Glowing Arc by Andromeda Might Be Remnants of a Dying Star

Discovered by amateur astronomers, a mysterious arc of gas might be the ghostly remains of a star shedding its outer layers. However, astronomers haven’t reached a consensus yet.

In 2023, a trio of amateur astronomers discovered a mysterious arc of glowing gas near the Andromeda Galaxy (M31). The glow comes from oxygen atoms that are missing two electrons, but where those atoms are — whether near Andromeda or in our own Milky Way — wasn’t clear. Now, a team of astronomers suggests that the arc might mark lingering aftershocks from an ancient dying star, whose outer layers are now plowing through our galaxy.

Astrophotographers Marcel Drechsler, Xavier Strottner, and Yann Sainty discovered the arc, dubbing it the Strottner-Drechsler-Sainty Object 1 (SDSO-1). Because of its faint glow and large size — spanning more than 1.5 degrees, or a little more than the width of your pinky finger if you held it up against the sky — professional surveys had missed it.

“With recent advancements of digital detectors, narrow passband optical filters, and sophisticated software programs including AI tools, today’s amateur astrophotographers have the necessary equipment — and time — to obtain deeper images of angularly large objects than professional telescopes,” says Robert Fesen (Dartmouth College), who coauthored a 2023 paper that proposed an extragalactic origin for SDSO-1. Not only do large telescopes often have a smaller field of view, but professional astronomers typically don’t spend more than a few hours imaging an object, while amateurs take dozens or even hundreds of images, going “exceptionally deep.”

But once the amateurs had found the object, no one was quite sure what it was. Some astronomers thought SDSO-1 might be shock waves resulting from the interactions between the haloes of sparse stars and gas that surround Andromeda and the Milky Way as the galaxies zip toward each other. Others thought the arc might be related to a stream of stars extending from Andromeda.

Now, Patrick Ogle (Space Telescope Science Institute) and colleagues are proposing something different: that the arc is a 400,000-year-old aftershock from a planetary nebula, the diffuse gas that expands as a red giant star sheds its outer layers. They also pinpoint the origin of the nebula: a nearby binary star system, EG Andromedae, contains a white dwarf and a giant star. The white dwarf’s outer layers, launched into space, create shock waves faster than the speed of sound, called a bow shock, as they encounter interstellar gas. (Names can be confusing: planetary nebulae are unrelated to planets, and EG Andromedae is named not for the galaxy but for the constellation the system is in.)

While a 400,000-year-old planetary nebula would normally be far too faint to see, the dying star’s remnants have become visible as they ram into interstellar gas. “That shock heats up the forward half of the shell to 160,000 K and makes it glow in [doubly ionized oxygen], while the back half of the shell remains invisible,” says Ogle. The results have been posted on the astronomy arXiv preprint server.

The team has proposed a name for this never-before-seen phenomenon: ghost planetary nebulae. If they’re right, this arc would be the first detection of these objects.

However, astronomers haven’t reached a consensus yet. A paper to appear in Astronomy and Astrophysics also found that SDSO-1 is located in the Milky Way, but that team proposes that the gas has only been ionized, rather than shocked, as photons from a nearby bright source bombard the gas and strip off its electrons. Given the arc’s unprecedented nature, only time (and with it, more data) will uncover the source of the mysterious emission.

Shocked or Ionized Gas?

“When the SDSO-1 discovery was announced in early 2023, I found it amazing, but found its interpretation as an intergalactic shock problematic,” Ogle says. Ultimately, he and his team landed on a shock wave that doesn’t lie between the two galaxies but rather within the Milky Way.

Ogle’s team collected imaging of SDSO-1 and its surroundings, joining forces with the discoverers to image the object for 525 hours in total. They were looking for the glow of doubly ionized oxygen ([OIII]) as well as emission from ionized hydrogen atoms (hydrogen-alpha), which would indicate a planetary nebula or supernova remnant.

The images reveal [O III] emission centered on EG Andromedae, with fainter filaments extending away from the star. The filaments indicate a fast-moving shock, Ogle’s team argues. The researchers also detected long, undulating bands of hydrogen-alpha emission, not associated directly with SDSO-1 but trailing behind it by about 1 degree. It could indicate turbulence in the object’s wake.

“EG Andromedae was the second-brightest far-ultraviolet source in the region, well placed to be the planetary nebula’s central star,” Ogle says. While the star’s motion aligned with the arc’s direction, its distance from SDSO-1 indicated that the arc was much larger than any other planetary nebula or bow shock ever seen, a staggering 65 light-years across. Planetary nebulae are typically less than 5 light years across — they’re made of tenuous, expanding gas and become too faint to see after a few tens of thousands of years. The object also sports a 145 light-year-long tail that crosses over Andromeda on the sky.

