The plan was deliberately dull. Point a new telescope at the Helix Nebula, take a test shot, check the lenses were focusing properly, move on. The Helix is one of the most photographed objects in the sky, a glowing ring about 650 light-years away in the constellation Aquarius, close and bright and mapped in fine detail since the days of glass plates. You do not aim a half-built instrument at a target like that expecting a discovery. You aim at it because you already know exactly what it should look like.
The test shot did not cooperate. Out in the faint outer halo, on the eastern edge where the famous eye-shaped ring fades to almost nothing, the image showed 22 glowing arcs nobody had ever catalogued. Curved, luminous, lined up like the ripples peeling off the front of a boat. They turned out to be the shredded remains of a dead star ploughing into the thin gas between the stars, caught in the act of being recycled back into the galaxy.
A calibration image that became a paper
The findings were published in Nature on 12 August, led by Yale astronomer Pieter van Dokkum, with co-authors including Imad Pasha and Roberto Abraham of the University of Toronto. The telescope doing the looking is called MOTHRA, the Modular Optical Telephoto Hyperspectral Robotic Array, and it is still under construction at the El Sauce Observatory in Chile. When it is finished it will carry 1,140 high-end telephoto lenses working together to catch light too faint for ordinary instruments to notice.
It was nowhere near finished when it took this image. The team chose the Helix precisely because it is famous and well understood, a sensible thing to point a part-built machine at while you calibrate it. “We thought we were taking a calibration image of one of the best-known nebulae in the sky,” said Abraham. “Instead, we found this extraordinary network of bow-shaped structures.”
What the arcs are telling us
The 22 arcs are bow shocks, the same shape a boat pushes ahead of itself through water. Clumps of debris flung off the dying star are moving fast enough to slam into the surrounding gas, and each collision lights up a curved shock front. Look further out and the pattern shifts. The arcs near the centre are large, thin and sharply drawn. The ones further away are smaller, fuzzier and more broken up.
That progression is a clock. It shows the clumps eroding as they travel, getting stripped and shredded the longer they are exposed to the gas around them. From the pattern, the team worked out that a single fragment survives intact for roughly 10,000 years after it first hits the surrounding gas. After that it is torn apart and blended into interstellar space, where one day it may help build fresh clouds, fresh stars and fresh planets.
A preview of the Sun’s own funeral
At the centre of the Helix sits a white dwarf, the burnt-out core of the star that made the whole thing. That is roughly where our own Sun is heading. In several billion years it will swell up, shed its outer layers and leave behind a similar smouldering ember, returning much of its material to the Milky Way. The Helix is a slightly morbid dress rehearsal for the neighbourhood.
The astronomers combined the new MOTHRA data with older images from the Hubble Space Telescope and the Kitt Peak 4-metre telescope to build the final picture. MOTHRA is part of the Dragonfly project, funded by Alex Gerko of the trading firm XTX Markets and run with the non-profit Convergent Research. For a machine that has not been switched fully on yet, catching a star’s afterlife in mid-shred is a decent opening act.
Did you know: the Helix Nebula’s uncanny resemblance to a giant iris and pupil has earned it a long-standing nickname, the “Eye of God”.
Sources
- Northwestern Now: astronomers witness cosmic recycling in action
- Nature: the study
- Phys.org: a dying star offers a rare glimpse of cosmic recycling
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