A mysterious bar-shaped cloud of iron has been discovered inside the iconic Ring Nebula by a European team led by astronomers at University College London (UCL) and Cardiff University.
The cloud of iron atoms, described for the first time in Monthly Notices of the Royal Astronomical Society, is in the shape of a bar or strip: it just fits inside the inner layer of the elliptically shaped nebula, familiar from many images including those obtained by the James Webb Space Telescope at infrared wavelengths1.
The bar's length is roughly 500 times that of Pluto’s orbit around the Sun and, according to the team, its mass of iron atoms is comparable to the mass of Mars.
The Ring Nebula, first spotted in 1779 in the northern constellation of Lyra by the French astronomer Charles Messier2, is a colourful shell of gas thrown off by a star as it ends the nuclear fuel-burning phase of its life. Our own Sun will expel its outer layers in a similar way in a few billion years' time.3
The iron cloud was discovered in observations obtained using the Large Integral Field Unit (LIFU) mode of a new instrument, the WHT Enhanced Area Velocity Explorer (WEAVE)4, installed on the Isaac Newton Group’s 4.2-metre William Herschel Telescope5.
The LIFU is a bundle of hundreds of optical fibres. It has enabled the team of astronomers to obtain spectra (where light is separated into its constituent wavelengths) at every point across the entire face of the Ring Nebula, and at all optical wavelengths, for the first time.
Lead author Dr Roger Wesson, based jointly at UCL and Cardiff University, said: "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."
How the iron bar formed is currently a mystery, the authors say. They will need further, more detailed observations to unravel what is going on. There are two potential scenarios: the iron bar may reveal something new about how the ejection of the nebula by the parent star progressed, or (more intriguingly) the iron might be an arc of plasma resulting from the vaporisation of a rocky planet caught up in the star’s earlier expansion.
Co-author Professor Janet Drew, also based at UCL, said: "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."
The team are working on a follow-up study, and plan to obtain data using WEAVE's LIFU at higher spectral resolution to better understand how the bar might have formed.
WEAVE is carrying out eight surveys over the next five years, targeting everything from nearby white dwarfs to very distant galaxies. The Stellar, Circumstellar and Interstellar Physics strand of the WEAVE survey, led by Professor Drew, is observing many more ionized nebulae across the northern Milky Way.
"It would be very surprising if the iron bar in the Ring is unique," explains Dr. Wesson. "So hopefully, as we observe and analyse 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 based 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."
ENDS
Media contacts
Sam Tonkin
Royal Astronomical Society
Mob: +44 (0)7802 877 700
Mark Greaves
University College London
Mob: +44 (0)7990 675 947
Science contacts
Dr Roger Wesson
University College London/Cardiff University
Professor Janet Drew
University College London
Images & captions
Figure 1: A composite RGB image of the Ring Nebula (also known as Messier 57 and NGC 6720) constructed from four WEAVE/LIFU emission-line images. The bright outer ring is made up of light emitted by three different ions of oxygen, while the ‘bar’ across the middle is due to light emitted by a plasma of four-times-ionised iron atoms. North is up and East is to the left in the image.
RGB key:- Red: the bar-shaped emission from four-times-ionized iron atoms in the [Fe V] spectral line at a wavelength of 4227 Angstrom (422.7 nm). Also shown in red, in the main ring, is emission in the [O I] 6300 Angstrom auroral line produced by neutral oxygen atoms. Green: emission in the [O II] 3727 Angstrom line pair emitted by singly-ionized oxygen atoms. Blue: emission in the [O III] 4959 Angstrom line of doubly-ionized oxygen atoms.
The angular dimensions of the image are 120 x 110 arcseconds on the sky (E-W x N-S), corresponding to physical dimensions of 95,000 x 87,000 Astronomical Units (AU) for the 787 parsec distance to the Ring Nebula. An Astronomical Unit is the mean distance from the Sun to the Earth.
Credit: University College London
Figure 2: An illustrative set of 8 individual WEAVE LIFU emission-line images of the Ring Nebula. The colour in each panel tracks the brightness of emission, with brown-red being the most intense, shading through yellow and green to blue for the faintest emission. North is up and east, left.
The 4 emission line images that are combined in Figure 1 are shown separately in the top row. Left to right, the emission lines are: the [Fe V] 4227 Angstrom (422.7 nm) line due to four-times-ionized iron atoms; the [O I] 6300 Angstrom auroral line due to neutral oxygen atoms; the [O II] 3727 Angstrom line pair due to singly-ionized oxygen atoms; the [O III] 4959 Angstrom line due to twice-ionized oxygen atoms.
Bottom row, from left to right: emission in the 4861-Angstrom line that is produced as ionized hydrogen atoms recombine in the nebula; emission in the [N II] 6548 Angstrom line of singly-ionized nitrogen; emission in the C II 4267 Angstrom line resulting from the recombination of twice-ionized carbon atoms; emission in the [Ar V] 6435 Angstrom line by four-times-ionized argon.
Notice the very different appearance of the emission from four times ionized iron atoms (top left) compared to the emission from four-times-ionized argon atoms (bottom right) – usually, these ions of argon and iron arise in the same volume, as they require the same physical conditions.
The angular dimensions of each of the 8 frames are 120 x 110 arcseconds on the sky (E-W x N-S), corresponding to physical dimensions of 95,000 x 87,000 Astronomical Units (AU) at the 787 parsec distance of the Ring Nebula. An Astronomical Unit is the mean distance from the Sun to the Earth.
Credit: University College London
Further information
The paper ‘WEAVE imaging spectroscopy of NGC 6720: an iron bar in the Ring’ by R. Wesson et al. has been published in Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/staf2139.
1 See e.g. https://www.ucl.ac.uk/news/2023/aug/second-james-webb-image-ring-nebula-hints-dying-stars-companion
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).
Notes for editors
About the Royal Astronomical Society
The Royal Astronomical Society (RAS), founded in 1820, encourages and promotes the study of astronomy, solar-system science, geophysics and closely related branches of science.
The RAS organises scientific meetings, publishes international research and review journals, recognises outstanding achievements by the award of medals and prizes, maintains an extensive library, supports education through grants and outreach activities and represents UK astronomy nationally and internationally. Its more than 4,000 members (Fellows), a third based overseas, include scientific researchers in universities, observatories and laboratories as well as historians of astronomy and others.
The RAS accepts papers for its journals based on the principle of peer review, in which fellow experts on the editorial boards accept the paper as worth considering. The Society issues press releases based on a similar principle, but the organisations and scientists concerned have overall responsibility for their content.
Keep up with the RAS on Instagram, Bluesky, LinkedIn, Facebook and YouTube.
Download the RAS Supermassive podcast