An international team led by Juan Diego Soler at the University of Vienna used two of the world's most powerful radio telescopes to uncover previously hidden structures within the Orion Nebula. The project produced the sharpest maps ever made of neutral hydrogen in that region of massive star formation. The findings expose the complex relationship of star-forming regions with their environment and suggest that the Orion Nebula has been shaped by multiple episodes of stellar feedback rather than a single expanding bubble. The study has currently been published in the journal Astronomy & Astrophysics.

The Orion Nebula is one of the most familiar objects in the night sky. Visible even to the naked eye, it has been studied for centuries and observed with nearly every modern astronomical instrument. Yet astronomers have now discovered that one of its most important components had remained largely hidden.

Using cutting edge some of the world's most powerful radio telescopes, an international team led by Juan Diego Soler at the University of Vienna has produced the sharpest maps ever made of neutral atomic hydrogen in the Orion Nebula. The observations reveal giant expanding shells, previously unseen cavities, and mysterious elongated structures surrounding the nearest region of massive star formation to Earth.

Hydrogen is the most abundant element in the Universe. In its neutral atomic form, it emits faint radio waves at a wavelength of 21 centimeters, allowing astronomers to trace otherwise invisible gas between the stars. To detect this emission in unprecedented detail, the researchers combined observations from the Karl G. Jansky Very Large Array (VLA) in the United States and the Five-hundred-meter Aperture Spherical Radio Telescope (FAST) in China.

Complex shell structure

Previous studies suggested that the shell surrounding Orion contains around one thousand times the mass of the Sun. The new hydrogen observations indicate a mass nearly ten times lower. "Measuring mass is fundamental," Soler says, "because it tells us about the efficiency of these newly formed stars shaping their environment with wind and radiation."

The new maps also reveal what appears to be a second expanding cavity inside the main shell, along with an elongated "protrusion" of atomic gas extending roughly four light-years outward from the bubble. These structures suggest that the Orion Nebula has been shaped by multiple episodes of stellar feedback rather than a single expanding bubble.

The complexity revealed by these observations challenges the current understanding of star formation, explains Daniel Seifried, co-author of the publication and researcher at the University of Cologne, "These stunning observations serve as a reference for many modern astrophysical simulations investigating the evolution of gas and stars in the Milky Way. These are the kind of images that challenge the theoretical models and numerical simulations that we use to understand how massive stars affect their immediate surroundings."

"This study is an exciting demonstration of the power of latest-generation radio telescopes to uncover new pieces to the star formation puzzle", say Co-author Claire Murray from the Space Telescope Science Institute (STScI) in Baltimore, USA.

"Orion is only the beginning. Our newly developed methods show how future interferometers will reveal the hidden structure and dynamics of the interstellar medium—even in regions that astronomers already believed they understood well," explains first author Soler from the University of Vienna.

Summary

• Hidden structures in the Orion Nebula discovered: high-resolution maps of neutral atomic hydrogen reveal expanding shells, cavities, and elongated gas structures.
• A combination of VLA and FAST observations provides the sharpest 21 cm maps of neutral atomic hydrogen to date.
• The mass of the surrounding shell is nearly ten times lower than assumed in earlier studies.
• The nebula was likely shaped by multiple episodes of stellar feedback, rather than by a single expanding bubble.
• The results challenge current star-formation models and motivate improved simulations.

The study is the first scientific result from the NeAtHood project, an international effort based at the University of Vienna and funded by the Austrian Science Fund (FWF). The project aims to map atomic hydrogen across nearby star-forming regions and connect the different phases of the interstellar medium — the diffuse gas and dust that fills galaxies and gives birth to stars.

Additional notes:

The Jansky Very Large Array (VLA) is located in Socorro, New Mexico, United States, and is operated by the United States' National Radio Astronomy Observatory (NRAO)
The Five-hundred-meter Aperture Spherical Telescope (FAST; 五百米口径球面射电望远镜) is located in Pingtang County, Guizhou, southwestern China, and is operated by the National Astronomical Observatories of the Chinese Academy of Sciences (NAOC).