Thermal Infrared Echoes: Illuminating the Last Gasp of a Dying Star

In the constellation Cassiopeia, the remnants of a massive star’s explosion, known as Cas A, form a hot, expanding cloud of gas that holds the secrets of a supernova event that occurred approximately 11,000 years ago. By analyzing the expansion velocity of the debris and tracing its fragments to a common origin, astronomers estimate the explosion’s visible remnants are about 360 years old.

The event may have been inadvertently observed by the English astronomer John Flamsteed in 1680, who recorded a faint star in the direction of Cas A, which no longer exists today. While the exact details of Flamsteed’s observation do not align perfectly with Cas A’s inferred age and location, his sighting might represent the fleeting afterglow of the explosion.

A supernova – the cataclysmic death of a massive start – marks the end of its peaceful evolution. Powered by the radioactive decay of iron and titanium isotopes, the explosion’s afterglow can last for years. A typical galaxy will have a few supernova events in a century.

With modern telescopes and satellites, astronomers can detect a few dozen supernova events in nearby galaxies. However, the most dramatic phase of a supernova, the breakout shock through the star's surface, occurs in less than a day (a star has a lifetime of around ten billion days – so this is a tiny fraction of its lifetime), making it incredibly rare to capture. Fortunately, nature gave us another opportunity to witness the precise moment in which the star of Cas A was obliterated.

Luckily, Cas A’s progenitor star left behind a lasting footprint: an intense flash of radiation heated nearby interstellar dust to temperatures around 100 degrees Kelvin (about minus 300 degrees Fahrenheit) creating thermal echoes that stand out against the cooler 30 degrees Kelvin dust. Using IDL image processing routines, astronomers converted infrared data from NASA’s Spitzer Space Telescope into maps of the region around Cas A, revealing these echoes as the final gasp of the dying star.

Figure 1. Mapping the Infrared Echoes of Cas A

Figure 2. The Geometry of an Echo

A pulse of light emitted from a source at S travels directly to an observer at O, arriving first. The same pulse is also absorbed and re-emitted as infrared radiation by discrete clouds of dust surrounding the source, creating hot spots that reach the observer later due to a longer travel path.

All the points that share the same time delay are located on an ellipse, (orange colored), with S and O as its two focal points. For example, the light arriving at O via point A1 is delayed by the light travel time from S to A1 and back to S. Similarly, light from B1 and C1 arrives with the same delay time as A1, meaning the observer sees simultaneous emissions from A1, B1, and C1.

As the light pulse expands outward, it lights up new regions of dust, creating new hot spots, (B2 and C2), that are located on a new ellipse (depicted in pink). When comparing images taken at different times, it appears the hots spots have moved across the sky. This variability in the sky maps is another hallmark of infrared echoes.

Figure 3. Determining the Properties of the Radiation Pulse

The hot spots shown in Figure 1 are located at well determined projected distances from the source S. For example, the distance d6 represents the projected distance of echo 6 from the source. The echo must also lie on an ellipse, where the distance S-A1 (Figure 2) corresponds to the 340 year-delay—twice the light travel time for that path. This places echo 6 at a unique distance from the exploding star, and the same method applies to all other echoes.

By knowing the distance to a source of radiation and the temperature of the surrounding dust, the pulse intensity required to heat the dust needed to be determined. The pulse intensity was calculated to be a hundred billion times more luminous that the sun, lasting only a few hours – a clear signature of a shock breakout.

Figure 4. Constructing a 3D Map of the Interstellar Medium

An IDL animation (figure 4) was created by combining images of nine successive observations of the area around Cas A over a five-year period (2003-2008). The apparent motion of the hot spots across the sky allows astronomers to construct a 3-D map of the surrounding interstellar dust and gas, similar to the way a CAT scan reveals internal structures in the human body.

Repeat observations of Cas A, illustrated in the animation, show the evolving positions of these hot spots. This ongoing project has now been expanded with NASA's James Webb Space Telescope (JWST). Preliminary, JWST results have revealed even more hot spots, helping astronomers further refine the 3D structure of the medium around the supernova remnant and place tighter constraints on the properties of the exploded star.

Acknowledgments: This work utilized NV5’s powerful IDL image processing routines to convert the infrared data from NASA's Spitzer Space Telescope into detailed maps of the Cas A remnant. Figure 1 shows the 24-micron image generated from this data. The animation presented in Figure 4 was produced by stacking the images taken at different epoch into a GIF file.

Figures 2 and 3, which illustrate the underlying calculations, were produced by IDL plot routines.