University of Michigan astronomers begin with the study of massive stars

As soon as the first stars in the early universe were born, metal elements began forming. As the universe has evolved, metal content in the universe has grown, driven by stellar winds and supernova explosions from massive stars.

It has become clear over recent decades that the amount of metallic elements, or metallicity, changes the fundamental physical properties of stars, including temperature, luminosity, and wind power. New research provides observations of individual massive stars that indicate that the early, low-metallicity universe is fundamentally different from our own galaxy.

Understanding the metallicity of galaxies is important because it tells astronomers about a galaxy’s evolution as well as the evolution of our universe, says University of Michigan astronomer Sally Oey.

“Metals are elements heavier than hydrogen and helium and are byproducts of stars. So low-metallicity environments are relatively pristine whereas high-metallicity environments are polluted and therefore presumably older or more evolved, having had more stellar generations,” she said. “Thus, the low-metallicity universe corresponds to early cosmic times, closer to the Big Bang, and high-metallicity is more like here and now in our Milky Way Galaxy.”

Oey is part of a team called the XShootU consortium, a worldwide collaboration of astronomers that has launched a groundbreaking series of studies into the most massive stars in our local dwarf galaxies, the Small and Large Magellanic Clouds.

Now, the group has released its first study, published in Astronomy & Astrophysics. The paper is the first release of a benchmark dataset combining ground-based data with ultraviolet data from the Hubble Space Telescope via its UV Legacy Library of Young Stars as Essential Standards (ULLYSES) Legacy project. This is the largest-ever legacy program carried out by Hubble through the Space Telescope Science Institute. The data were gathered during 1,000 Hubble orbits, observing 250 massive stars as well as newly formed stars under the ULLYSES program.

Oey led an advisory committee that conceived of and recommended the program to the Space Telescope Science Institute. U-M astronomer Nuria Calvet was also a member of the advisory committee and science collaboration for the young star side of the program.

The XSHOOTU consortium, led by Jorick Vink of Armagh Observatory and Planetarium in Ireland, combined the Hubble data of massive stars with ground-based optical data observed using the European Southern Observatory’s Very Large Telescope in Chile.

“We now have a very high quality dataset that spans both UV and optical ranges that will be foundational in understanding stars, galaxies, and the universe at low metallicity,” Oey said. “This is critical since it applies to the universe at early cosmic times and also to nearby energetic starbursts in dwarf galaxies, the origin of binary black holes that cause gravitational waves, kilonovae and a whole host of extreme explosions.”