This article comes from the journal Sciences et Avenir – Les Indispensables n ° 206, July / September 2021.
Anne Decourchelle is an astrophysicist with the French Atomic Energy Commission (CEA). It evokes the star observed on November 11, 1572 by the astronomer Tycho Brahe (1546-1601) and which gave the graceful touch to the ancient conceptions of the Universe.
Sciences et Avenir: Why is Tycho’s star called SN 1572?
Anne Decourchelle: Le chiffre vient de l’année où Tycho Brahe l’a découverte, 1572. I reported the temporary variations of l’intensité de sa nouvelle étoile dans De Stella Nova in 1573, puis dans Progymnasmata, published in 1602, après his death. As for the term supernova (SN), it was not until the 1930s that it was introduced by the German Walter Baade and the American Fritz Zwicky to designate these very bright novae that correspond not to new stars, but to the explosion of stars in the end of his life.
Based on Tycho Brahe’s measurements, Walter Baade concluded in 1938 that the one observed by the Dane was a supernova and clarified, in 1945, that it was type Ia. This corresponds to a double system where a small aging star, a white dwarf, swallows so much material from its companion that it explodes and there is nothing left except the ejected material that can be observed today. As for the accurate evaluation of the distance of SN 1572, it ranges from single to quadruple. A summary, carried out in 2010 by a Japanese team led by Asami Hayato of the Riken Institute, yields values ranging from 4,800 to more than 19,000 light years, depending on the models used.
Have we found your companion star?
A few candidates have been identified, but the debate remains open even if one of them, named Tycho-G, is privileged. Meanwhile, it is the residue of the explosion that is the most studied. It has been observed at all wavelengths, from radio waves, since 1950, to very high-energy gamma rays since 2011.
Why this interest?
It is one of the few historical supernovae, and one of the few in which we were able to observe the echoes of the light emitted during the explosion over the interstellar clouds, which confirmed its nature as a normal type Ia supernova. During these types of events, matter from the star, ejected at tens of thousands of kilometers per second, collides with interstellar matter, generating a double-collision structure. These shocks compress and heat the X-ray emitting material to tens of millions of degrees. In this way, we can observe the emission lines of silicon, sulfur and iron, which allows us to quantify the elements produced and expelled during the process. Explosion, and even the signature of accelerated particles, source of the detected gamma rays. The observations of the rest of SN 1572 and their comparison with the models therefore contribute to the understanding of the physics of the explosion, the physics of collisions and the acceleration of cosmic rays.
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