Star dust

Distribution of chemical elements reveals details about the structure and evolution of the Milky Way

ROGER SMITH/AURA/NOAO/NSFWhite as milk: Milky Way segment (on the right), as seen from the Inter-American Observatory of Cerro Tololo, in the Chilean AndesROGER SMITH/AURA/NOAO/NSF

In these Spring months, if you are far from urban centers, you can see a vast bright band over the horizon just after sunset. This luminous streak in the sky is part of the Milky Way, the galaxy that is home to the Sun and the planets that revolve around it. With almost 200 billion stars, it is shaped like an immense octopus, spinning like a cosmic whirlpool. But it was not always like that. When the Universe began 14 billion years ago, the Milky Way was no more than a giant cloud of gas that, little by little, became denser in certain places, thus generating the stars and planets. Despite the advance of astronomy in the last century and the production of increasingly powerful telescopes, astronomers all over the world are still trying to understand how this transformation occurred and how the Milky Way achieved its current form, with three very different regions: the bulge, a central zone in the shape of a globe where hundreds of millions of stars are concentrated; a vast flattened disc of stars, gas and dust; and a third spiral structure that surrounds the other two, the halo, where stars are rarer and there is little gas or dust.

In an attempt to understand how these foundations of the galaxy originated, the team of astronomer Walter Junqueira Maciel, from the University of São Paulo (USP), has been investigating the chemical composition of different points of the Milk Way for more than ten years. Over the last few years the group has reached conclusions that may not explain everything, but they give us a more precise idea of how these three structures were formed and have evolved since they appeared, about 1 billion years after the Big Bang, the explosion which created the Universe.

How did this evolution take place? “In principle, quite differently, for each of these three regions of the galaxy”, says Maciel. The scattered stars that are now part of the halo formed very quickly about 13 billion years ago, extinguishing almost all the gas from around the Milky Way. Almost at the same time the bulge began to form. Hundreds of times smaller than the halo but with a very much higher gas density, the bulge possibly had two periods of star formation: the first that lasted some millions of years and the second that lasted very much longer. It was only some billions of years later that the gas, from which the disc originated, started getting denser, the researchers concluded from their observations of the galaxy’s chemical composition.

The reason why the researchers from the Institute of Astronomy, Geophysics and Atmospheric Sciences (IAG) from USP decided to measure the concentration and variety of different chemical elements is simple. Everything that exists in the Cosmos and can be observed (planets, stars, clouds of gas, dust, and living beings) is formed from different combinations of the 116 chemical elements that are known and organized in the periodic table taught in school chemistry lessons. These elements did not appear all at the same time. In the first moments after the Big Bang, hydrogen atoms were formed; hydrogen is the most abundant chemical element in nature and also the simplest, consisting of one positively charged particle (proton) and one negatively charged particle (electron). This original explosion also generated part of the helium, comprising two protons, two electrons and two particles with no electrical charge (neutrons), and an infinitely small amount of lithium 7 (three protons, four neutrons and three electrons). The other chemical elements were born very slowly, mostly through nuclear fusion, the forced recombination of protons that only occurs at extremely high pressures and temperatures, like those reached inside stars or when they explode.

As in other galaxies, in the Milky Way there are also hundreds of billions of stars that function like nuclear reactors, which transform hydrogen and helium atoms into heavier elements, internally. The consequence of this uninterrupted process is the progressive increase in the number of these elements in the galaxy, producing raw materials for more stars, planets and the life that some of them may harbor. Therefore, if the quantity of these heavier elements at different points in the galaxy and at different moments in their life is known, one can discover how the composition and form of the Milky Way evolved over time, since the speed at which stars are born and die is known.

Therefore, in the saga of reconstructing the Milky Way’s past, Maciel needed to find more suitable sources of heavy chemical elements among the 200 billion stars in our galaxy, which in the sky are easily confused with those from other nearby galaxies. These chemical elements abound in the planetary nebulae. Of rare beauty, these objects, which may be shaped like an eye, an hour glass or a sting-ray, are the record of the dying moments of a star that has transformed all the hydrogen in its nucleus into heavier chemical elements.

VLT/ESOFar from home: a spiral galaxy similar to the Milky Way, the home of the Solar System VLT/ESO

Planetary nebulae (so called by English astronomer William Herschel because through a telescope they are reminiscent of the planet Uranus) have nothing to do with planets. They are important because they carry information about the Universe’s distant past. After burning its stock of hydrogen for periods that generally range from 1 billion to 10 billion years, stars such as the Sun swell rapidly and expel their outer layers into the interstellar environment, releasing a cloud of gas and dust that is rich in carbon, nitrogen and oxygen. Generated from stars whose mass is similar to or a little greater than the Sun’s, planetary nebulae contain elements produced by the star that formed them. The other elements they expel into space were produced by the previous generation of stars that lived up to 10 billion years earlier. “To analyze the composition of planetary nebulae is to look at the galaxy’s distant past, close to when it started to form”, says Gaucho astronomer Roberto Dias da Costa, from IAG, who since 1987 has been working in partnership with Maciel.

