b> published with <b><a href="http://www.bv.fapesp.br/en/auxilios/89706/the-frequency-of-field-blue-straggler-stars-in-the-thick-disk-and-halo-system-of-the-galaxy/" target="_blank">support from FAPESP</a></b> in <em>The Astrophysical Journal Letters</em>.</p>
<p>Santucci is a PhD student at the University of São Paulo’s Institute of Astronomy, Geophysics & Atmospheric Sciences (IAG-USP) in Brazil, supervised by Professor <b><a href="http://www.bv.fapesp.br/en/pesquisador/4078/silvia-cristina-fernandes-rossi/" target="_blank">Silvia Rossi</a></b>. Placco is a professor at the University of Notre Dame in the United States and has studied the galactic halo, also with <b><a href="http://www.bv.fapesp.br/en/bolsas/68647/the-milky-way-halo-revisited/" target="_blank">FAPESP’s support</a></b>.</p>
<p>To understand the significance of their research, it is necessary to imagine the shape of the Milky Way, which is a spiral galaxy with arms that spread out from the center to form a disk with a diameter of 100,000 light years. Star density – the number of stars per unit area – is very great in the center, known as the galactic nucleus or bulge.</p>
<p>Star density decreases in proportion to distance from the nucleus, so that the galactic disk is thinner near the edge.</p>
<p>However, this is a small part of the galaxy – the part that is visible to most astronomical observatories. The Milky Way’s disk is surrounded by the galactic halo, a spherical volume many times larger than the disk.</p>
<p>In fact, the halo is several hundred thousand light years across and consists mainly of dark matter, which is invisible and unknown but appears necessary to keep the galaxy in one piece. Without dark matter, it would fly apart. The halo also contains hydrogen clouds and stars.</p>
<p>Halo stars can be divided into three groups. The first comprises tens of thousands of stars in dense spherical multitudes known as globular clusters.</p>
<p>Some 150 globular clusters are orbiting the Milky Way, but there are also two other types of halo star. There are stars that have ended up in the halo because their escape velocity was sufficiently high for them to fly out of the galactic disk, and there are stars that originally belonged to smaller galaxies cannibalized by the Milky Way billions of years ago.</p>
<p>The second and third groups concerned the authors of the recently published paper. More specifically, the researchers studied a specific subgroup called “blue horizontal-branch stars”.</p>
<p>“They’re giant stars, ten times the size of the Sun on average, and they’re nearing the end of their lives,” Santucci said. “When young, they burned hydrogen. Now, they’re well into old age, fusing helium into carbon.”</p>
<p>When this fuel runs out, they shrink and become white dwarfs. This is what will happen to our Sun.</p>
<p>The researchers set out to study a large number of these blue horizontal-branch stars to estimate their ages based on the colors of their light.</p>
<p>A star’s age is calculated by analyzing the color and chemical signature of its light. “The color of a star is linked to its temperature, which in turn has to do with its mass, and the larger its mass, the shorter its life cycle,” Placco said.</p>
<p>According to Santucci, in the specific case of this research, “the age variations we describe in the paper were based on colors”.</p>
<p>Most young giant stars are white or blue, while medium-sized stars are yellow or orange. Old stars become red giants in the latter part of their lives and white dwarfs when truly senile. “However, blue horizontal-branch stars are an exception to the rule. They retain their blue color even in old age,” Santucci said.</p>
<p><strong>Second-generation stars</strong></p>
<p>The chemical signature of stars is determined by spectroscopy. The presence of chemical elements in the star’s atmosphere can be detected by analyzing the light from the core as it escapes through the atmosphere into outer space. When astronomers observe the light emitted by a distant star, one of the first things they analyze is its spectrum.</p>
<p>When the universe began expanding, there were only three chemical elements: hydrogen, helium, and a small fraction of lithium. All the other elements were forged in the first generation of stars, which died in cataclysmic explosions known as supernovae. Detritus from the explosions seeded the interstellar medium with the elements that now fill up the periodic table.</p>
<p>The seeding went on and continues even today, thanks to the supernovae of subsequent star generations. Based on its chemical composition, our own Sun is believed to be the outcome of the evolution of several generations of stars. Its atmosphere contains a large variety of chemical elements.</p>
<p>If the aim of spectroscopic analysis is to identify very old stars, astrophysicists will look for chemical signatures that indicate the presence of only a few elements besides hydrogen, helium and lithium, especially carbon and nitrogen.</p>
<p>When they detect this composition, they can be practically certain the stars in question are so ancient that they belong to the second generation. “They may be as old as the Milky Way or even older,” Placco said.</p>
<p>The first study of the subject, published in 1991, attempted to estimate the distances and ages of 150 stars. It was unsuccessful as far as age was concerned, as the quality of the available data was insufficient. “Twenty-four years later, Santucci set out to perform another selection of stars,” Placco said.</p>
<p>To accomplish this, Santucci plunged into the gigantic database built up by the US-based Sloan Digital Sky Survey (SDSS), which aims to catalog hundreds of thousands of distant galaxies. “The Milky Way’s stars are on the way to these far-off galaxies, so an important by-product of the SDSS was the discovery of many thousands of stars in its halo,” Santucci said.</p>
<p>Searching the database, he identified 4,700 stars that could be studied to produce the first map of the oldest stars in the Milky Way. “There’s far more data available today, and its quality is much better than in 1991,” Placco said.</p>
<p>The map of the Milky Way’s two hemispheres (above and below the galactic disk) showed that the oldest stars were formed either before or at the same time as “the gravitational collapse of the immense gas cloud from which the stars at the center of the galaxy were formed,” Santucci said. “We can tell from our map that the objects closest to the galactic bulge are approximately 13 billion years old.”</p>
<p>Since then, stars have continued to emerge in chronological order from the center outwards. “Our study confirmed longstanding theories of galactic evolution according to which stars at the center are the oldest, so they become steadily younger as you move out to the halo. No one had shown that before,” Santucci said.</p>
<p>The authors did not foresee this result, and they are writing a new paper for submission to the journal <em>Science</em>. “It’s a much larger and more precise map based on a sample of 100,000 stars,” Santucci said.</p>
<p>Evidence of their study’s originality can be found in the work of their academic competitors. At a meeting of the American Astronomical Society (AAS) held in Kissimmee, Florida, in the first week of January, scientists presented a map of the Milky Way showing the ages of 70,000 red giants.</p>
<p>The focus was on the galactic disk instead of the halo. The study confirmed expectations regarding the galaxy’s expansion: it started in the middle and grew outwards. The proof is the abundance of old stars in the middle of the disk, according to Melissa Ness from Germany’s Max Planck Institute for Astronomy, lead author of the study.</p>
<p>The article “Chronography of the Milky Way's Halo System with Field Blue Horizontal-Branch Stars” (doi:10.1088/2041-8205/813/1/L16) by Rafael Santucci, Vinicius Placco et al. can be read at <a href="http://iopscience.iop.org/article/10.1088/2041-8205/813/1/L16" target="_blank"><b>http://iopscience.iop.org/article/10.1088/2041-8205/813/1/L16</b></a>.</p>
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