Scientists map chronological distribution of stars in Milky Way

Some 130,000 blue stars in the Galaxy's halo were observed by researchers at the University of São Paulo, in collaboration with colleagues in the United States. The results were published in Nature Physics.

By José Tadeu Arantes  |  Agência FAPESP – An article published in late 2015 in The Astrophysical Journal Letters by a group of researchers at the University of São Paulo’s Institute of Astronomy, Geophysics & Atmospheric Sciences (IAG-USP) in Brazil, in collaboration with colleagues in the United States, shows that the Milky Way formed stars from the inside out. After the gravitational collapse that produced stars in the central region, the process moved gradually toward the edge, reaching the outer extremities of the Galactic halo.

The same group has now published a new chapter of their research story in Nature Physics, entitled “The age structure of the Milky Way's halo”.

The authors are Rafael Miloni Santucci and Silvia Cristina Fernandes Rossi at IAG-USP, Vinicius Moris Placco at the University of Notre Dame, and other researchers in the US. In Brazil, their work is mainly linked to the Thematic Project “MOSAIC: the multi-object spectrograph for the ESO extremely large telescope”, with Rossi as principal investigator and supported by FAPESP. Rossi, an associate professor with habilitation, supervised Placco’s PhD and is currently supervising Santucci’s.

The new study used a database almost 30 times more robust than the previous one. Instead of 4,700 stars, some 130,000 were considered. “The abundance of points enabled us to produce a detailed map showing the chronological distribution of the stars in the Galaxy,” Rossi told Agência FAPESP.

The 130,000-odd stars that have now been mapped are located in the Milky Way’s halo and belong to the class of blue horizontal branch (BHB) stars. They are old stars at an advanced stage of their evolution, in which intense and constant brightness is generated by the fusion of helium into carbon. Our own Sun, which is at a much younger stage of the evolutionary cycle, belongs to a different class known as main sequence (MS) stars, which are powered by the fusion of hydrogen into helium in their cores – an earlier stage of the thermonuclear fusion process.

It should be stressed that the terms “main sequence” and “horizontal branch” have nothing to do with stars’ spatial positions. They refer to their positions in the Hertzsprung-Russell Diagram (HRD), created circa 1910 by Danish chemist and astronomer Ejnar Hertzsprung (1873-1967) and US astronomer Henry Norris Russell (1877-1957) to classify stars into evolutionary stages. The diagram plots the position of each star on a graph measuring brightness against color, which indicates the star’s effective temperature. The horizontal branch corresponds to a stage that accounts for about 10% of a star’s lifetime.

A burning question that has long remained unanswered by researchers who study the Milky Way is whether the Galaxy was formed from the outside in or from the inside out. Two competing scenarios have been proposed. According to the “monolithic scenario,” a galaxy is formed by the sudden gravitational collapse of a giant gas cloud, which gives rise to a huge number of protostars at the same time. In the “hierarchical scenario,” the starting point is a relatively small galaxy, which grows larger as even smaller galaxies are attracted by its gravitational force and merge with it.

“Our study doesn’t confirm either scenario exclusively, but it suggests the large-scale formation of objects from the inside out,” Santucci said. “In other words, gravitational force initially collapses the gas located at the center, giving rise to a first generation of stars, and over a period of some 3 billion years, this stellar formation extends to the most peripheral regions.”

The sample used, which encompassed stars spread throughout all parts of the halo, was extracted from the Sloan Digital Sky Survey (SDSS), a public database of roughly 2 million galaxies. “From all the stars recorded in this collection, we selected those that had previously been identified as being in the evolutionary stage of interest,” Rossi said. “We began our analysis from there.”

“When we measured the objects’ colors and the distances between them, we found that the bluest BHB stars in the halo were predominantly located in the central region and that their color became slightly redder as their distance from the Galaxy’s center increased,” Santucci said. “The shift is very subtle, but our color measurements using special filters show it exists.”

Color variation

Endeavoring to explain this very subtle color variation, the researchers ruled out the hypotheses raised by the literature, such as the existence of dust in the interstellar medium or different chemical compositions of the stars.

The redder color also has nothing to do with the Doppler effect, which causes a blue shift when a star is approaching the observer and a red shift when it is receding, because this phenomenon is relevant only at far higher speeds of travel, as in the case of stars in other galaxies – but not in the case of the Milky Way.

“The only factor left to explain the color variation we observed was the age of the object,” Santucci said. “In this stage of their evolution only, the redder these stars are, the younger they are.”

The bluer stars are in or near the center, while the redder stars are in or near the periphery, hence the suggestion that the Milky Way expanded from the inside out.

“Let me stress yet again that the association between redness and youth is valid only for the evolutionary stage in question here,” Rossi said. “Redder stars are usually older, but not in the horizontal branch, which is the only stage in the life of a star where redness indicates relative youth. Our hypothesis is that the mass of these objects varies slightly over time, so that younger objects have slightly less mass, and this explains why they emit redder light.”

Age and mass

A basic principle of stellar evolution is that the fate of a star depends on its mass. Stars with large and very large masses remain in the main sequence for less time, during which hydrogen fuses into helium, and move sooner into the horizontal branch, where helium fuses into carbon. They also spend less time in this second stage of their evolution, because the thermonuclear reactions in their core are much more intense.

The Brazilian research group produced a 3D animation to help visualize the spatial distribution of stars according to their age (www3.nd.edu/~vplacco/map). Older and therefore bluer BHB stars are seen in the central region of the Galaxy and are estimated to be about 12 billion years old. BHB stars farther from the center are younger and therefore become gradually redder. At the edge of the halo, their age is estimated to be 9.5 billion years. These age estimates are valid only for BHB stars. Other classes of stars can be far younger – our Sun’s age is estimated to be 4.6 billion years, for example.

“A surprising discovery, which can easily be seen in the map and in the animation, and which we were able to make thanks to the much bigger database available, is that the oldest objects are located in a vast region around the Galactic nucleus, extending as far as the sector of the halo near the Sun – that’s about 28,000 light-years or 8.5 kiloparsecs from the center of the Milky Way,” Santucci said.

This ancient region contains a precious archive of old stars that can offer up important information for researchers that can help them to understand the chemical composition of the young Universe and its evolution through time.

“It also shows us that we can find stars that are very old, and therefore metal-poor, even in regions close to the Solar System. In other words, we can include bright objects in any search for the first celestial bodies of the Universe,” Santucci said. “This kind of search is also part of our group’s focus, and the results so far have been promising.”

The article “The age structure of the Milky Way's halo,” published in Nature Physics, can be retrieved from nature.com/nphys/journal/vaop/ncurrent/full/nphys3874.html.