Supernovae in the Milky Way: an Unobserved Certainty

The remnant of Kepler’s star (or SN1604) as observed by the Hubble Space Telescope in the visible. Credit: NASA/ESA.

On October 9th, 1604, Italian scholar Lodovico delle Colombe was observing the night sky when, in the foot of the Ophiuchus constellation, he noticed a very bright star that had not been there the night before. A firm believer in the Ptolemaic-Aristotelian model of the cosmos, according to which stars are eternal and immutable, he concluded the object had to be a permanent star that was only visible occasionally, and he didn’t pay it much mind.

Johannes Kepler had heard about this new object, but due to cloudy weather over Prague he was not able to begin observation until October 17th; over the next 18 months, he saw the star increase in luminosity until it was brighter than all the planets, and then wane, until it was no longer visible to the naked eye by March 1606. In his book De Stella Nova in Pede Serpentarii (On the New Star in the Foot of Ophiuchus), he argued this had to be a new star, in direct opposition to both religious dogma and the scholarly consensus of his time [1].

Although neither Kepler nor delle Colombe could have known, they would be the last to observe a phenomenon of this kind happening in the Milky Way.

Illustration in Kepler’s De Stella Nova. SN1604 can be seen near the foot of Ophiuchus, labelled “N”. Courtesy of the Caltech Library, California Institute of Technology.

This transient object, now known as Kepler’s star or SN1604, was a supernova, a bright explosion which occurs either in the last stages of the life of massive stars, or as a result of a white dwarf accreting mass from a binary companion, eventually triggering a runaway nuclear reaction.

Although today we discover thousands of them every year thanks to improvements in our monitoring and observation capabilities, since SN1604 no supernovae have been observed in the Milky Way. While we know from their dimmer remnants that other supernovae have occurred in our galaxy, none of them were noticed around the peak of their luminosity [2].

Cassiopeia A (seen above as imaged by the Hubble Space Telescope) is the remnant of a supernova that, according to calculations, exploded in the late 1600s. No confirmed historical records of its observation exist. Credit: NASA/ESA.

This is made even more peculiar if one considers the fact that, from both the observation of galaxies similar to our own and the detection of gamma rays emitted by aluminum-26 (which, like many other elements, is formed almost exclusively in these explosions), we would expect 1 to 3 supernovae to occur every century in the Milky Way [3].

And yet we have seen none for the past 421 years. How is that possible?

The answer has to do with where most of the stars are found in our galaxy, and as a result where supernovae are likely to appear.

An artist’s rendition of the Milky Way, showing the galactic disk in blue. The position of the Sun is labelled, as are some other important features. Credit: NASA/JPL-Caltech.

The Milky Way is a barred spiral galaxy, with the solar system at a distance of about 26,000 light years from the Galactic Centre. Most stars, including the Sun, lie in the galactic disk, which is a fairly thin, circular region around the centre. However, stars are not the only component of the disk—cool gas and dust make up a large portion of it. This “interstellar medium”, or ISM, has substantial effects on observations: the dust grains especially can very efficiently scatter light around the bluer side of the visible spectrum, and enough of them can eventually block most light at optical wavelengths.

This means that a lot of stars in our galaxy are simply hidden from our view by the ISM; when these stars eventually go supernova, their light simply cannot reach us and the explosion, as bright and powerful as it is, goes undetected. In fact, out of the 20 or so supernovae one would naively expect to have appeared in our galaxy in the last millennium, only 5 can be found in the historical record [2], the latest being the aforementioned SN1604. To be able to witness such an event in the future, we need it to occur not only fairly close to the solar system, but also along a relatively clear line of sight.

Thankfully, according to recent research, we have around a 33 to 50% chance of being able to observe the next supernova in the Milky Way with the naked eye [4]; the only thing we have to do is wait. And so, in spite of our incredible technological and scientific progress since the days of Kepler and Galileo (who also observed and wrote about SN1604), all we have been missing for the past 400 years… is a little bit of luck.

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