We humans were born in stars. Our elements were forged in the crucible of exploding stars, ones that had come to the end of their life and then erupted in a single cataclysmic event. This process has been going on for billions of years which is why we find our universe full of many of the elements that make up the periodic table and not just a melange of hydrogen. Like stars supernovae come in a variety of shapes and sizes and a recently observed one, dubbed ASASSN-15lh, sets the record for the brightest one ever observed. In fact it was so bright that we’re just barely able to explain how it might have happened.
ASASSN-15lh was first observed just over a year ago and initially showed up as a transient spot on observations conducted by the All Sky Automated Survey for SuperNovae. Further observations, conducted by the du Pont Telescope in Chile and the South African Large Telescope, confirmed that it was a noteworthy event that required further investigations. The final observation was then conducted by the Swift Space Telescope which then resulted in Central Bureau of Astronomical Telegrams designating it SN 2015L. The observations confirmed that this was the most luminous supernova ever to occur, something which pushes the boundaries of our understanding about how big events like this can get.
Now most blips don’t warrant the level of scrutiny that ASASSN-15lh received however the spectrum of the supernova, provided by the du Pont Telescope, was incredibly unusual. The spectrum would match that of a previously seen superluminous supernova but only if the light had been significantly red-shifted (I.E. that it happened so far from Earth that the wavelengths of light had been stretched by the expansion of space to look more red). This is where the observation from the African Large Telescope comes into play as it confirmed that the light had undergone significant redshifting. This then meant that they were looking at an incredibly bright supernova, 3 times brighter than the previous record holder.
How supernova can get this bright is an incredibly interesting process. Essentially it relies on the star shedding its outer layers first and then forming whats called a Magnetar core. These neutron star variants are shrouded in a magnetic field so powerful that it’s lethal to life at distances even up to 1000km away from it. This magnetar would then have to spin incredibly fast, completing a full revolution every millisecond (the theoretical maximum for these kinds of stars). Then, as the star began to slow, giant magnetic winds would billow forth, slamming into the outer hydrogen layers and producing a shockwave of incredible luminance.
To put it in perspective just how bright ASASSN-15lh is if it were to have happened anywhere in our galaxy it would be visible by the naked eye during the day. If it happened in our cosmic backyard it would be as luminous as the moon. At its peak ASASSN-15lh shone 20 times brighter than all the stars in the Milky Way combined.
This explanation however relies on everything happening at a perfect maximum in order to produce something as bright as this. Whilst it’s quite possible that the magnetar explanation is sufficient it’s right on the edge of our understanding and so it’s very possible that there’s other mechanics at work here that influenced the final outcome. It’s taken a year of obsverations and research to get to this point so it’s likely that the data gathered on ASASSN-15lh has numerous more insights to give us on how such incredible events occur.
For me the incredible scale of things like this fill me with a sense of wonder and amazement. To think a single entity could dwarf an entire galaxy like that, even if for only a brief moment, gives you an incredible amount of perspective on all things. Indeed the fact that the atoms and molecules that constitute me were born in such places gives me a sense of connectedness to the universe and all the wonders that dwell within it.
