When stars die, they don’t die quietly but prefer to go out with a bang! This is known as a supernova, which occurs when a star has expended all of its fuel and undergoes gravitational collapse. In the process, the outer layers of the star will be blown off in a massive explosion visible from billions of light-years away. For decades, NASA has been monitoring galaxies beyond the Milky Way and detected numerous supernova taking place.
For instance, over the past 20 years, the Hubble Space Telescope has been monitoring the galaxy NGC 5468 – an intermediate spiral galaxy located roughly 130 million light-years from Earth in the constellation Virgo. In that time, this galaxy has experienced 5 supernovae and, thanks to its orientation (perpendicular to our own), astronomers have been able to study this galaxy and its supernovae in glorious detail.
In some cases, stars experience a supernova near the end of their lifespans once they have consumed all their hydrogen and helium fuel – known as a Type II supernova. Depending on the mass of the star, it will either leave behind a remnant known as a neutron star or a black hole. However, astronomers have found that in most cases, stars will go supernova as a result of a binary companion “siphoning” material from them.
This scenario, known as a Type I supernova, will occur when one of the binary pair has already gone supernova and become a neutron star or a black hole. As the companion star exits its main sequence and expands to become a Red Giant, the gravitational force of the white dwarf/black hole companion will begin to siphon material from the Red Giant’s surface and pull it into a disk that slowly accretes onto it.
Over time, the Red Giant star will lose more mass to its companion than it is able to support, causing a runaway nuclear fusion in its core that will kick-off the supernova process. In both cases, the explosion will result in an intensely bright object that will temporarily shine as brightly as the entire galaxy that hosts it.
In the case of NGC 5468, both types of supernovae have been observed during the past 20 years – which include SN 1999cp, SN 2002cr, SN2002ed, SN2005P, and SN2018dfg. Thanks to the galaxy’s orientation relative to us, astronomers were able to spot each of the bright objects that resulted from these five supernovae the moment they became visible.
The observation of supernovae in another galaxy raises an important question. How often do stars go supernova in the Milky Way, and what contributes to the rate at which a galaxy’s stars go supernovae? Suffice it to say, the Milky Way doesn’t experience a lot of supernovae, at least not ones that our astronomers have been able to observe. In fact, the last time anyone witnessed a supernova in the sky was over 400 years ago!
One of the people who bore witness to this event was famed astronomer Johann Kepler. On October 9th, 1604, he spotted the bright object in the sky from his observatory in Prague and tirelessly monitored until it faded from view two years later. His observations were recorded in a treatise titled De Stella Nova in Pede Serpentarii (“The New Star in the Foot of the Serpent Handler“), which was released in 1606.
Thereafter known as Kepler’s Supernova (or Kepler’s Star), the appearance of this bright object would go on to bolster the case being made by Galileo for the heliocentric model. However, it also stands alone as the most recent example of a supernova that was observed in our galaxy. Since then, only one supernova has occurred close to home, which happened in 1987.
This event, known as SN 1987A, was a type II supernova that took place in the Large Magellanic Cloud, the dwarf galaxy located nearly 168,000 light-years from Earth. Part of the problem has to do with perspective. One might get the impression that observing supernovae in our own galaxy would be easier than spotting them in distant galaxies, but they would be wrong.
Observing supernova in our galaxy is more difficult for the exact same reason that astronomers have a harder time gauging the true size and density of the Milky Way. In short, were are inside of it! Since we are lodged in the disk of the Milky Way, it is difficult for astronomers to see the many, many stars that also call the galaxy’s disk home.
Those stars that are brighter and closer to the Solar System tend to obscure the ones that are fainter and farther away. Also, the bulge at the center of the Milky Way prevents us from seeing what’s on the other side of the galaxy altogether. Therefore, it’s much harder to get an accurate assessment of our own galaxy and what goes on in it.
Luckily, back in 2006, an international team led by the Max Planck Institute for Extraterrestrial Physics used data from the European Space Agency’s Integral satellite to calculate how often supernovae occur. Based on their analysis, they determined that a massive star explodes about once every 50 years in the Milky Way on average.
So in other words, NGC 5468 experiences in 20 years what the Milky Way takes 250 years to experience (aka. a factor of twelve and a half). One can’t help but feel a little humbled by that fact. Luckily, scientists have a pretty good idea of when the next supernova in our galaxy will occur – a triple star system located 8,000 light-years from Earth.
This star system is officially designated 2XMM J160050.7-514245 but has been nicknamed “Apep” by astronomers (after the Egyptian serpent deity). Because this system is an example of a rapidly-rotating Wolf-Rayet star – consisting of a large star with two companions, surrounded by a massive pinwheel of dust – it is expected to produce a long-duration Gamma-Ray Burst (GRB) when it undergoes gravitational collapse.
When the star system goes supernova in a few hundred thousand years, it will be a momentous occasion for two reasons. Not only will it be the first GBR in our galaxy to be observed by astronomers, but it will also be visible long enough for astronomers to study it. Let’s just hope humanity or some offshoot thereof is around by that time to appreciate it.
As always, observations of other galaxies in the Universe tell us more about the galaxy which we inhabit. Until the day comes when we can step outside of our galaxy and look back at it, we will be forced to get a better feel of our surroundings this way.
Further Reading: NASA