Why Haven't We Observed Any Supernovas Since the Last One in 1604?

According to astronomical studies, on average, three supernovas are expected to occur in the Milky Way Galaxy every century. But, why have we not observed any since the last one in 1604?

The Obscuring Dust and the Milky Way Galaxy

Large parts of the Milky Way are shrouded in dust, making it difficult to observe even bright supernovas. Despite the low frequency of such cosmic events, two supernovas have been identified in our galaxy post 1604.

Cassiopeia A

Cassiopeia A, the remnant of a supernova that exploded around 1670, initially went unnoticed. It was possibly cataloged as star 3 Cassiopeia by John Flamsteed in 1680. However, it was only noticed in 1947 as one of the brightest radio sources in the sky. This highlights the challenge of observing supernovas obscured by dust clouds.

G1.90.3

G1.90.3, the remnant of a supernova that exploded around 1868, was undetected due to the dust in the center of the Milky Way. This region is so obscured that even significant events like supernovas might go unnoticed.

Our Position in the Milky Way

We reside on one side of the Milky Way, approximately one-third the distance from the center to the edge. Our galaxy is flat, resembling a pancake, with us situated in its middle. On a clear night, from a truly dark location, one can observe around 2,000 discrete stars. The majority of these stars are within a radius of 2,000 light-years (ly) from Earth, with the farthest visible star being approximately 16,000 ly from Earth. The Earth itself is 25,000 ly from the center of the Milky Way.

Given these facts, it is apparent that we can only see a small portion of the Milky Way. Even if supernovas were visible, obscuring dust and dense regions, like the galactic center, would block our view.

The Role of Dust and Clusters

Dust clouds play a crucial role in obstructing our view of the Milky Way. Consider the Horsehead Nebula as a prime example—dark clouds that impede our ability to see beyond them. There are certainly regions in the Milky Way beyond our detection, as evidenced by the discovery of a galactic cluster by NASA's Hubble Space Telescope last year (NGC 6752).

Average vs. Reality

The term "average" in supernova occurrences is an estimation based on data and statistical analysis. It is possible to experience significant fluctuations. For instance, you could witness 300 supernovas in a single year before having to wait an exceptionally long time—potentially up to 10,000 years—without observing any.

Additionally, the advent of advanced telescopes in the recent decades (capable of observing in the infrared, ultraviolet, X-ray, and gamma-ray spectra) and the use of radio telescopes have improved our ability to detect supernovas. However, there is no guarantee that every supernova will be observed, particularly those obscured by vast dust clouds.

Thus, the absence of observed supernovas since 1604 can be attributed to a combination of natural phenomena like dust clouds, the vast dimensions of our galaxy, and the role of statistical fluctuations in the occurrence of such dramatic cosmic events.