Cassiopeia A: Echoes from the Past

But now, an international scientific team has performed an impressive work of celestial archaeology: they have used the interstellar dust as a look-back mirror that allowed them to receive news from the past.

Some light from that explosion was reflected by dust clouds placed at some distance from the dying star, and this reflection has been detected and analysed. This way, modern astronomy witnesses the cataclysmic display that our ancestors did not study, for some reason, in the 17th century. Calar Alto staff, telescopes and instruments have contributed to this finding.

Supernovae in our Galaxy
The word supernova refers to several different processes that lead to the cataclysmic, explosive destruction of a star. These phenomena count among the most energetic displays found in nature, and a typical supernova can reach a brightness similar to that of a whole galaxy during a short period of time.

Throughout the history of mankind Galactic supernovae (i.e., those that have exploded inside our Galaxy) have been among the most spectacular astronomical events witnessed. These flashes have been seen and written down starting in China more than two thousand years ago. No doubt, the most famous example is the explosion observed in year 1054, certainly seen by different cultures all across the Earth, but accurately registered only in the far East. This supernova led to the Crab nebula (in constellation Taurus). More recent supernovae were detected in 1572 (Tycho's supernova) and 1604 (Kepler's supernova). In both cases extremely bright, “new” stars were seen shining during several weeks at places where no previous object had been located before.

The celestial object known as Cassiopeia A (or, also, 3C 461) was the first astronomical source of radio waves detected in constellation Cassiopeia, around year 1940. It was identified soon as a supernova remnant, the remains of one of these giant explosive processes. The astronomical community was very puzzled, because the expansion rate of the gas inside Cassiopeia A indicates that the explosion had to be very recent (around 1680), but it seemed that the burst had never been observed.

Knowing the approximate outburst date from the gas motion inside Cassiopeia A, a search was started looking for observational records from 17th century, and finally a possible observation was found, done by the English astronomer John Flamsteed. The identification was not absolutely sure but, if true, it would imply that the Cassiopeia A supernova showed very faint in the Earth skies.

Behind the dust
The observation of other galaxies indicates that a normal spiral galaxy as our own would experience, in average, one supernova explosion every 50 years or so. But the scarcity or even lack of recent bright supernovae in our Galaxy seemed a mystery until recently, when it was found that galactic supernovae are as frequent as expected, but several of them have happened at places where they lay hidden behind thick layers of interstellar dust, what reduces their apparent brightness, or even makes them completely invisible from Earth. Now it is known that this was the case of the two last galactic supernova events: the most recent explosion could have been seen around 1870 (supernova G1.9+0.3), if interstellar absorption would not have completely hidden it out of view, and one more (Cassiopeia A supernova) blinked in the Earth's skies in 1680. The 1680 event was also very obscured by interstellar dust, but it seems that it could have became bright enough to have been glimpsed by Flamsteed.

But the paths of science are often surprising and a team of researchers have now found the way to retake a part of the light that went straight past Earth more than three centuries ago, clarifying the nature and circumstances of the Cassiopeia A supernova. The key lays in the process known as light echoing.

Light echoing
Let's imagine supernova Cassiopeia A exploding some 11 000 years ago, in a region of our Galaxy full of interstellar gas and dust. More or less 300 years after the explosion, part of the light emitted by the supernova lighted up a dusty blob. This cosmic lump reflected a fraction of the radiation towards Earth, but this reflected light started its journey towards us with a delay of 300 years. 11 000 years after the explosion, the main bulk of light emitted by the supernova reached Earth, but it had been so softened by the interstellar dust, that the event went almost unnoticed and no scientific study was done. Finally, 300 years more later, the small quantity of light reflected by that humble dust knot arrives to our planet. Three centuries have been enough time to develop observatories such as Calar Alto, Subaru and Spitzer Space Telescope. And here was the international team of scientists, led by Oliver Krause (Max-Planck-Institut für Astronomie, Germany), to use these facilities to perform a study of the supernova not through its direct emission, but from the radiation bounced by this dusty look-back mirror.

The light echo effect allows today to study in detail what happened 300 years ago: this process offers an action replay of the supernova which created the most spectacular supernova remnant on sky, and which has at most been witnessed by Flamsteed as a faint star in a time when the first telescopes had just been invented.

So, we are placed in front of this surprising paradox: interstellar dust, the main reason that prevented the study of the explosion in the 17th century, makes it possible to recover part of the lost light, but several centuries later, when human kind has developed much more powerful telescopes, instruments and theories, what allows a better understanding of this event.

The research
“Cassiopeia A lies in our cosmic backyard and offers the sharpest view of what is left hundreds of years after a supernova explosion,” says Oliver Krause, who adds: “The echoes of light we found around Cassiopeia A provide us with a time machine to go back and see its past.” Another member of this research team, astronomer Tomonori Usuda, considers that “This result is exciting because the telescope acted like a time machine.”

The analysis of the light echo showed the signatures of the atoms present when Cassiopeia A exploded. The resulting spectrum of light revealed hydrogen and helium — telltale signs that Cassiopeia A was once a huge red supergiant star whose core collapsed in a rare supernova referred to as “type IIb.” Previously, it was not clear the supernova class to which Cassiopeia A belonged. The result contributes to expalain why the explosion went unnoticed in 1680. Beside the absorbing effect of interstellar dust, now it is necessary to add the specific behaviour of this kind of explosions: “Type IIb supernovae fade quickly,” says co-author George Rieke (University of Arizona, United States of America). “This, plus a few cloudy nights, might explain the historical enigma around Cassiopeia A,” explains Rieke, referring to the reason why it was not recorded by more observers.

Part of the observations leading to this discovery were performed at Calar Alto observatory in October 2007, with the 2.2 m telescope. Key observations were done, too, at Hawaii with the ground-based Subaru telescope, and with the orbiting Spitzer Space Telescope. But, as in any scientific pursue, all this work could be just the beginning. Oliver Krause announces: “We are already excited about future results from the echo research. Beside a detailed study of the Cassiopeia A supernova based on additional Calar Alto and Subaru observations this summer, we are also thinking about a variety of questions about the interstellar medium which can be addressed by the unique light echo experiment Nature has provided us with Cassiopeia A.”