But the arc’s huge size makes Fesen skeptical of the bow shock scenario. “While I find their suggestion that the binary star EG Andromedae is the cause of this large and puzzling emission arc near M31 interesting, I don’t think the case of its origin is as settled as they claim it is,” says Fesen. He still prefers an extragalactic origin, in the interaction of the two galaxies’ halos.

The other team, while agreeing with Ogle’s team that the filament is in the galaxy, is doubtful about the shocked scenario. “The ghost planetary nebula scenario is complicated to accept,” says team lead Alejandro Lumbreras-Calle (Aragón Center for Physics of the Cosmos, Spain). As the dying star travels through interstellar gas, the outer layers it has shed could be quickly stripped away, “making it impossible to reach the large size we see in the images,” he says. That’s why Lumbreras-Calle prefers a scenario where the gas is more gently ionized rather than violently shocked.

In addition to imaging, the team measured thespectrum, or the intensity of light over different wavelengths, at different regions in the arc. They looked at peaks of light emitted by specific molecules in order to probe those molecules’ motions.

Lumbreras-Calle and colleagues found that the gas motions were less than 20 kilometers per second (45,000 mph) across the entire region, which they argue is too slow for a bow shock. Instead, they suggest that the gas is most likely ionized rather than shocked — excited enough to emit a glow, but not bright enough to blast past the speed of sound.

Ogle pushes back: “Ghost planetary nebulae are a unique new class of nebulae,” he says, so their properties are unknown. “When exploring new phenomena, there always has to be a first, and other evidence was mounting to indicate this is indeed a very large source in our own Milky Way,” Ogle says.

Ogle’s team determined the nebula’s age by comparing EG Andromedae’s speed (107 kilometers per second, or 240,000 miles per hour) and its distance from the arc. Winding back the clock, they find that, if SDSO-1 is indeed a bow shock from a ghost planetary nebula, it would have originated from the parent star around 400,000 years ago.

A Galactic Mystery, Unsolved

The result is compelling, but astronomers haven’t agreed on the ghostly scenario just yet. Fesen remains cautious on the bow shock scenario because, he notes, the team didn’t detect hydrogen-alpha emission that curves along with the arc — the undulating bands the researchers see are not as curved as expected, and located some distance away from SDSO-1.

“However, on the positive side, the proposed detection of the outer structure around a highly evolved planetary nebula is an interesting and novel idea,” he adds.

Fesen is also unsure whether the additional emission lines measured by Lumbreras-Calle’s team are associated with the arc either, arguing that they display a very different structure and appearance. Furthermore, there must be a bright object nearby to ionize the gas, such as a young, massive star, which the team did not yet find.

Lumbreras-Calle acknowledges that finding this source may be difficult. “Because the nebula is very extended on the sky and may be nearby, the ionizing source could be located many degrees away,” he says. Carrying out such a vast search is beyond the team’s capabilities, “but we will closely follow new results from both amateur and professional efforts and will be ready to pursue promising candidates as they emerge,” he adds.

Accurate measurements of the gas motion in different parts of the arc could help confirm a scenario once and for all, according to Fesen, though this data will be difficult to obtain due to the arc’s faintness.

Looking beyond SDSO-1, Ogle and his team have already identified seven more ghost planetary nebula candidates, based on their similar characteristics to this one. Many were discovered by other amateur astronomers in the last five years. “We just didn’t know what they were,” Ogle says. If confirmed, shock-powered ghost planetary nebulae would be a new phase of these nebulae’s evolution.

“We are looking forward to finding more so that we can better understand this new class of object,” Ogle says. “Ghost planetary nebulae also open a previously inaccessible window into the late time evolution of planetary nebulae and how they eventually merge with their surroundings, returning their star stuff to the Milky Way.”

About Arielle Frommer

Arielle Frommer has been writing for Sky & Telescope since April 2024. She covers news stories ranging from newly-discovered exoplanets to local astronomy events. She is a recent graduate of Harvard University, where she obtained her bachelor's degree in Astrophysics and Physics and researched massive star formation and exoplanets. Arielle is currently studying extrasolar atmospheres at Leiden Observatory in the Netherlands. In her free time, she enjoys hiking, crocheting, drinking coffee, and reading and writing fiction.