Benedetta CappelliniEditoriale

A few days ago I went downstairs to the cafeteria of the Space Telescope Science Institute to get a “Grande Non-Fat Latte”, by now my preferred American drink. In the half-desert hall, I noticed three colleagues, Adam Riess, Mario Livio and Mike Hauser, sitting at a table and intent in a discussion. We are dealing, let’s be clear, with three scientific personalities of a certain caliber. Adam Riess is the one who discovered the acceleration of the universe, and therefore the enigmatic “dark energy” which represents the greatest unsolved mistery of this last decade. He may get the Nobel prize for this. Mario Livio is an eminent theoretical astrophysicist, and besides having a monstrous publication and citation record, he is a writer of best sellers translated all over the worlds. Mike Hauser, our deputy director, was previously the responsible of one of the three instruments that flew on the COBE satellite to map the cosmic background radiation, the primordial light emitted soon after the Big Gang. The Nobel prize for this measure has been assigned only to the responsible of the two other COBE instruments, John Mather e George Smoot. Call it bad luck, but anyway, we are at those levels.
What called my attention to them was the peculiarity of their behavior. Adam had a few sheets of paper in his hands, and was excitedly showing them to the other two. They, on their turn, were staring at the pages with their mouth open, nodding from time to time. The evident excitation reminded me, let me pass the comparison, a scene from a Caravaggio’s painting, made even more incredible by the otherwise usual composure especially of Mario and Mike, who are in their 60’s and don’t get easily impressed when someone has a story to tell.  Holding my mug, I walked closer to them, ‘fourth amid so much wisdom’, saying openly that I could not resist to the curiosity of knowing what was going on, what they were looking at so astonishing. Patiently, Adam smiled turning the pages from the beginning, while the other two, not at all annoyed, rather encouraged him like saying, “please go, tell us again!”. And what I heard is very surprising.
In the year 1572, exactly on the 11th of  November, a new star appeared in the constellation of Cassiopea. In a few days it became as bright as Venus, visible also in the daylight, but soon started to dim and eventually disappeared to the naked eye after a bit more than one year. The greatest astronomer of his time, Tycho Brahe, established that the star was not moving with respect to the others, therefore it was not a planet but it belonged to the sphere of fixed stars, which were supposed, with Aristotle and Ptolemy, perfect and immutable. A shoking discovery, at that epoch.
Tycho Brahe had observed what we call today a supernova. Supernovae represent the most formidable event in stellar astrophysics, the final explosion of a star. The amount of energy released in the explosion is comparable with the energy emitted by all the stars in a galaxy, about 100 million suns. Being so bright, supernovae can be detected up to enormous distances, comparable with the size of the observable universe. But those close to us are also extremely rare (we may say luckily!). By chance, a few years later, in 1604, Tycho’s pupil Johannes Kepler discovered another  supernova. After these two in short sequence, nothing for 400 years. We are still waiting.
Let’s also say that there are two types of supernovae: those of type I and those of type II. Type II supernovae are produced by supergiant stars, who explode at the end of a short and intense life, sort of cosmic James Dean. Type I supernovae have more modest origins. Let’s imagine two stars like our Sun, but close to the point that they nearly touch while they are orbiting around each other. During their life they exchange material up to the point that one gets completely naked, with the exception of its central “hard core”, whereas the other swells enormously. Technically, the first one is a “white dwarf”, the second one a “red giant”. The giant wants to return some material to the dwarf, but a white dwarf cannot accept more than a certain amount: beyond a critical threshold it gets unstable and explodes catastrophically.
Supernovae of this type, that more than James Dean remind us of a mediocre mid-life couple crisis, still emit more light than type II supernovae. But the reason why they are especially important it that their explosions are all, more or less, equal, i.e. they release the same amount of energy (I must skip the details here). Therefore, they represent fundamental distance indicators: if I know how much light is emitted and I measure how much light I receive, I get the distance. It is exactly using this method that Adam Riess and coworkers have discovered that something is strange in the way the universe expands. In fact, it is accelerating.
What type of supernova was that one of Kepler? Unfortunately, we don’t know. Its light came to us a few years before Galileo’s discovery of the astronomical telescope, in 1609 (we celebrate this year the 400th anniversary). Three more centuries had to pass before we could learn how to study the light emitted by celestial bodies through spectroscopy, at the end of the 1800. Kepler’s supernova therefore exploded too early, civilization had not yet develop to the point of allowing us to study it. It is a shame, because it would be fundamental to measure with exquisite precision a nearby supernova, especially a type I, to better understand the explosions that occur in the furthest corners of the universe, and from which we get only faint signals.
In fact, the light emitted from Tycho supernova propagated like a wave through all space. Depending on the direction, on its path it intercepted other celestial bodies, stars and nebulae. And here is the unexpected event: one of those anonymous clouds of interstellar gas and dust, reached at a certain point by the ephemeral flash, lightened up reflecting the light of the supernova all around, sending therefore its late signal also to the Earth. It is what astronomers call a “light echo”. The delay of this echo is about 400 years. Four centuries that allowed us to discover the telescope, astrophysics and so on. In this way, on September 24 2008, a group of German and Japanese astronomers have observed from Hawaii the light echo of Tycho supernova and got its spectrum. It is just a spectacular type I supernova spectrum. Finally!
Nature, we can say, has been patient with us, compensating our slowness and giving us a second chance. Reality is generous. This does not diminish the fact that one has to be a brilliant scientist to look for these things, to be “ready”. In this episode there is the whole dynamic of the adventure of scientific discovery. But in these days I keep thinking especially to the faces of my three friends, able only to repeat themselves “amazing!”, to our talking and chatting perceiving that the universe is filled in any corner by a vanishing echo of all lights appeared in its past, and to this evidence that the great mistery of reality has –literally- the goodness of revealing  himself and never satiate us, like a true lover. These are beautiful things. And we are privileged, because it is beautiful to grow older and begin a new year (the International Year of Astronomy!) more and more able to amaze ourselves.