Scientists investigate oldest stars in the Milky Way

Remnants of the universe's infancy that are concentrated in the galaxy's bulge are estimated to be more than 13 billion years old.

By José Tadeu Arantes

Agência FAPESP – The first structure to form in a spiral galaxy is a bulge, according to most models as well as to evidence provided by recent observations, so it must be at least as old as the halo that surrounds it. The bulge in our own galaxy, the Milky Way, contains its oldest stars, which are more than 13 billion years old.

These very old stars are considered second-generation stars, as first-generation stars must have disappeared a long time ago, exploding and enriching the universe with chemical elements synthesized inside them. There is great interest in studying second-generation stars because they are remnants of the universe’s infancy.

This research is one of the objectives of the Thematic Project entitled “Chemical evolution and galactic and extragalactic stellar populations, by means of spectroscopy and imaging,” led by Beatriz Barbuy, Full Professor at the University of São Paulo’s Institute of Astronomy, Geophysics & Atmospheric Sciences (IAG-USP) in Brazil. Begun in 2011 and scheduled for completion this year, the project is supported by FAPESP.

“First-generation stars are believed to have been very massive, with relatively short lives, rapidly exploding and ejecting chemical elements. The stars that formed after that, which had normal masses, must have absorbed these chemical elements from the first generation. The metal-poor second-generation stars are our principal object of study,” Barbuy told Agência FAPESP.

Because the bulge is assumed to be the oldest structure in a galaxy, the search for old stars focuses on this region, but second-generation objects are also sought in the halo, which contains stars that are very poor in metals. Silvia Cristina Fernandes Rossi, a researcher affiliated with IAG-USP and also a member of the Thematic Project group, has systematically studied these halo stars.

As a rule of thumb, metallicity can be considered a marker of a star’s longevity. The fact that a star is metal-rich indicates that it incorporated materials synthesized by the previous generation.

“But this isn’t a mechanistic rule because the high concentration of stars in the galactic bulge makes enrichment with chemical elements occur much faster there than in the halo,” Barbuy said.

The Milky Way’s bulge is indeed extremely dense. This spheroidal region has a radius of only 10,000 light-years, but it contains some 10¹⁰ solar masses, whereas the Milky Way’s total mass, estimated at 5.8×10¹¹ solar masses, is spread across an enormous disk with a diameter of some 100,000 light-years.

“Stars form ten times faster in the bulge than in the vicinity of the Sun, so the interstellar medium is enriched very quickly and there are few metal-poor stars in the region,” Barbuy said.

“In partnership with Cristina Chiappini, currently at the Leibniz Institute for Astrophysics Potsdam in Germany, and other researchers, we’re developing a chemical evolution model to show that rapid enrichment led to a very old bulge population with high metallicity. The stars in this population may be the oldest in the Galaxy.”

“In 2011,” she added, “we published an article in Nature sustaining the hypothesis that these old stars were enriched by fast-rotating massive stars, which we called ‘spinstars.’ And recently, with Cesar Henrique Siqueira Mello Junior, we obtained confirmation of this hypothesis.”

Hypernovae

Spinstars are thought to have formed shortly after the Big Bang, so they are approximately 13.7 billion years old. A spinstar has a mass eight or more times that of the Sun, and it spins around its own axis at a surface speed of 1.8 million kilometers per hour (kph), or 250 times that of the Sun, which is 7,200 kph.

Spinstars are thought to have lasted only 30 million years, a very short life span on the stellar timescale. When they finally exploded, they ejected heavy materials into the interstellar medium, increasing its metallicity.

“Other evidence obtained by myself, Amâncio Friaça and Carlos Roberto da Silveira points to the possibility that bulge stars may have been enriched by hypernovae, stars that explode with ten times more energy than supernovae. According to Japanese researcher Kenichi Nomoto and collaborators, the hypernova’s very high explosive energy derives from the fact that it has a fast-rotating dark hole at its center,” Barbuy said.

Spinstar and hypernova may merely be different names for the same object, although this has yet to be verified. Friaça, Silveira and Barbuy used Nomoto’s model to explain the zinc-iron ratios of 56 stars in the galactic bulge. A report of their study has been submitted for publication in the journal Astronomy & Astrophysics.