The largest and most detailed photo ever made of the Galactic Centre in visible light was taken by the Euclid mission. For just one day (23rd March 2025), Euclid turned its gaze towards the extremely bright inner region of our Milky Way galaxy, known as the galactic bulge. The extraordinary picture that resulted is not part of Euclid’s main cosmological survey, which is designed to look at objects far outside our own Galaxy, but was made in response to a special request from astronomers who were after what Euclid does best: capturing large areas of the sky in crisp detail. Packed with more than 60 million stars, this image opens the door for scientists to confirm the existence of any exoplanet found in this region and measure its mass using tiny changes in starlight over time. The observational data obtained in this project were released today as Euclid’s Q2 data release; for details of Q1 please see here.
Designed to observe billions of faraway galaxies, the space telescope’s visible light camera is sensitive enough to tell apart individual stars in our super-crowded galactic bulge, without being blinded. This ability is crucial for what scientists want to use this image for: studying planets around other stars using a special technique called gravitational microlensing.
For comparison, Euclid’s sharpness and sensitivity in visible light is similar to the NASA/ESA Hubble Space Telescope’s wide field camera. But each pointing that Euclid captures in a few hours spans an area 270 times larger than Hubble’s field of view. To observe the same Euclid mosaic, the Keck Observatory would need around 2000 hours. Euclid is faster, and able to capture details from fainter stars that would be otherwise missed when observing from the ground. This single mosaic also encompasses the entire region that the upcoming Roman space telescope will monitor for planet hunting.
In a post last week I hinted that I hoped soon to be able to provide an update on progress with the European Space Agency’s Euclid Mission. People within the Euclid Consortium have known for some time, but it has now been officially announced by ESA, that the timeline for the first full Data Release (DR1; originally scheduled for October 2026) has now been revised.
The plan now is for DR1 to happen in stages, with a first tranche to occur in November 2026 (precise date yet to be announced). The complete data release will take place in mid-2027 (probably in June). The sky area covered by DR1 will be about covering a large sky area of about 1900 deg².
To give a little more background, the data products from the Euclid survey are divided into three levels of data processing function:
LE1 – the “raw” data frames, prior to calibration, generated at the Science Operations Centre from the time-ordered data and telemetry received from the spacecraft
LE2 – the calibrated and corrected data for the two instruments (VIS and NISP) – images, spectra, catalogues of point sources, etc
LE3 – the high-level data products (galaxy catalogues, cosmic shear maps, etc) designed for cosmological analysis
The first release will comprise LE1 and LE2 only. This will be called DR-Foundation. The LE3 data will be added next year to make the full DR1. Since LE3 is required for the cosmological analysis that is the prime motivation for the Euclid mission, it follows that there will be no official cosmology results from Euclid DR1 until mid-2027 at the earliest. Other results based on “Foundation” data may of course emerge before then.
I hope this clarifies the situation.
P.S. Irish physicists and astronomers will be particularly interested to know that the LE1 data includes information about the spacecraft and instrument pointing orientations, which is stored in the form of quaternions…
I’ve been slow onto a result which was announced last week concerning the detection weak gravitational lensing in the cluster Abell 2390 by the Euclid spacecraft and its use to determine the distribution of dark matter in the cluster. You can find a full discussion of the result here and the scientific paper is here.
The analysis was based on Early Release Observations of the cluster, a pretty picture of which are shown here:
Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre (CEA Paris-Saclay), G. Anselmi.
(The little blue patches are artefacts caused by internal reflections in the VIS instrument and can be dealt with in software.)
According to general relativity, the presence of any mass bends the path of light passing near it, producing gravitational lensing. The most famous examples of this are the giant arcs and multiple images associated with strong gravitational lensing, but these are very rare as they require good alignment between observer, lens and source.. Most lines of sight in the universe do not satisfy this condition so are in the weak lensing regime. Even in such cases, however, the presence of the foreground mass can be detected, by way of a systematic alignment in the orientation of background sources around the lensing mass. A circular background image would be distorted into an ellipse by this process. Unfortunately galaxies aren’t circular but are approximately elliptical, so the shape of each source is changed from an ellipse to differently shaped ellipse. The distortion is therefore impossible to detect in a single background source because we don’t know the intrinsic orientation of the galaxy, but the distortion of different sources is correlated in a particular way. Weak gravitational lensing is thus an intrinsically statistical measurement, but it provides a way to measure the masses of astronomical objects without requiring assumptions about their composition or dynamical state. Weak gravitational lensing observations are, however technically difficult to carry out and analyse, as one has to be very careful that no correlations are introduced by systematic errors in the optics.
