how the 1572 supernova was discovered

The Mechanism

Tyge Ottesen Brahe — Latinised as Tycho Brahe — was born on 14 December 1546 at Knutstorp Castle in Scania, then a Danish province (now southern Sweden), the eldest son of a Danish privy-councillor and a lady-in-waiting to Queen Sophie. He was raised from about a year old by his childless uncle Jørgen Brahe, a naval officer who drowned in 1565 saving the future King Frederick II of Denmark from falling into a canal off a bridge in Copenhagen. As a student at Rostock in 1566, Tycho lost the front of his nose in a duel over a mathematical dispute with a fellow Danish student, Manderup Parsberg, and wore for the rest of his life a metal prosthesis — long said in the popular literature to have been silver or gold, but which post-mortem chemical analysis of his 2010-exhumed remains showed to have been brass. By November 1572 he was 25, staying at Herrevad Abbey — the former Cistercian monastery in Scania that his other uncle Steen Bille had taken over after the Reformation, and where Tycho had built a small alchemical laboratory. On the evening of 11 November 1572, walking home from his uncle's supper, he glanced up and saw in the constellation Cassiopeia — the recumbent "W" of five stars overhead in the autumn Nordic sky — a sixth star of the first magnitude that had not been there the night before, or any night before that. He called his household out to confirm he was not hallucinating. The star brightened over the next fortnight to a magnitude of about –4, brighter than Venus at its brightest, and remained visible in daylight for perhaps two weeks. Tycho did the one thing that mattered: he measured its position, night after night, against the fixed stars of Cassiopeia with a large wooden sextant of his own construction, and found that it did not move relative to them. The Aristotelian cosmology in force in every European university in 1572 taught that the celestial sphere beyond the moon was perfect and unchanging — that any transient phenomenon in the sky, a comet or a new star, must be a sublunary meteorological event in the Earth's upper atmosphere, and would therefore show a measurable parallax as it drifted across the sky over hours. Tycho's measurements set an upper limit on the parallax of the new star of a few arcminutes: the new star was farther from Earth than the moon; farther, in fact, than any of the planets — it was among the fixed stars. He published his findings the next year, in Copenhagen, as *De Nova et Nullius Aevi Memoria Prius Visa Stella* (On the New and Never Previously Seen Star), 52 pages long, imprint 1573, dedicated to the Danish court. The word "nova," from the title of that book, entered every European language as the astronomical term for a new star. It was the founding shot in a two-century demolition of the Aristotelian doctrine of celestial immutability: five years later, in 1577, Tycho would measure the parallax of a great comet and show that it too was farther away than the moon and moved through the supposed crystalline spheres of the planets without disturbing them. In 1576 King Frederick II gave him the island of Hven in the Øresund to build the observatory Uraniborg — Europe's first purpose-built research observatory — where over the next twenty years Tycho and his assistants would make the pre-telescopic naked-eye astronomical measurements that Johannes Kepler, arriving in 1600 as Tycho's mathematical assistant, would use to derive the elliptical orbits of the planets and open modern astronomy. Tycho died in Prague on 24 October 1601 at 54, of a bladder disorder aggravated by a court dinner at which he was too polite to excuse himself before the meal was over. The 1572 "new star" is now known to have been a Type Ia supernova — the thermonuclear detonation of a white dwarf in a binary system — approximately 8,000 to 10,000 light-years away in Cassiopeia, its expanding remnant, catalogued as 3C 10 and now called Tycho's Supernova Remnant, still visible today as a shell of shocked gas about 8 arcminutes across, imaged in X-rays by the Chandra observatory.