A dying star offers a rare glimpse of how the universe recycles its raw materials

Astronomers have captured an unprecedented image of a cosmic recycling project in progress—fragments of a dying star being busted up, stripped and absorbed into interstellar space.

A research team led by Yale astronomer Pieter van Dokkum made the discovery while taking images of the Helix Nebula, a much-studied planetary nebula located about 650 light-years from Earth. The researchers presented their findings in a new study published 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 van Dokkum, the Sol Goldman Family Professor of Astronomy and professor of physics in Yale’s Faculty of Arts and Sciences and lead author of the study. He is also a member of the Dragonfly Focused Research Organization, a new type of nonprofit group that aims to build high-impact scientific infrastructure at the pace of a tech startup.

“That handoff—from recognizable stellar debris to the diffuse gas between the stars—has been very difficult to observe,” van Dokkum said. “Far in the future, the sun will go through a similar process, and its material will enter the same cycle.”

The researchers made the discovery by accident. They had chosen the Helix Nebula as a good spot to calibrate their new MOTHRA telescope, which combines images from hundreds of telephoto lenses to detect and study diffuse ionized gas located between galaxies. MOTHRA (the Modular Optical Telephoto Hyperspectral Robotic Array) is being constructed at the El Sauce Observatory in Chile’s Rio Hurtado Valley and will have an array of 1,140 telephoto lenses when completed.

“We thought we were taking a calibration image of one of the best-known nebulae in the sky,” said study co-author Roberto Abraham, of the Dragonfly Focused Research Organization and 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 Helix Nebula is characterized by a bright ring formed from material expelled by a dying white dwarf star at the center. The new observations reach far beyond that bright inner nebula, into an extraordinarily faint outer halo. There, on the eastern side of the Helix Nebula, the researchers found 22 complete or partial “bow shocks.”

A bow shock is similar in shape to the wave that forms in front of a boat moving through water. In the Helix Nebula, mostly invisible clumps of stellar debris are moving rapidly through the thin gas between stars. The glowing arcs mark where those clumps collide with the surrounding gas.

The shocks change strikingly with distance from the central star. Those nearer the center are large, thin and sharply defined. Farther out, they become smaller, fuzzier and increasingly fragmented.

The researchers interpret that progression as evidence that the clumps are being steadily eroded as they travel. They estimate that an individual fragment remains coherent for only about 10,000 years once it is exposed to the surrounding gas. After that, its material is largely shredded and mixed into interstellar space.

“This is only the beginning of what we’ll be able to see with MOTHRA,” van Dokkum said.

MOTHRA stands as a major expansion of concepts that van Dokkum and Abraham first incorporated into their earlier Dragonfly Telescope. That telescope, located in the mountains of New Mexico, combines images from multiple lenses to detect the dim glow of faint stars and galaxies.

MOTHRA will dramatically increase the number of lenses being used, along with new filters and computational power.

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