Anyway, they say that a picture paints a thousand words so here are two pictures. On the left we see the shear axes as extracted from the above image and on the right the inferred dark matter distribution. You can slide the bar backwards and forwards to see how the two images relate.
Shear map (left) and inferred dark matter distribution (right)
You can see that the shear tends to be aligned tangentially to a line connecting the image to the cluster centre (in the plane of the sky), which is what theory would predict.
There’ll be much more of this sort of analysis in the full Euclid Survey. I hope to be able to give an update about this reasonably soon.
A few weeks ago the Euclid Consortium released a printable 3D model of the Euclid Spacecraft. Being a mere theoretician, I don’t know how to operate a 3D printer so I had to ask our technicians if they would make a version for use at open days, etc. The larger version is indeed quite large and everyone has been busy with labs etc, so took a while to print, but thanks to Pat Seery (who did the printing) and Ian McAuley (who assembled and painted the result), here is our model:
The scale for the model is 1:16, so its actual dimensions are 29.5 × 19.5 × 19 cm for the model, and 33 × 20 × 20 cm including the stand. The real thing is over 4.5m tall. There is a little model of an astronaut that comes with the kit (not pictured above) to give an idea of the real size. It’s going to get a protective coat of varnish on it before we use it in public, but it will be a nice addition to our open-day stand and will otherwise be on show in a display case in the Physics Department.
Anyway, if you have access to a 3D printer and would like to make your own version, you can download full instructions here.
A collage of fourteen by eight squares containing examples of gravitational lenses. Credit: ESA/Euclid/Euclid Consortium/NASA, image processing by M. Walmsley, M. Huertas-Company, J.-C. Cuillandre.
I’m sharing the text of a press release from Euclid here to encourage readers to join in this new Zooniverse project.
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In brief
With the launch of Space Warps, a new citizen science project on the Zooniverse platform, you can now join in the search to find rare and elusive strong gravitational lenses in never-before-seen images captured by the European Space Agency’s Euclid space telescope. The project aims at shining a light on dark matter in galaxies and providing clues about mysterious dark energy.
In-depth
Warps in spacetime do not only show up in science fiction movies like Interstellar. In real life, we can see the warping effect that gravity has on spacetime in the form of gravitational lensing.
The enormous gravity of a massive object – such as a galaxy or cluster of galaxies – distorts the shape of spacetime and can bend the light rays coming from a distant galaxy behind. By warping spacetime, the foreground galaxy acts like a magnifying glass.
Light from the background object that would be obscured doesn’t travel in a straight line anymore. Instead, it curves around the intervening mass, often producing multiple images, stretched arcs, or even a complete ring known as ‘Einstein ring’, like the one recently discovered by Euclid.
Strong gravitational lenses offer a striking demonstration of Einstein’s theory of general relativity, showing that matter in the Universe can act as a natural telescope, bringing distant objects into sight.
ESA’s Euclid telescope is revolutionising the studies of strong gravitational lensing by providing very sensitive imaging over large swaths of the sky in unprecedented detail. This is exactly what is needed to identify rare gravitational lenses.
In March 2025, 500 galaxy-galaxy strong lenses were found nestled in just the first 0.04% of Euclid data, most of them previously unknown. This pioneering catalogue was created thanks to the combined effort from citizen scientists, artificial intelligence (AI) and researchers.
Early glimpse of new Euclid images
As Euclid continues its survey, sending around 100 GB of data back to Earth every day, ESA and the Euclid Consortium once again need help from citizen scientists to identify strong gravitational lenses in a large data set.
For this, the Space Warps team has launched a citizen science project based on new Euclid images, which will be part of the future Euclid Data Release 1. While this data is not public yet, by participating in this new citizen science project you can get an early glimpse of these new images of galaxies captured by the telescope.
For this project, you will be inspecting new high quality imaging data from Euclid in which many previously unknown strong lenses are hiding. About three hundred thousand images pre-selected by AI algorithms will be shown, which are fine-tuned with the results from the initial citizen-science Euclid strong lens search. These are the highest ranked candidates from a whopping 72 million galaxies from DR1 that were classified by the AI algorithms. Scientists expect that this exquisite high-quality data will reveal more than 10 000 new lenses.
