It turned out that supernovae had been exploding in every single place, we simply could not see them

When the poet Horace said, “We are only dust and shadow,” he probably did not think that dust itself could create a shadow. But it can, and this shadow can even obscure some of the most powerful explosions in the universe. At least that’s the result of new research by an international team using data from the recently decommissioned Spitzer telescope. It turns out that dust in galaxies far away can obscure supernovae.

Existing theoretical models have predicted almost twice as many supernovae as observed in the wider universe. More specifically, they overestimated the number of supernovae observed in more distant parts of the universe. Scientists assumed that the missing supernovae existed, they could only not be captured in the visible light spectrum. You were right.

NASA video of the Spitzer Space Telescope exploits
Credit – NASA YouTube Channel

Spitzer is an infrared telescope, which means it can see through otherwise opaque material that other telescopes cannot see through. When it turned its attention to 40 relatively nearby galaxies, it found 5 new supernovae that were not captured by any optical telescope.

Five new supernovae may not sound like much, but it was for the relatively short time Spitzer spent with the observation program and for the relatively small part of the universe it was focused on. If one extrapolates this number to the entire universe, the number of supernovae increases almost perfectly, which corresponds to the theoretical expectations.

Spitzer isn’t the only infrared telescope useful for finding hidden supernovae. Here are three more under construction.

So the supernovae were actually obscured by something, and pretty quickly it became clear that the obscured material was “dust”. Not the same type of dust that you find in a house, not even really the same type of dust that you find on Earth, but interstellar dust, which is made up of particles about the size of a grain that together have the same effect as smoke.

This dust permeates many galaxies, especially those that are farther away and “younger” than the Milky Way, which means that the dust has not yet been able to settle. The further away and younger a galaxy is, the more likely it is that its supernovae will be obscured by dust. It is noteworthy that the observed number of supernovae in nearby galaxies, which do not have as much star-forming activity and are generally less dusty, was already more in line with the theoretical predictions.

Supernovae not only emit light in the visible and infrared spectra – they would potentially also generate sound waves.

Fortunately, dust does not obstruct the infrared light, which means that Spitzer can pick up infrared radiation that would otherwise be indistinct to optical telescopes. Supernovae gave off their fair share of infrared light (which can also be thought of as heat) so Spitzer could absorb them too. It was still challenging, however, as supernovae in galaxies really far away may not be powerful enough to accommodate on Earth.

To explain this, the 40 galaxies the researchers studied were located closer to Earth, but were in certain types of galaxies that are known to be heavily enveloped in dust. Known as Luminous and Ultra-Luminous Infrared Galaxies (LIRG & ULIRG), these galaxies were perfect candidates and were close enough to capture a supernova with a relatively short observation time and easily observed with visible light spectral platforms.

There are still many supernovae in other galaxies that we can see. Here is a photo taken by the Hubble Space Telescope of the 1994D supernova in the galaxy NGC 4526.
Credit – NASA / JPL-Caltech

With this new data come new models of both star death and star formation. Such stellar activity is one of the pillars of astrophysics, and a clearer look at this activity will lead to better understanding and hopefully models that can be used in other parts of the scientific world. So dust may not be as effective as a shadow after all.

Learn more:
JPL stars explode in dusty galaxies. We just can’t always see them
MNRAS – A Spitzer study for dust-covered supernovae
Sci-News – Spitzer discovers five dust-covered supernovae
UT – The way these stars look, a supernova is inevitable

Mission statement:
Image of a galaxy by Spitzer and Hubble, with Spitzer’s contribution in the white circle, where the infrared light reveals a previously obscure supernova.
Credit – NASA / JPL-Caltech

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