Astronomers pinpoint how Milky Way Galaxy was formed

Using colors to identify the approximate ages of more than 130,000 stars in the Milky Way’s halo, astronomers have produced the clearest picture yet of how our galaxy formed more than 13.5 billion years ago.

The astronomers, based at the University of Notre Dame, are part of JINA-CEE – the Joint Institute for Nuclear Astrophysics – Center for the Evolution of the Elements – which is headquartered at Michigan State University.

The researchers’ findings are published in the journal Nature Physics, which includes a chronographic map that supports a model of galaxy formation. Click on

“We haven’t previously known much about the age of the most ancient component of the Milky Way, which is the halo system,” said astrophysicist Daniela Carollo, research assistant professor in the Notre Dame Department of Physics. “But now we have demonstrated conclusively for the first time that ancient stars are in the center of the galaxy and the younger stars are found at longer distances.”

The new results confirm predictions of models that suggest the Milky Way formed by merging and accretion of small mini-halos containing stars and gas. In these models the center of the galaxy forms first and therefore contains the oldest stars, while younger stars and small galaxies that were drawn in by gravity later, over billions of years, are found farther from the center.

Using data from the Sloan Digital Sky Survey – of which JINA-CEE is a partner – the scientists identified more than 130,000 blue horizontal-branch stars, which burn helium in their cores, and exhibit different colors based on their ages. The colors, when the stars are at that stage, are directly related to the amount of time that a star has been alive, so the age can be estimated.

MSU is one of four core institutions comprising JINA-CEE, a National Science Foundation Physics Frontiers Center dedicated to interdisciplinary research at the interface of nuclear physics and astrophysics.

JINA-CEE conducts research in nuclear astrophysics that is carried out at MSU’s National Superconducting Cyclotron Laboratory, Department of Physics and Astronomy, Department of Chemistry, and Department of Computational Math, Science, and Engineering, as well as its 20 associated institutions worldwide.

These new research results will allow JINA-CEE collaborators to develop improved computer simulations of the formation and evolution of Milky Way-like galaxies. These models provide a basis for stellar astrophysicists and nuclear scientists to implement data from nuclear experiments at facilities like NSCL, Notre Dame, Argonne National Laboratory, and, in the future, the MSU-based Facility for Rare Isotope Beams.

“This result gives critical clues to the formation of the Milky Way’s stellar halo, and presents us with the only way to directly determine when the Milky Way’s progenitors formed,” said Brian O’Shea, professor in computational mathematics, science and engineering, physics and astronomy, and the NSCL, who also is a member of JINA-CEE. “It will be invaluable for informing our computational models of galaxy evolution that we are developing here at MSU."

The Notre Dame research team included Carollo; Timothy Beers, Notre Dame chair of astrophysics; and research assistant professor Vinicius Placco.

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