What can we learn from strong lenses
The Euclid mission explores how the Universe has expanded and how its structure has changed through cosmic history using mainly two methods: weak lensing and baryonic acoustic oscillations. From this, scientists can learn more about the role of gravity and the nature of dark matter and dark energy.
Strong gravitational lenses can also provide insights into these central questions. For example, strong lensing features can ‘weigh’ individual galaxies and clusters of galaxies. This reveals the total matter (whether dark or light) and traces the distribution of dark matter. By studying strong lenses across cosmic time, scientists can trace the expansion of the Universe and its apparent acceleration. This will provide additional insight into the role of dark energy.
“We’ve already seen the success of combining AI with visual inspection by citizen volunteers and scientists on Space Warps, efficiently finding hundreds of high‑probability lens candidates in an initial small Euclid search in 2024”, explains Aprajita Verma, Space Warps’ co-founder and project lead at the University of Oxford, UK. “In this brand new DR1 data, 30 times larger than the initial search and together with our improved AI algorithms, we are expecting to find more than 10 000 high quality lens candidates. This is more than four times the number of lenses than we have been able to find since the first gravitational lens was discovered nearly 50 years ago.”
This step-change is possible thanks to Euclid. The mission can map large areas of the sky with unique sharpness, an ideal combination for finding rare objects like strong gravitational lenses.
“We can’t wait to see what we will find within this unprecedented dataset. Join us on Space Warps to take part in this exciting search!” concludes Aprajita.
Euclid is a European mission, built and operated by ESA, with contributions from NASA. The Euclid Consortium – consisting of more than 2000 scientist from 300 institutes in 15 European countries, the USA, Canada, and Japan – is responsible for providing the scientific instruments and scientific data analysis. ESA selected Thales Alenia Space as prime contractor for the construction of the satellite and its service module, with Airbus Defence and Space chosen to develop the payload module, including the telescope. NASA provided the detectors of the Near-Infrared Spectrometer and Photometer, NISP. Euclid is a medium-class mission in ESA’s Cosmic Vision Programme.
In a vain attempt to convince my readership that I know anything about observational astronomy, I thought I’d share this image of the central regions of the Perseus Cluster (also known as Abell 426) made by my final-year project students:
Picture Credit: Ben Doyle
The image was taken last November using the 1.20m reflecting telescope at the Observatoire de Haute-Provence where the final-year astrophysics students from Maynooth spent a week last November on a field trip taking various observations. The exposure was 240 seconds and the field of view is about 15 arcminutes on a side. Most of the objects in the image are galaxies, rather than stars.
I asked my students to look at this cluster (which is about 10 degrees across), partly because it appears near the Zenith in November so would be a good target, partly because it is nearby so the galaxies in it are therefore quite bright, and partly because it was observed by Euclid and featured among the Early Release Observations. The Euclid telescope is also 1.20m in diameter, but because it has a very fancy camera and is in space, Euclid reveals far more galaxies but I was nevertheless impressed at how well this turned out!
This extraordinary planetary nebula in the constellation Draco has captivated astronomers for decades with its elaborate and multilayered structure. Observations with ESA’s Gaia mission place the nebula at a distance of about 4300 light-years.
Planetary nebulae, so-called because of their round shape when viewed through early telescopes, are in fact expanding gas thrown off by stars in their final stages of evolution. It was the Cat’s Eye Nebula itself where this fact was first discovered in 1864 – examining the spectrum of its light reveals the emission from individual molecules that’s characteristic of a gas, distinguishing planetary nebulae from stars and galaxies.
Here, the nebula is showcased through the combined eyes of the NASA/ESA Hubble Space Telescope and ESA’s Euclid, highlighting the remarkable complexity of stellar death.
Though primarily designed to map the distant Universe, Euclid captures the Cat’s Eye Nebula as part of its deep imaging surveys. In Euclid’s wide, near-infrared and visible light view, the arcs and filaments of the nebula’s bright central region are situated within a halo of colourful fragments of gas zooming away from the star.
This ring was ejected from the star at an earlier stage, before the main nebula at the centre formed. The whole nebula stands out against a backdrop teeming with distant galaxies, demonstrating how local astrophysical beauty and the farthest reaches of the cosmos can be seen together in modern astronomical surveys.
