On Friday, May 19, renowned Japanese supernova hunter Koichi Itagawa added another discovery to his tally: a massive explosion in M101—the famous Pinwheel Galaxy.
It was quickly apparent that this supernova, named SN 2023ixf, was the largest seen in more than a decade.
While such an explosion would spell certain destruction for any alien civilization within a radius of a couple of dozen light-years, astrophotographers on Earth suddenly had the chance to observe a rare phenomenon from a safe distance of 21 million light-years away.
In fact, I had been photographing M101 for several weeks prior to the eruption, so I was well-positioned to shoot a before-and-after sequence of the event.
In the astrophotography world, spring is known as “galaxy season.” During this time of year, the Earth at night is pointed not inward towards our own galaxy but rather outwards towards the far reaches of intergalactic space.
Imagine pulling over on an elevated overpass in the suburbs of a major city.
You might turn and look back towards the skyscrapers of the urban center – our Milky Way’s core, which we can see in summer – or you can turn and look away towards the rural landscape beyond – intergalactic space, which we see in late winter and spring.
For galaxy season, I usually focus on very small targets – galaxies tens of millions of light-years away. Resolving these small galaxies demands large focal-length telescopes and sensitive, cooled cameras.
It also requires many hours of exposure, as there is a trade-off between long focal lengths and light-gathering power (focal ratio or f-stop).
This means that to make one low-noise high-resolution photo from my backyard in central Virginia, I need multiple clear, moonless nights.
This spring, with lots of cloud cover, rain, and pervasive smoke from massive Canadian wildfires, the imaging nights were few and far between.
So it was quite lucky that I happened to have my telescope on M101 just hours prior to Itagawa’s discovery, which professional spectroscopic studies quickly confirmed to be a Type II supernova – the catastrophic destruction of an ageing supergiant star.
As soon as I learned about the cosmic event on Friday, I looked at the forecast and saw… more clouds and rain!
It wasn’t until Sunday evening that I finally had a clear night. As soon as I could calibrate my telescope’s position, run my focus routine, and begin guiding for long exposures, I downloaded my first image.
The supernova was immediately apparent. Indeed, it shone brighter than the galaxy itself!
By Sunday, supernova SN 2023ixf had grown in intensity from magnitude 14.9 to around 11.1
Rememeber that astronomical magnitude, the measure of the brightness of celestial targets, is a “backwards” logarithmic scale, defined such that a magnitude 1 star is exactly 100 times brighter than a magnitude 6 star.
So, by Sunday when I photographed the supernova at mag 11.1, it was approximately 33 times brighter than on Friday when it was discovered.
The images I produced show the before and after of an explosion that contained the energy of billions of average stars, but at such an unfathomable distance, it appears only as a bright dot of light.
Each monochrome image combines 120 two-minute exposures, shot with a Green filter (I happened to be collecting Green wavelengths to build an RGB image on the night prior to the supernova).
I was subsequently able to collect post-supernova exposures in Red and Blue filters as well to create the final color image.
I used a Celestron EDGE HD 9.25″ Schmidt-Cassegrain telescope, with Chroma RGB filters and a ZWO 2600MM-Pro mono, cooled astro-camera.
I processed each photo using Astro Pixel Processor and Pixinsight, calibrating the light exposures with 20 dark frames, 15 flat frames, and 15 dark flat frames.
Supernovae vary in duration and brightness, so astronomers, professional and amateur alike, have their telescopes and cameras trained on the Pinwheel Galaxy to see what happens next.
In late May, the magnitude is hovering around 11, already remarkable as it is rare for supernovae to break mag 12, but since then, the supernova has faded significantly.
That means that for a fleeting moment, we had the chance to get out and observe a cataclysmic explosion from a long time ago in a galaxy far, far away.
