🔭 astrophysics

The outer rings of SN 1987A from year 1994 to 2024: morphology, light curves, and optical to mid-infrared spectra

This study characterizes the physical properties, morphology, and spectral evolution of the outer rings of SN 1987A from 1994 to 2024 using multi-instrument data, revealing a steady optical decline driven by the fading of the initial UV flash with no evidence of ejecta interaction, while providing new constraints on temperature, density, and decay times.

Original authors: Sophie Rosu, Elko Gerville-Reache, Steven Thomas, Josefin Larsson, Patrick J. Kavanagh, Jason Spyromilio, Claes Fransson, Christa Gall, Robert D. Gehrz, Alec S. Hirschauer, Olivia C. Jones, Robert P.

Published 2026-05-29

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Sophie Rosu, Elko Gerville-Reache, Steven Thomas, Josefin Larsson, Patrick J. Kavanagh, Jason Spyromilio, Claes Fransson, Christa Gall, Robert D. Gehrz, Alec S. Hirschauer, Olivia C. Jones, Robert P. Kirshner, Peter Lundqvist, Mikako Matsuura, Margaret Meixner, Beth Sargent, Jesper Sollerman

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a cosmic crime scene that happened nearly 40 years ago. In 1987, a massive star in a nearby galaxy (the Large Magellanic Cloud) exploded, creating a supernova known as SN 1987A. But this wasn't just a simple explosion; the star had been "preparing the scene" for thousands of years before it died.

About 20,000 years before the explosion, the star ejected a massive, triple-layered necklace of gas and dust into space. This paper focuses on the two outermost loops of that necklace, called the Northern Outer Ring (NOR) and the Southern Outer Ring (SOR).

Here is the story of what the astronomers found, using the Hubble Space Telescope (HST), the Very Large Telescope (VLT), and the James Webb Space Telescope (JWST) to watch these rings over the last 30 years.

1. The "Flashlight" Effect

When the star exploded, it sent out a blinding flash of ultraviolet light. Think of this like a giant camera flash going off in a dark room. That flash hit the outer rings, causing them to glow brightly.

For the last three decades, astronomers have been watching these rings like a slow-motion movie. The main finding? The rings are slowly fading away.

Just like a flashlight battery dying out, the glow of the rings is dimming. The astronomers measured this dimming in two specific "colors" of light:

The "Green" Glow ([O III]): This faded relatively quickly.
The "Red" Glow (H-alpha): This is fading much more slowly.

The paper confirms that the explosion's debris (the "ejecta") has not yet hit these outer rings. If it had, the rings would have suddenly lit up again, like a car crash causing a spark. Instead, they are just quietly fading, waiting for the debris to catch up to them.

2. The "Ghostly" Rings vs. The "Solid" Ring

The supernova has three rings: a bright inner ring (the Equatorial Ring) and the two fainter outer rings.

The Inner Ring: This is like a solid, dense wall that the explosion debris slammed into years ago, creating a massive, glowing firework show.
The Outer Rings: These are more like ghostly, wispy clouds. The paper shows that the outer rings are physically different from the inner ring. They have different temperatures and densities. It's as if the inner ring is made of thick concrete, while the outer rings are made of thin, wispy smoke.

3. The Detective Work: Measuring the Gas

The astronomers used powerful telescopes to act like cosmic detectives, measuring the "temperature" and "density" of the gas in these rings.

Temperature: They found the gas in the rings is incredibly hot, ranging from about 12,000 to 17,000 degrees. That's hotter than the surface of our Sun!
Density: The gas is very thin. They calculated that there are only about 600 to 800 atoms in every cubic centimeter of space within these rings. For comparison, the air we breathe has about 100 billion billion atoms in that same space.

4. The "Time Travel" Aspect

Because the rings are tilted relative to us, the light from different parts of the rings takes different amounts of time to reach Earth.

The Northern part of the rings is closer to us.
The Southern part is farther away.

This means when we look at the rings, we are seeing the North and South parts at slightly different stages of their "life." The paper found that the Southern ring is fading slightly faster than the Northern ring, likely because it is further away and we are seeing it at a slightly different point in its cooling process.

5. The Future: Waiting for the Collision

The most exciting part of the paper is the prediction of what happens next.

The Debris is Coming: The explosion debris is expanding outward like a shockwave. It has already hit the inner ring, but it hasn't reached the outer rings yet.
The Big Impact: The astronomers estimate that the debris will finally crash into the outer rings in about 5 to 10 years (though it could be as late as 2040).
The Result: When the debris hits, the rings will light up again. However, the paper suggests this won't be as spectacular as the first collision with the inner ring. Because the outer rings are further out and the debris has spread out and thinned over time, the "crash" will be less violent.

Summary

In simple terms, this paper is a 30-year diary of two giant, invisible gas rings around a dead star.

They were lit up by the star's death flash.
They have been slowly dimming ever since.
They are made of very hot, very thin gas.
They are different from the inner ring.
They are waiting for the explosion debris to finally catch up and hit them, which will happen sometime in the next decade or two.

The astronomers are now keeping a close eye on these rings, ready to capture the moment the "ghosts" get hit by the "shockwave."