Within this broad view of the nebula and its surroundings, Hubble captures the very core of the billowing gas with high-resolution visible-light images, adding extra detail in the centre of this image. The data reveal a tapestry of concentric shells, jets of high-speed gas and dense knots sculpted by shock interactions, features that appear almost surreal in their intricacy. These structures are believed to record episodic mass loss from the dying star at the nebula’s centre, creating a kind of cosmic “fossil record” of its final evolutionary stages.
Combining the focused view of Hubble with Euclid’s deep field observations not only highlights the nebula’s exquisite structure but also places it within the broader context of the Universe that both space telescopes explore. Together, these missions provide a rich and complementary view of NGC 6543 – revealing the delicate interplay between stellar end-of-life processes and the vast surrounding space.
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For more information, see here. There’s also this video which shows the Nebula in context in Euclid’s extraordinarily impressive wide field capability and Hubble’s superb resolution in the optical band:
P.S. I put the following on my office door in Maynooth University to demonstrate the true scale (!) of my own involvement in Euclid.
For last year’s words belong to last year’s language And next year’s words await another voice. And to make an end is to make a beginning.
From Four Quartets, ‘Little Gidding’ by T. S. Eliot.
So it’s New Year’s Day. Athbhliain faoi mhaise dhaoibh!
For me this brings the festive season to an end. I’ve been eating and drinking too much for the last week as one is supposed to. Last night I brought in the new year with a dish of roast duck and the last of the Christmas vegetables. I think I’ll be buying any sprouts and parsnips for a while. When the iron tongue of midnight told twelve, I had a glass of excellent Irish Whiskey in the form of Clonakilty Single Pot Still (46%). It has been a most enjoyable week, but heightened level of self-indulgence has been rather exhausting, and I’ll be taking things a bit easier for a few days before I go back to work on Monday. It’s hard work being a glutton.
Anyway, I thought I’d mention a few things looking forward to the New Year.
January will, as usual, be dominated by examinations, and especially the marking thereof. The first examination for which I am responsible is on January 12th. The examination, incidentally, will be held in the Glenroyal Hotel in Maynooth as the Sports Hall on campus – usually a major exam venue – is out of commission due to building work.
I have a couple of writing deadlines, in addition to having to correct the examinations, so it will be a busy January.
Then February sees the start of a new semester. I’ll be teaching Particle Physics again. I was a bit surprised to be asked to teach this again, as I was filling last year in for our resident particle physicist who was on sabbatical. I’m glad to be able to continue with it given the work I put in to do it last time. My other module is Computational Physics which I have taught at Maynooth every year since 2018, apart from 2024 when I was on sabbatical. This time, however, I will have to think hard about how to deal with the use of generative AI in the coursework.
Will I get to teach any astrophysics or cosmology at Maynooth before I retire? That’s looking very unlikely. I think it’s probable that the new academic year, starting in September, will find me teaching the same modules as last year.
The year ahead will also see the first data release (DR1) from the European Space Agency’s Euclid Mission. The date for that will be October 21st 2026. This is a hard deadline. There’s a huge amount of work going on within the Euclid Consortium to extract as much science as possible from the observations so far before the data becomes public, but you’ll have to wait until October to find out more!
This reminds me that I forgot to share this nice image from Euclid that was released just before Christmas.
Galaxy NGC 646 looking like a cosmic holiday garland in this image from the European Space Agency’s Euclid space telescope.
Once upon a time, WordPress used to send an email about the year’s blog statistics, etc, but it stopped doing that some time ago. I checked this morning, however, and learned that traffice on the blog in 2025 was up by 2.6% since 2024. I’m not sure how meaningful this is, because there is so much scraping going on these days. That figure doesn’t include the people who get posts via email or RSS or via other platforms such as the Fediverse.
While I’m on about social media I’ll mention a stat about my Bluesky account. I joined Bluesky in 2023 when I abandoned Xitter. As of today I have 8,078 BlueSky followers, which is more than I ever had on X, and with far higher levels of engagement and much friendlier interactions.
I’m also on Mastodon, although with a much smaller following (1.4k). This blog also has a separate existence on Mastodon here. I very much like the federated structure of Mastodon (which, incidentally, accords with my view of how academic publishing should be configured) and am a bit disappointed that it doesn’t seem to have caught on as much as it should.