Using the 1.60 meter telescope from the National Laboratory of Astrophysics in Brasópolis, Minas Gerais, and data from catalogues, Maciel and the astronomer Hélio Rocha Pinto, ten years ago, started looking for these chemical element plants in the neighborhood of the Solar System, located in the galactic disk, a little more than half way between the center and the outer edge. More recently, with the help of a telescope from the European Southern Observatory (ESO) in Chile, Maciel, Costa and astronomers Monica Uchida, André Escudero, Leonardo Lago and Cíntia Quireza expanded this search to include all of the disk region of the Milky Way that can be seen from the Southern hemisphere. They determined, with fairly high precision, the concentration of the chemical elements oxygen, sulfur, neon and argon in 240 planetary nebulae of the nearly 2000 ones known in the galaxy, which spread out from the bulge almost as far as the outer edge of the disk.

However, the chemical concentrations or chemical abundance revealed by the nebulae refer to periods that range from 10 billion to 2 billion years ago. To find out what they are like today, the IAG team compared the planetary nebulae data with those from other structures in the galaxy, called H II regions. “From the physical point of view, planetary nebulae and H II regions are very similar, since both are clouds of gas warmed by stars”, says Maciel. But that is where the similarity ends. 20 to 40 times larger than planetary nebulae, the H II regions harbor dozens of stars in formation and show what the chemical composition of the galaxy has been like over the last few million years, which are recent times for astronomers. Analyzed together, the information about planetary nebulae and H II regions reveals details about the galaxy’s chemical evolution that have been presented in a series of articles published over the last few years, several of them in the journal Astronomy and Astrophysics.

Escudero, Costa and Maciel, in evaluating the composition of the almost 500 planetary nebulae in the bulge (around 100 observed by them and 400 by other researchers) saw that in this structure, whose diameter corresponds to a tenth of the length of the galaxy, there were families of stars that were much more varied than had been previously imagined, of very different ages, ranging from those with a mass dozens of times greater than that of the Sun and with life cycles of a few million years, to stars with very low mass and slow evolution, almost contemporaries of the start of the Universe.

Skilled in IT, Escudero developed a computer program capable of simulating how this region of the galaxy might have developed. The scenario that best corresponded to the concentrations of chemical elements observed is one indicating that the bulge developed in two main stages. Initially, there was a rapid collapse of gas, which in a few million years gave rise to a large number of high-mass stars. According to calculations, some of these stars evolved quickly and exploded, releasing heavy chemical elements towards the halo and the galactic disk, which was still in an embryonic stage. Between 1 billion and 3 billion years later, part of this ejected material was attracted back to the bulge, feeding the slower formation of a new generation of stars that are richer in chemical elements than the prior generation, suggest the researchers in an article to be published shortly in the Monthly Notices of the Royal Astronomical Society.

“But this model of the bulge doesn’t tell us how much gas was ejected from the bulge, or how long this phase lasted. It will only be possible to define this with more accurate data and more realistic models”, writes Escudero. Maciel hopes to obtain more accurate data as soon as the device used for identifying the chemical elements (spectrograph) of the Soar telescope in Chile starts working.

JPL/NASAHelix nebula: in blue, dust cloud expelled by a star in the final stage of its life JPL/NASA

In the disk, development seems to have been far slower, but continuous. Monica, Costa and Maciel compared the concentration of heavy chemical elements of planetary nebulae with different evolution times – 10 billion, 6 billion and 1 billion years. They found that the concentration of heavier elements drops progressively from the center to the outer part of the galaxy. Additionally, the rate at which this reduction occurs changed over time: it was stronger in the past than more recently. Based on what happened with this group of stars, the astronomers believe that the disk was formed from the center out. This idea ties in with observation of nebulae. The oldest are closer to the center, while the youngest are found close to the bulge, but also very far away from it. “In this region that is further from the galaxy’s center, the formation of stars must have been slower”, comments Costa.

These data corroborate the forecasts of two mathematical evolution models of the Milky Way – one developed by French astronomers, which attributes to the galaxy’s chemical evolution a greater weight in determining its current structure, and the other created by Italians, according to whom the Milky Way achieved its shape as a result of the movement of stars, planets and clouds of gas and dust. All are in agreement, but only regarding the neighborhood of the Solar System. The main doubt is how the disk behaves from there on, outward, in the direction of the galaxy’s outer limits. “We need information about more planetary nebulae from this region, which can be seen more easily from the Northern hemisphere”, comments Costa. Until these data become available Monica Uchida will try to understand what happens in this region of the galaxy using another strategy. In 2008 she will spend some time with Francesca Matteucci’s team in Trieste, Italy, studying the chemical composition of spiral galaxies similar to the Milky Way. From the observation of other galaxies she will try to discover how the Milky Way reached its current stage.

The Project
Photo-ionized nebulae, stars and the chemical evolution of galaxies (nº 10/18835-3); Modality: Thematic Project; Coordinator: Walter Junqueira Maciel – IAG/USP; Investment: R$ 95,194.57