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 theDanish 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
An international research team has now used light echoes from the ancient supernova outburst to precisely classify the supernova witnessed by Tycho 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", Dr 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 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 the backyard of our own Galaxy, but also provide a wealth of new information which can be now compared in great detail to observations of both the explosion and the remnant at the same time.
The results of these studies have been published in the scientific journal Nature, 4th December 2008 issue. The article is authored 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).
Light echoes from Tycho Brahe's 1572 supernova
Animation showing the evolution of the light echoes from the supernova explosion.
Animation 1: This animation illustrates how a light echo works and how it can be used for time travel. A supernova explosion acts like a cosmic flashbulb. The wave of light from the explosion zips through space. When the light wave is hitting dust particles of an interstellar cloud, some light is reflected back. This reflected light forms a secondary wave of light which is delayed relative to the original one by some time – this is called light echo. In the year 1572 the direct light wave from a supernova explosion swept past Earth and was observed by Tycho Brahe and others. Now, more than 400 years later a secondary wave of light of the supernova was observed. Using the scientific instruments of the 21st century, the mystery of the famous 16th century supernova could be solved.
Animation showing the evolution of the supernova remnant over time until today's view.
Animation 2: Artist's illustration of how a supernova remnant is born. The animation starts with the progenitor of Tycho Brahe's supernova of the year 1572. Following a titanic thermonuclear blast, which has blown apart a white dwarf star, material is ejected into interstellar space at an incredibly high velocity of up to 30,000 kilometres per second – or one tenth of the speed of light! Over the last 4 centuries the debris have expanded to a diameter of more than 20 light years. Million degree hot gas as well as heated dust particles are seen in an composite image of the remnant today, which has been obtained with the Chandra and Spitzer Space Telescopes and the Calar Alto observatory. This real image concludes the animation.
Multi-wavelength image of Tycho's Supernova remnant in its present form.
Figure 1: This composite image of the Tycho supernova remnant combines infrared and X-ray observations obtained with the Spitzer and Chandra space observatories and the Calar Alto observatory. It shows the scene more than 4 centuries after the brilliant explosion witnessed by Tycho Brahe and other contemporary astronomers as "Stella Nova". The thermonuclear explosion of the white dwarf star has left a several million degree hot cloud of expanding debris (green, yellow). The location of the blast's outer shock wave can be seen as blue sphere of ultra-energetic electrons. Newly synthesized dust in the ejecta as well as heated pre-existing dust from the circumstellar medium of the supernova radiates at a wavelength of 24 micron (red). Fore- and background star in the image are white.
Credit: Prof. John P. Hughes,
Dr. Jeonghee Rho and Dr. Oliver Krause
Tycho Brahe witnesses the supernova of the year 1572
An engraving showing the observation of the supernova by Tycho Brahe in 1572.
Figure 2: 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 great Danish astronomer Tycho Brahe reported in his book "Stella Nova" revealed that the star was located far beyond the Moon. This was inconsistent with the Aristotelian tradition that the "Eighth Sphere" of stars would be unchangeable and eternal. This tradition had dominated western thinking for nearly 2000 years. The supernova of 1572 was therefore a cornerstone in the history of science. It set the stage for the work of Kepler, Galileo, Newton and others and is today known as Tycho's supernova. The engraving shows Tycho Brahe observing the new star in the Cassiopeia constellation (upper left corner).
Credit: Camille Flammarion,
Astronomie Populaire, Paris 1879
Comparison of the newly obtained spectrum of Tycho's Supernova with other type Ia supernovae of different luminosities.
Figure3: All important characteristic spectral lines of the light echo from Tycho's supernova (red) are consistent with the spectra of other classic Type Ia supernovae. The strong absorp- tion of Si II is particularly clear. The calcium line labeled "HV Ca II" at a wavelength of 798.0 nanometers (HV stands for "high velocity") probably originated in matter ejected asymmetrically in the explosion.
Comparison of the newly obtained spectrum of Tycho's Supernova with different subgroups of type Ia.
Figure4: Das Spektrum der Supernova des Jahres 1572 (jeweils schwarz) zeigt weitgehende Übereinstimmung mit einer normalen Supernova vom Typ Ia (orange, Mitte). Etwa ein Drittel aller Supernovae vom Typ Ia zeigen eine deutlich höhere oder eine deutlich niedrigere Leuchtkraft. Wie ein Vergleich mit den Vertretern niederer Leuchtkraft (rot, unten) zeigt, wurde eine für diese Objekte typische Absorptionsbande bei einer Wellenlänge von 4300 A, die vom Element Titan herrührt, im Spektrum von Tycho Brahes Supernova nicht gefunden. Besonders leuchtkräftige Supernovae vom Typ Ia zeichnen sich durch die schwächere Absorption durch Silizium bei 6100 A aus. Das Fehlen einer solchen schwachen Siliziumlinie schließt auch die Zugehörigkeit zur leuchtkräftigen Untergruppe aus.