Astronomers have used light echoes as a time machine to unearth secrets of one of the most influential events in the history of astronomy –a stellar explosion witnessed on Earth more than 400 years ago.
By using a Galactic cloud as interstellar “mirror” an international team led by Oliver Krause of the Max Planck Institute for Astronomy in Germany has now re-analysed the same light seen on Earth in the 16th century and have, for the first time, determined the exact type of the explosion that happened. Calar Alto Observatory has contributed to this discovery and these results were published in the scientific journal Nature, 4th December 2008 issue.
A brilliant new star appeared on the sky in early November 1572. The new star outshined all other stars in brightness and was even visible during daylight. It was widely observed by astronomers all around the world and it helped to change our understanding of the Universe forever. Precise measurements of the star position by the Spanish scientist Jerónimo Muñoz, professor at the University of Valencia, and the Danish astronomer Tycho Brahe, revealed that the star was located far beyond the Moon. This was inconsistent with the Aristotelian tradition that had dominated western thinking for nearly 2000 years. The supernova of 1572 was a cornerstone in the history of science and is today known as Tycho's supernova.
Supernovae of different types
The 16th century astronomers did not know what kind of star they had observed. Only in 1940 it was concluded that it must have been a supernova –an explosion blasting apart a star at the end of its life. Supernovae, in general, comprise the most intense stellar explosions. But not all supernovae are of the same kind. Some of them are related to the sudden collapse of very massive stars at the end of their lives, the so-called type II events. Other supernovae are produced in a process of cataclysmic interaction between the members of a binary stellar system. The most important of this group are the so-called type Ia supernovae, explosions of white dwarf stars.
Light echoes as probe of historic supernovae
Tycho's supernova was observed on Earth in 1572. Too early to use the tools of modern astrophysics for its study? Not really: an international research team has used the technique of light echoing to analyse today some light from that ancient cataclysm witnessed by Muñoz, Brahe and others more than 430 years in the past. Although the direct photons from Tycho’s supernova went past Earth in 1572, they spread out through space in a constantly expanding sphere. When the light hits a cloud of dust and gas off to the side (in the sky) of the supernova, some photons are reflected towards Earth, and they reach us years later. Think of dropping a rock into a still pond –the waves go outwards uniformly until they hit (say) a pier; new waves are generated, which also travel outwards. An observer on the far shore of the pond would first see the direct waves from the rock, and some time later the reflected waves from the pier.
By using such a galactic cloud as interstellar “mirror”, Krause's team could re-observe the same light witnessed on Earth in the 16th century –shortly before the invention of the telescope– with the powerful scientific tools of the 21st century available at modern observatories such as Calar Alto and Subaru. The situation is somewhat similar to that found in the case of supernova Cassiopeia A, studied by this same research team using a similar strategy. The supernova of 1572 is again placed in the Cassiopeia constellation, and at a similar distance: at around 11 000 light-years from the Solar System. The explosion really happened, thus, more than 11 000 years ago.
Crucial data on type Ia supernovae
The spectroscopic analysis of the light echo showed the signatures of the atoms present when the supernova exploded. The resulting spectrum of light revealed silicon but no hydrogen, telltale signs that Tycho's supernova resulted from a type Ia explosion of a white dwarf star. All supernovae of type Ia show practically the same intrinsic luminosity and, for this reason, they are used as cosmological probes to measure the large distances among the galaxies in the vastness of the Universe. The observation of type Ia supernovae in other galaxies has led to the discovery of the accelerated expansion of the Universe, what suggests the existence of the mysterious dark energy that puzzles astronomers and challenges fundamental physics since more than a decade.
Despite their importance, many details of type Ia supernovae remain to be fully understood. All recent type Ia supernovae have occurred in external galaxies. To describe the physics of these events in the greatest detail, it would be ideal if we could observe one of them in our own Galaxy: this is what has been done now in the study performed by Krause's team. The results not only qualify Tycho's supernova as a normal type Ia in our own Galaxy, but also provide a wealth of new information.
In Oliver Krause’s words, "We show that the supernova of 1572 was a standard type Ia event that shows indications of an aspherical explosion. This puts new stringent constraints on explosion models, which can be now compared in great detail to observations of both the explosion and the remnant at the same time." The new data include an exact classification of the event, and new estimates of the power of the explosion, its geometry, and an independent measure of its distance.
An international effort
Now, observations made in the 21st century add to the list of data gathered by Muñoz in his report Libro del nuevo cometa [Book on the New Comet] in 1573, Brahe in his brief book De Stella Nova [On the New Star] and others in the past. Part of the observations leading to this research were performed at Calar Alto (Spain) in August and September 2008, using both the 2.2 m and 3.5 m telescopes of this observatory. Later data were obtained at Hawaii (USA) with the Japanese Subaru telescope.
The results of these studies have been published in the scientific journal Nature, 4th December 2008 issue. The article is signed by Oliver Krause (Max-Planck-Institut für Astronomie, Max Planck Institut for Astronomy, Germany), with the following co-authors: Masaomi Tanaka (University of Tokyo, Japan), Tomonori Usuda (National Astronomical Observatory of Japan), Takashi Hattori (same institution), Miwa Goto (Max-Planck-Institut für Astronomy, Max Planck Institut for Astronomy, Germany), Stephan Birkmann (same institution and European Space Agency), and Ken'ichi Nomoto (University of Tokyo, Japan).
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