An international team of scientists, including Northwestern University astrophysicists, has captured an unprecedented view of a dying star's remains in the act of being recycled.
A striking new image of the Helix Nebula reveals a star's fragments plowing through surrounding gas. By studying the image, astronomers traced the star's debris as it is stripped, broken apart and gradually mixed into interstellar space. From there, the Helix's material may one day become part of new clouds, new stars and perhaps even new planets.
The finding captures a long-elusive step in the cosmic recycling process that supplies raw materials for the formation of new planets and stars. It also offers a preview into the potential fate of our Sun, which eventually will shed its outer layers and return some of its own material to the Milky Way.
The study was published today (Aug. 12) in the journal Nature.
"We are seeing material shed near the end of a star's life being broken apart and returned to the galaxy," said Pieter van Dokkum, the study's lead author. "That handoff - from recognizable stellar debris to the diffuse gas between the stars - has been very difficult to observe. Far in the future, the Sun will go through a similar process, and its material will enter the same cycle."
An expert in galaxy formation and evolution, van Dokkum is co-founder of the Dragonfly Focused Research Organization (FRO), the Sol Goldman Family Professor of Astronomy and Physics at Yale University and a visiting scholar at Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA). In January 2027, he will join Northwestern as the Kimberly Querrey Professor of Astrophysics at the Weinberg College of Arts and Sciences.
A familiar nebula, seen in a new way
Located roughly 650 light-years from Earth in the constellation Aquarius, the Helix Nebula is one of the closest and best-studied planetary nebulae. Resembling a giant human eye, its familiar bright ring formed from material expelled by a star near the end of its life. The remnant of that star, a white dwarf, lies at the nebula's center.
The new observations reach far beyond the nebula's bright innerworkings - extending to its extraordinarily faint outer halo. There, on the eastern side of the Helix, the scientists found a collection of 22 complete or partial arc-shaped, glowing shock waves. Called bow shocks, the waves resemble those that form in front of a boat moving through water. In the Helix Nebula, mostly invisible clumps of stellar debris dart rapidly through the thin gas among stars. The glowing bow shocks mark where those clumps collided with the surrounding gas.
"The shocks change dramatically with the distance from the central star," said study co-author Imad Pasha, a member of Dragonfly FRO and visiting scholar at CIERA. "Those nearer the center are large, thin and sharply defined. Farther out, they become smaller, fuzzier and increasingly fragmented."
According to the researchers, that progression shows that the clumps of stellar debris steadily erode as they travel. With this evidence, the team estimates that an individual fragment remains intact for just 10,000 years once it encounters surrounding gas. After that, its material is largely shredded and mixed into interstellar space.
Discovery hidden in a calibration image
The team did not initially observe the Helix Nebula in search of a new discovery but to calibrate a new telescope. Because the Helix is large, bright and extensively studied, the team selected it as a familiar target for testing MOTHRA (Modular Optical Telephoto Hyperspectral Robotic Array), which is under construction at the El Sauce Observatory in Chile.
When completed, MOTHRA will use 1,140 high-end telephoto lenses and specialized filters to detect the extremely faint glow of gas across large swaths of the sky. Astronomers captured the Helix observations when MOTHRA was still in the early stages of construction and not fully operating. Despite its limitations, MOTHRA was sensitive enough to reveal previously unseen structures - offering a glimpse into what the new telescope will be able to accomplish.
"We thought we were taking a calibration image of one of the best-known nebulae in the sky," said co-author Roberto Abraham, a member of the Dragonfly FRO and a professor of astronomy at the University of Toronto. "Instead, we found this extraordinary network of bow-shaped structures. It was immediately clear that the faint outer Helix was telling us a story that had largely been missed."
The development of MOTHRA and Dragonfly FRO have been made possible by the funding and continued strategic support of Alex Gerko, Founder and CEO of XTX Markets. Dragonfly FRO was launched in partnership with Convergent Research.