Problem
Supernova (SN) 1987A in the Large Magellanic Cloud is the most thoroughly studied astronomical object in the Universe, yet detailed investigations have predominantly focused on the supernova ejecta, the equatorial ring (ER), and reverse shocks. In contrast, the outer rings (ORs)—specifically the northern (NOR) and southern (SOR) rings ejected approximately 20,000 years prior to the explosion—have received limited attention. The last major analysis of the ORs dates back to Tziamtzis et al. (2011). Understanding the physical properties of the ORs is crucial for constraining the progenitor star's properties and the pre-supernova mass-loss history. A significant gap exists in the long-term monitoring of the ORs' morphology, light curves, and spectral evolution, particularly regarding whether the expanding supernova ejecta have begun to interact with them.

Methodology
The authors compiled and analyzed a comprehensive dataset spanning three decades (1994–2024) from multiple space and ground-based observatories:

Hubble Space Telescope (HST): Utilized narrow-band filters (F502N for [O III] $\lambda$ 5007 and F657N for H $\alpha$ + [N II]) from WFPC2, ACS, and WFC3 instruments. Data covers epochs from day 2537 to day 12,980 post-explosion. Fluxes were corrected for instrumental differences between WFPC2, ACS, and WFC3 using MUSE spectra as a baseline.
VLT/MUSE: Integral Field Unit (IFU) observations taken in March 2023 (day 13,182) provided optical spectra (4750–9350 Å) with adaptive optics, allowing for spatially resolved spectroscopy of the NOR and SOR.
James Webb Space Telescope (JWST):
- NIRCam: Deep imaging in eight filters (F150W to F444W) obtained in September 2022.
- MIRI/MRS: Medium Resolution Spectrograph IFU observations from Cycle 1 (July 2022) and Cycle 2 (August 2024), covering the mid-infrared (6.5–28 $\mu$ m).

The study involved measuring emission fluxes in defined elliptical annuli around the NOR and SOR, correcting for contamination from diffraction spikes and the central ejecta, and deriving physical parameters (temperature and electron density) from line ratios.

Key Contributions

Long-term Light Curves: Provided three decades of optical light curves for the NOR and SOR, the longest temporal baseline to date.
First Spectra: Presented the first mid-resolution optical spectra (MUSE) and the first complete mid-infrared spectra (JWST/MIRI/MRS) of the outer rings.
Morphological Evolution: Analyzed the evolution of the ORs' clumpy morphology over time, confirming that the rings remain distinct and have not yet been swept up by the supernova ejecta.
Physical Constraints: Derived electron densities and electron temperatures for the NOR and SOR using optical line ratios ([N II] and [S II]).

Results

Light Curve Evolution: The optical light curves in both H $\alpha$ $α$ +[N II] and [O III] show a steady decline over the last 30 years, consistent with fading photoionization from the initial UV flash of the supernova. No brightening indicative of ejecta interaction was observed. The decay times ( $t_{decay}$ $t_{d ec a y}$ ) were estimated as:
- [O III]: 900 days (NOR) and 630–730 days (SOR).
- H $\alpha$ +[N II]: 15,870 days (NOR) and 7,160 days (SOR).
Spectral Properties:
- The ORs spectra differ significantly from the ER spectrum in both detected lines and line ratios.
- Temperature: Constrained from [N II] lines to 13,400–16,900 K (NOR) and 11,800–14,500 K (SOR).
- Density: Constrained from [S II] lines to 610–670 cm $^{-3}$ (NOR) and 720–790 cm $^{-3}$ (SOR).
- Line Ratios: The ratio of [N II] $\lambda\lambda$ 6548, 6583 to H $\alpha$ has decreased from $\sim$ 2.0–2.4 (1994–2003) to $\sim$ 1.0–1.3 (2023). Conversely, the [Ar III] $\lambda$ 7136 line flux has increased relative to H $\beta$ , suggesting it originates from lower-density regions that recombine more slowly.
Morphology: The NOR is consistently $\sim$ 10% brighter than the SOR in HST and NIRCam data, partly due to continuum contribution. The western part of the NOR and the eastern part of the SOR are systematically brighter than their counterparts, but all regions fade at similar rates.
Interaction Status: The line widths of the ORs remain narrow (consistent with pre-shock expansion velocities of $\sim$ 20–25 km s $^{-1}$ ), while the SN ejecta lines are thousands of km s $^{-1}$ . The lack of brightening and the continued fading indicate that the SN ejecta have not yet interacted with the ORs.

Significance
The paper establishes that the outer rings of SN 1987A are currently fading due to the cessation of the initial UV flash ionization, with no evidence yet of interaction with the supernova ejecta. This contrasts with the equatorial ring, which began interacting with the ejecta in 1995. The authors estimate that the ejecta may reach the outer rings in 5–10 years, though uncertainties could push this to $\sim$ 2040. Continued monitoring from X-ray to mid-infrared is deemed essential to capture the moment of interaction, which will provide critical insights into the progenitor's mass-loss history and the physical connection between the different ring structures. The study fills a critical gap in the understanding of SN 1987A by providing the first comprehensive multi-wavelength characterization of the outer rings over a 30-year baseline.