With dozens of large telescopes scattered across the planet, some of them so far above, you’d think we’d discovered almost everything in the sky. But literally, that’s an overly narrow view.
The recent discovery of a massive cloud of gas floating near the Andromeda galaxy — one of the most extensively studied objects in the sky — is the latest evidence that the sky still offers plenty of room to sift through. For decades, this cloud has been hidden in plain sight. The best part is that its origin is a mystery.
The advent of cheap but high-quality digital detectors has made astrophotography easier than ever. This has sparked a new trend among astronomy enthusiasts to focus on a selected region of the sky and take effectively long exposures in the hope of finding any faint blur that might be there.
amateur astronomers and researchers Marcel Drexler and Xavier Strotner It was decided to take advantage of these recent technological improvements.They wanted to scan part of the sky for faint nebulae — clouds of gas — so they introduced Yan Shuntian, French amateur astronomer and astrophotographer. Sainty decided to target the Andromeda Galaxy, a large spiral galaxy similar to our Milky Way, only 2.5 million light-years away. From a cosmic perspective, its proximity has made Andromeda a popular target for astronomers for more than a century. With its most obvious wealth map, astronomers usually assume that anything waiting to be discovered will have small, independent objects, such as nebulae inside galaxies themselves.
Sainty sent his observations to Drechsler and Strottner for processing and analysis. While poring over the images, they found something unexpected: a giant extension that appears to be nearly as large as Andromeda itself, and right next to it. The nebula only appeared when Sainty obtained the image with a filter that blocks all light except for the blue-green glow from doubly ionized oxygen — oxygen atoms that have lost two of their outer electrons , which is common in giant gas clouds. It was essentially human curiosity that drove the use of this filter; no large-scale map of the very deep sky around Andromeda had ever been made before.
During the same observation, Sainty also took depth images using another filter tuned to the light emitted by the hydrogen atoms. While he saw plenty of such gas clouds around Andromeda (likely nebulae from our own Milky Way galaxy superimposed on the sky near Andromeda), none matched the size and shape of the exotic oxygen-rich nebula.
However, the team wondered whether the clouds might be some kind of artifact in Sainty’s images—for example, bouncing light in his telescope. To find out, the researchers asked another accomplished amateur astronomer, Bray Falls, making more observations with his own telescope. He saw the same nebula in his data, independently confirming the cloud’s existence.
In the end, observations from five telescopes in France, California, and New Mexico convinced the team that the object was real. It is now known as Strottner-Drechsler-Sainty Object 1, or SDSO-1.
But the question remains: what is it?
To find out, the team reached out to professional astronomers Robert Faison, Michael Schur and Stefan Kimswinger for a deeper analysis.Published in the Journal of the American Astronomical Society (AAA) Research Notes of the American Association for the Advancement of Science, This The result of this professional-amateur collaboration Fascinating, even if the clues to the origin of the nebula are still very vague.
Astronomers have investigated many possibilities, but at this point, gas clouds are unexplained. Its proximity to the Andromeda galaxy in the sky strongly suggests it is somehow related to the Milky Way, and the clouds are slightly curved as if bulging out of the Milky Way. Neither of these properties can conclusively link clouds to galaxies physically, but both are certainly provocative. If SDSO-1 is indeed part of Andromeda, but outside the main body of the galaxy, that would mean the cloud is tens of thousands of light-years long, making it one of the largest coherent structures in Andromeda.
If it exists in Andromeda’s great halo — a roughly spherical collection of stars surrounding the Milky Way — it could be gas thrown off by stellar streams there. But if so, large amounts of hydrogen should also be seen, since it’s the star’s main constituent. However, as Sainty demonstrated with his hydrogen detection filter, there is none—or at least too little—to detect in the cloud.
The Andromeda galaxy is moving toward our Milky Way, and SDSO-1 lies roughly between them, providing another potential clue. Andromeda is so close to the Milky Way that as the two galaxies pass by in space, their respective halos could interact, colliding with each other. The diffuse gas in the halos of the two galaxies would compress upon the collision, creating a curved structure like the bow wave of a ship sailing through water. However, if this were the case, the cloud should not appear so close to Andromeda. Instead, it will appear about halfway across the sky between Andromeda and the center of the Milky Way. And that still doesn’t explain the lack of hydrogen.
Another possibility is that the cloud is physically much smaller but much closer to us, meaning it’s a nebula in the Milky Way that just happens to be in the vicinity of Andromeda. Planetary nebulae are gaseous shells shed by dying Sun-like stars, which are often rich in hydrogen and oxygen. With central stars energizing the gas, these nebulae tend to appear bright in light from both elements. Therefore, the lack of hydrogen in SDSO-1 is again puzzling.
SDSO-1 may be the remnant of a Milky Way star that exploded in a supernova, but it should also glow in ultraviolet and radio waves. However, astronomers searching for earlier observations of the Andromeda Galaxy found that the cloud was devoid of any other wavelengths of light, including X-ray, visible and infrared light.
So currently, there is no known mechanism that fits all data. While puzzling, it’s also something scientists love. Solving hard problems is why we want to be scientists in the first place.
The fact that something so massive in our sky has remained undetected so far is surprising, if not entirely surprising, to astronomers. Large telescopes tend to have narrow fields of view, making it difficult to observe objects of large apparent size—especially something as massive as SDSO-1, which is as wide as three full moons across the sky. Larger instruments simply miss it, missing the forest for the trees.
Also, the clouds are very faint and require long exposures to spot. The total observation time for discovery and confirmation images using only doubly ionized oxygen filters was a staggering 160 hours. Even the filters and detectors used by professional observatories are designed to see differently than amateur telescopes. This also increases the difficulty of finding objects like SDSO-1.even 3.8m Canada-France-Hawaii Telescopewhich comes with Incredible 378-megapixel camera and oxygen filter, it seems at SDSO-1 is located in a part of the sky where nothing can be seen. It’s an impressive piece of equipment, but it wasn’t designed to see something so large and faint in the sky.
Solving this mystery requires spectroscopy — breaking down a cloud’s light into small wavelength parts, similar to how raindrops break up sunlight into rainbows. By carefully examining SDSO-1’s spectrum, the velocity of its gas can be determined through the Doppler effect — a slight blue shift in color if the gas is moving toward us, and a red shift if it is moving away. If the cloud is moving at a similar speed to Andromeda, then it’s likely part of that galaxy. Conversely, if it’s moving more slowly, then it’s likely within ourselves. Such spectroscopic observations are already underway, but until these observations are complete, the cloud’s origin and behavior will remain a mystery, the study’s authors report.
While SDSO-1 presented a conundrum, it was also a beacon of hope: It showed us that there are still many treasures to be found in the skies. We just have to use the right tools to spot them.
This is an opinion and analysis article and the views expressed by the author or author do not necessarily represent Scientific American.
Editor’s note (1/12/23): This article was edited after publication to correct descriptions of the respective roles of Marcel Drechsler, Xavier Strottner, and Yann Sainty in the discovery.
