AOP’s Dr. Andreas Sander has been involved in a scientific first unravelling the mysteries of rare stars!

Within the constellation Cygnus, an elderly star and its massive companion are having one last hurrah, flinging off mass at an incredible rate before they explode as supernovae and collapse into a black hole.

Now, researchers have mapped the elderly star’s orbit around its oversized and equally ancient partner. In a scientific first, they have also determined the dynamical mass of both stars that make up a binary system called Wolf-Rayet 133. This marks the first-ever visually observed orbit of a rare type of star called a Nitrogen-rich Wolf-Rayet (WN) star. The WN star in question is half of the starry dance duo in the WR 133 binary.

Location of this system in the cygnus constellation
Credit: Andreas Sander, background image: Digitized Sky Survey (STScI/NASA/CDS)

The WN star pirouettes around its partner star, an O9 supergiant, every 112.8 days – a relatively brief orbit, indicating that the two stars are close together, researchers reported. The WN star has 9.3 times more mass than our Sun, while the O9 supergiant is a whopping 22.6 times more massive, the team found.

Imagining the Early Universe

The research opens a new window to the distant past when stars and planets were first beginning to form.

Wolf-Rayet type stars, so named for the astronomers who discovered them in 1867, are massive stars near the end of their lives, said Dr. Noel Richardson, assistant professor of Physics and Astronomy at Embry-Riddle. They’re very hot, a million times more luminous than the Sun, and stellar winds have stripped off their hydrogen envelopes. That has made it difficult to measure their mass – a vital step toward modeling the evolution of stars – until now.

Because the pair of stars in the WR 133 binary are tightly coupled, they’ve likely exchanged mass, Richardson noted. “In the early universe, we think most stars were very, very massive and they probably exploded early on,” he said.

Dr. Noel Richardson, assistant professor of Physics and Astronomy at Embry-Riddle. Credit: Embry-Riddle/Jason Kadah

“When these types of binary stars are close enough, they can transfer mass to each other, possibly kicking up space dust, which is necessary for the formation of stars and planets. If they’re not close enough to transfer mass, they’re still whipping up a huge wind that shoots material into the cosmos, and that can also allow stars and planets to form. This is why we want to know more about this rare type of star.”

`A Blue Marble in Space’

Armagh Observatory & Planetarium are one of the many institutions involved in the project. Our own Andreas A.C. Sander said the team’s findings were somewhat surprising and will prompt researchers to rethink key assumptions. “The results are very interesting as they yield a lower mass than expected for such a star,” Sander noted.

“While this might sound like a detail, it will change our perception of the Black Holes resulting from collapsing Wolf-Rayet stars, a crucial ingredient in the astrophysical context of gravitational wave events.”

AOP’s Dr. Andreas Sander

Gail Schaefer of the CHARA Array noted that Richardson’s observations using the Georgia State University (GSU) telescopes on Mount Wilson – made possible through an open-access program at the facility – “will help improve our understanding of how binary interactions impact the evolution of these massive stars.”

Astronomer Jason Aufdenberg of Embry-Riddle, who has also used the CHARA Array, said that “the kind of work Noel is doing, establishing orbits, is very important because they can get the masses of these things. Knowing about these very hot stars, how many there were and their luminosities is all part of understanding what happened in our universe after the Big Bang.”

The research project included collaborators from many parts of the world and multiple institutions. In addition to Embry-Riddle, GSU’s Chara Array, the Armagh Observatory & Planetarium and the Keck Observatory, the work involved Michigan State University, the University of Arizona, the University of Denver, the University of Exeter, the Institute of Astronomy in Belgium and the Institute of Planetology and Astrophysics in France.

The Astrophysical Journal Letters paper is entitled “The First Dynamical Mass Determination of a Nitrogen-rich Wolf-Rayet Visual and Spectroscopic Orbit.”

Journal Reference:

The First Dynamical Mass Determination of a Nitrogen-rich Wolf-Rayet Star using a Combined Visual and Spectroscopic Orbit – NASA/ADS (


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