That disappointment pales into insignificance, however, with the outrage I feel that my employer – along with most other universities – persists in using Xitter. Touting for trade in a far-right propaganda channel is no way for a institution of higher education to behave. You can read my views on this matter here.
And finally there’s the Open Journal of Astrophysics. The year ahead will see the 10th anniversary of our first ever publication – on an experimental prototype platform, long before we moved to Scholastica. It will be next Monday before we resume publishing, starting Volume 9. Which author(s) will be the first to get their final versions on arXiv in 2026? Stay tuned to find out!
Last night I received a message via the Euclid Consortium conveying the very sad news of the death, at the age of 67, of the French astrophysicist and cosmologist Yannick Mellier (pictured left). Among many other things, Yannick was the Euclid Consortium Lead in which role he took on enormous responsibility for getting the project started and, with his team, keeping everything running. His loss is incalculable.
Yannick’s research work focussed on cosmology and the search for dark matter using gravitational lensing. Back in 1987 he was part of the observational team that discovered the first giant arc produced by strong gravitational lensing. He also did pioneering work in the field of weaking gravitational lensing with the Canada-France Hawaii Telescope in that regard starting back in 2000.
For well over a decade now Yannick had been involved with the European Space Agency’s Euclid mission. He was a major force right from the beginning, making the proposal, and after it was accepted leading the Consortium assembled to bring the project into being, preparing for launch, and dealing with the first data. The Euclid Consortium is a huge collaboration and it is impossible to overestimate the scale of the task facing the Lead. The first full data release (DR1) from Euclid will take place towards the end of next year (2026). It is sad beyong words that he did not live to see this.
During the period when I was Chair of the Euclid Consortium Diversity Committee I had a number of interactions with Yannick, sometimes dealing with difficult and confidential matters. I found him to be a man of great wisdom and sensitivity. Despite having many other things to deal with, including a long-term illness, he was unfailingly supportive and his advice was always sound.
The following is an excerpt from the message sent out yesterday:
Yannick’s death leaves a huge void within the consortium and our community. Those of us who have been here the longest know how hard he worked to make the Euclid project a success. He became its embodiment, working tirelessly to ensure its success; we owe him an immense debt of gratitude, and we will surely have the opportunity to reflect in detail on all that we owe him.
Indeed. I hope the Euclid Consortium – and the international cosmological community generally – will, at some stage, organize an appropriate tribute to Yannick.
By way of a quick follow-up to yesterday’s post, here’s another Euclid Q1 product. This one is an updated version of the famous “Tuning Fork” representation of galaxy morphology:
Credits: Diagram: ESA/Euclid/Euclid Consortium/NASA, Diagram by J.-C. Cuillandre, L. Quilley, F. Marleau. Images alone: ESA/Euclid/Euclid Consortium/NASA, image processing by J.-C. Cuillandre, E. Bertin, G. Anselmi
You can click on the image to make it (much) bigger.
A galaxy’s structure is a sign of its formation history and the environment in which it resides. Since early on, astronomers have ordered galaxies according to their visible structure – as a basis to understanding the underlying physics: This panorama of galaxies’ structure shows the ‘classical’ morphological sequence from ellipticals (E, left) to lenticulars (S0) through spirals (S) to irregulars and dwarfs (right). The fork divides barred and unbarred spiral families: originally only SA (unbarred) and SB (barred) galaxies were arranged in a ‘tuning fork’ layout, the addition of SAB (weakly barred) galaxies as a third branch is making this term increasingly challenging to use. Lowercase letters a to d indicate progressively later spiral stages (tighter to looser arms), the trailing m (e.g., SAm) denotes Magellanic, very-late-type systems (patchy, often one-armed). The Milky Way is classified as an SBc galaxy.
Below the main sequence there are three auxiliary panels showing objects not represented in the fork: (1) spiral galaxies seen edge-on, with varying bulge-to-disk ratios and warps; (2) interacting and merging galaxies illustrating gravitationally driven morphological change; and (3) the morphological diversity of dwarf galaxies.
You can read more about this image and the other Q1 results here. You can also find an interactive version of the plot here.
The views presented here are personal and not necessarily those of my employer (or anyone else for that matter).
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In the Dark 