The Ring Nebula is hiding a giant structure made of iron

The iron cloud is being reported for the first time in Monthly Notices of the Royal Astronomical Society. It forms a long strip that fits neatly inside the nebula's inner region, which has an elliptical shape seen in many well known images, including those captured by the James Webb Space Telescope at infrared wavelengths.¹ The structure is immense. Its length is about 500 times greater than Pluto's orbit around the Sun, and the total amount of iron it contains is roughly equal to the mass of Mars.

What Makes the Ring Nebula Special

The Ring Nebula was first observed in 1779 by French astronomer Charles Messier in the northern constellation Lyra.² It is a glowing shell of gas produced when a star reaches the end of its nuclear fuel burning stage and ejects its outer layers into space. Astronomers expect the Sun to shed its outer material in a similar way several billion years from now.³

How the Iron Bar Was Found

The iron cloud was revealed through observations made with the Large Integral Field Unit (LIFU) mode of a new instrument known as the WHT Enhanced Area Velocity Explorer (WEAVE).⁴ WEAVE is mounted on the Isaac Newton Group's 4.2-meter William Herschel Telescope.⁵

LIFU is made up of hundreds of optical fibers working together. This setup allowed the researchers to collect spectra (where light is separated into its constituent wavelengths) from every point across the face of the Ring Nebula, covering all optical wavelengths for the first time.

Seeing the Nebula in a New Way

Lead author Dr. Roger Wesson, who is based at both UCL's Department of Physics & Astronomy and Cardiff University, described how the finding emerged. "Even though the Ring Nebula has been studied using many different telescopes and instruments, WEAVE has allowed us to observe it in a new way, providing so much more detail than before. By obtaining a spectrum continuously across the whole nebula, we can create images of the nebula at any wavelength and determine its chemical composition at any position.

"When we processed the data and scrolled through the images, one thing popped out as clear as anything -- this previously unknown 'bar' of ionized iron atoms, in the middle of the familiar and iconic ring."

Competing Ideas About Its Origin

The researchers say the origin of the iron bar is still unknown. More detailed observations will be needed to understand how it formed. One possibility is that the structure preserves new information about how the dying star expelled its material. Another, more speculative explanation suggests the iron could be part of a curved arc of plasma created when a rocky planet was vaporized during an earlier expansion of the star.

Co author Professor Janet Drew of UCL Physics & Astronomy stressed that key information is still missing. "We definitely need to know more -- particularly whether any other chemical elements co-exist with the newly-detected iron, as this would probably tell us the right class of model to pursue. Right now, we are missing this important information."

What Comes Next for the Research

The team is now preparing a follow up study and plans to gather new data using WEAVE's LIFU at higher spectral resolution. These observations should help clarify how the iron bar formed and whether other elements are present alongside it.

WEAVE is scheduled to conduct eight major surveys over the next five years, studying objects that range from nearby white dwarfs to extremely distant galaxies. One part of the project, the Stellar, Circumstellar and Interstellar Physics survey led by Professor Drew, is already observing many additional ionised nebulae across the northern Milky Way.

Dr. Wesson noted that similar structures may turn out to be common. "It would be very surprising if the iron bar in the Ring is unique. So hopefully, as we observe and analyze more nebulae created in the same way, we will discover more examples of this phenomenon, which will help us to understand where the iron comes from."

Professor Scott Trager, WEAVE Project Scientist at the University of Groningen, added: "The discovery of this fascinating, previously unknown structure in a night-sky jewel, beloved by sky watchers across the Northern Hemisphere, demonstrates the amazing capabilities of WEAVE. We look forward to many more discoveries from this new instrument."

Notes

1. 1 See e.g. https://www.ucl.ac.uk/news/2023/aug/second-james-webb-image-ring-nebula-hints-dying-stars-companion https://www.cardiff.ac.uk/news/view/2739414-astronomers-spy-structures-that-no-previous-telescope-could-detect-in-new-images-of-dying-star
2. The Ring Nebula is also known as M 57 -- the 57th listing in Messier's catalogue of 'Nebulae and Star Clusters'. John L E Dreyer also included it in his New General Catalogue, first published in 1888 by the Royal Astronomical Society, where it appears as NGC 6720.
3. Once a star like the Sun runs out of hydrogen fuel, it expands to become an extreme red giant and sheds its outer layers, which then coast out to form a glowing shell. A shell created in this way is known in astronomy as a planetary nebula. The leftover stellar core becomes a white dwarf, which, though no longer burning any fuel, continues to shine as it slowly cools over billions of years. The Ring Nebula is a planetary nebula located 2,600 light years (or 787 parsec) away, that is thought to have formed about 4,000 years ago. Planetary nebula ejection returns matter forged in a star to interstellar space and is the source of much of the Universe's carbon and nitrogen -- key building blocks of life on Earth. Stars more than about eight times the mass of the Sun age differently, ending life abruptly in a powerful explosion called a supernova as they collapse to form a black hole or neutron star.
4. Funding for the WEAVE facility has been provided by UKRI STFC, the University of Oxford, NOVA, NWO, Instituto de Astrofísica de Canarias (IAC), the Isaac Newton Group partners (STFC, NWO, and Spain, led by the IAC), INAF, CNRS-INSU, the Observatoire de Paris, Région Île-de-France, CONACYT through INAOE, the Ministry of Education, Science and Sports of the Republic of Lithuania, Konkoly Observatory (CSFK), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Lund University, the Leibniz Institute for Astrophysics Potsdam (AIP), the Swedish Research Council, the European Commission, and the University of Pennsylvania. The WEAVE Survey Consortium consists of the ING, its three partners, represented by UKRI STFC, NWO, and the IAC, NOVA, INAF, GEPI, INAOE, Vilnius University, FTMC -- Center for Physical Sciences and Technology (Vilnius), and individual WEAVE Participants. The WEAVE website can be found at https://weave-project.atlassian.net/wiki/display/WEAVE and the full list of granting agencies and grants supporting WEAVE can be found at https://weave-project.atlassian.net/wiki/display/WEAVE/WEAVE+Acknowledgements.
5. The William Herschel Telescope is the leading telescope of the Isaac Newton Group (ING), which in turn is part of the Roque de los Muchachos Observatory on La Palma, in the Canary Islands. The ING is jointly operated by the United Kingdom (STFC-UKRI), the Netherlands (NWO) and Spain (IAC, funded by the Spanish Ministry of Science, Innovation and Universities).