Astronomers capture first image of star that exploded twice

A star has died in a way that astronomers have long suspected but never seen so clearly. Its remains show two distinct layers of debris, the calling card of a double explosion.

The object is located in the Large Magellanic Cloud, a nearby dwarf galaxy located about 160,000 light years away. Its relic shell, named SNR 0509-67.5, still glows centuries after the explosions.

In a peer-reviewed paper, the team reports two distinct calcium-rich shells, with a single sulfur shell nestled between them.

The pattern matches what models predict when a white dwarf experiences two linked detonations. One starts in an outer layer of helium and the other fires in the star’s carbonaceous oxygen core.

The calcium is found in two concentric layers because each detonation forged calcium in a different part of the star. Sulfur peaks between these layers, where conditions favored its production.

Understanding Type Ia Supernovae

A Type Ia supernova, a powerful stellar explosion that briefly eclipses an entire galaxy, occurs when a white dwarf in a binary system ignites in a thermonuclear runaway. The explosion detaches the star and floods space with heavy elements.

These events shine with predictable brightness after calibration, so they act as distance markers across the cosmos. Their use revealed the accelerated expansion of the universe.

Because Type Ia supernovae constitute the anchor of our cosmic criterion, their triggering mechanism is important. While there are multiple ways to light them, astronomers must take this diversity into account.

This study of SNR 0509 adds direct visual evidence that at least some Type Ia explosions occur before the white dwarf reaches a critical limit. This pushes the theory toward more than one path to detonation.

How SNR 0509 exploded twice

In the double detonation scenario, a thin layer of helium forms on the surface of the white dwarf. The helium becomes unstable and explodes first.

This surface explosion sends a shock wave around the star and inward. The shock compresses the core and triggers a second, deeper detonation.

The result is a single supernova event with a two-step trigger. The calcium fingerprint of SNR 0509, two shells with sulfur in between, preserves the sequence in the expanding debris.

The team explained that white dwarf explosions are central to astronomy because they help trace the expansion and chemical composition of the universe, but the precise mechanism that triggers these explosions has remained a mystery for decades.

Calibrations and Calcium Cards

If some Type Ia supernovae explode below the Chandrasekhar mass, the maximum mass a white dwarf can reach before collapsing under its own gravity, their brightness and colors may differ in subtle ways. Calibrations must capture these differences to maintain accurate distance estimates.

The SNR 0509-67.5 calcium map acts as a forensic photograph. It shows that the white dwarf exploded in two stages, not by slow mass growth to a single critical threshold.

This helps explain why Type Ia supernovae are not perfectly uniform. It also guides models that convert explosion physics into light curves and spectra.

This feedback loop, observation to model and vice versa, improves how we use these events to measure cosmic distances. It also specifies the amount of nickel, iron and other elements produced by each path.

Inside the rest of SNR 0509

The remainder is an almost perfect sphere because it expands into low density gas. The shock waves peeled away the outer layers and exposed deeper ejecta structures.

When the inward-moving reverse shock ionizes the debris, the highly ionized calcium glows in two rays. Sulfur peaks between them, corresponding to hydrodynamic simulations of a double detonation.

The SNR 0509 shell spans approximately 23 light years and travels at more than 11 million miles per hour.

The vestige being only a few centuries old, its interior stratification is still legible. The two calcium shells did not mix, thus preserving the imprint of the explosion.

Chemistry written in light

Calcium traces different combustion regimes in the star. The helium layer produces calcium at lower densities, while core detonation forges calcium deeper.

The sulfur highlights the zone of intermediate density between these calcium layers. Its location is a key control over models that predict a single sulfur shell.

Seeing both at once removes the ambiguity that can persist only in spectra. Spatial maps resolve debates that light curves and one-dimensional models could not resolve.

The researchers pointed out that the results show that white dwarfs can explode before reaching the Chandrasekhar mass limit, which clearly proves that the double detonation mechanism occurs in nature.

What the data says

The small difference in Doppler shift between the two calcium shells shows that they are distinct layers and not a projection of a single shell. This supports a two-stage formation story.

Computer models that simulate how stars explode under fluid dynamics show the same structure seen in the remains.

When the models use a white dwarf lighter than Chandrasekhar’s mass, they produce two calcium shells with a layer of sulfur between them, just like the observations.

This match suggests that the models capture the true physical process behind the double explosion.

This agreement does not prove all the details, but it secures the basic trigger sequence. The tomography of the remainder highlights the case.

Lessons from SNR 0509

Type Ia supernovae always originate from multiple channels, including different types of companion stars. A double detonation explains a significant fraction, but probably not all.

Future observations will look for the same pattern of calcium and sulfur in other young remains. A larger sample will allow testing the frequency of this path and its impact on luminosity diversity.

Better three-dimensional radiative transfer models will relate ejecta maps to observed colors. This will strengthen the standardization that underpins distance measurements.

As methods converge, the cosmic distance scale gains strength. This allows you to stay focused on the physics and not just the data.

The study is published in Natural astronomy.

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