The New Star Tycho Brahe Saw From Denmark in 1572

On the evening of 11 November 1572 the 25-year-old Tycho Brahe was walking back from the alchemical laboratory at his uncle’s estate of Herrevad Abbey — then in Danish Skåne, now in southern Sweden — to the abbey’s main residential building when he looked up at the northern sky and saw something that was not supposed to be there. In the constellation Cassiopeia, directly above the abbey’s south tower, there was a star approximately as bright as Venus that had not been there the previous night.

The standard Aristotelian cosmology of 16th-century European astronomy held that the heavens were unchanging. The fixed stars sat on a single transparent crystalline sphere that rotated around the Earth once every twenty-four hours; the spheres were perfect, eternal, immutable, ungenerated, incorruptible. Any change in the visible sky — comets, meteors, atmospheric optical effects, any apparent appearance or disappearance of stars — had to be explained as a phenomenon of the sublunar atmosphere, occurring below the lowest celestial sphere (which carried the Moon). The heavens above the Moon did not, by the standard physical theory of the universe, contain anything that could become brighter or dimmer.

Tycho, who at twenty-five had spent the previous decade as an unusually able amateur astronomer, called over the servants to make sure he was not hallucinating.

The measurement

The next morning he wrote down everything he had measured: the new star’s position relative to the eleven principal stars of Cassiopeia, its brightness (estimated as approximately equal to Venus at its brightest, which would have been about apparent magnitude -4), its colour (initially white, with possible slight blue tinge). He repeated the measurements the next several evenings and produced the most precise documented contemporary positional record of the object.

The critical measurement was the new star’s parallax — the small apparent shift in its position relative to the background of fixed stars caused by the observer’s motion around the Earth as the Earth rotated through the night. If the new star was, as Aristotelian doctrine implied, a phenomenon of the sublunar atmosphere — a meteor, an atmospheric optical anomaly — it would have shown substantial nightly parallax against the fixed-star background. If it was beyond the Moon, in the supposedly unchanging celestial regions, it would show no measurable parallax at all.

Tycho measured the parallax. It was within his measurement uncertainty of zero. The new star was beyond the Moon. The heavens were not, in fact, unchanging.

The publication

Tycho’s pamphlet De Nova Stella — “On the New Star” — was published in Copenhagen on 5 May 1573, eight days short of the new star’s six-month anniversary. It was the most consequential scientific publication of the 16th century not titled De Revolutionibus. The 52-page treatise included Tycho’s positional measurements, his brightness observations, his parallax analysis, his conclusion that the new star was beyond the Moon, and a substantial polemical argument that the Aristotelian doctrine of unchanging heavens was inconsistent with the observational evidence.

The reaction across European scholarly astronomy was rapid. Multiple independent astronomers had observed the new star themselves (it was visible to the naked eye for sixteen months, gradually fading from initial Venus-brightness through autumn 1572 to invisibility in March 1574). The English mathematician Thomas Digges, the German astronomer Michael Maestlin (Kepler’s later teacher), the Czech court astronomer Thaddaeus Hagecius, and the Wittenberg astronomer Wolfgang Schuler all independently confirmed Tycho’s parallax measurement and reached substantially similar conclusions about the supralunar character of the object. By the summer of 1574 the substantive scholarly astronomical consensus across Europe had accepted that a new star had appeared in the supposedly fixed heavens.

The Aristotelian-philosophical mainstream of European universities took substantially longer to absorb the implication. The theologically-charged question of whether the heavens were unchanging was substantially independent of whether astronomical observations supported the doctrine. The doctrine itself remained the official teaching at most European Catholic universities into the 17th century and at most Lutheran universities into the early 18th. The progressive dismantling of Aristotelian cosmology in the European intellectual mainstream took, depending on the institution, between thirty and two hundred years after Tycho’s 1573 pamphlet.

The companion event

The new star of 1572 was the more famous of two consecutive Aristotelian-cosmology-shattering observational events in Tycho’s lifetime. The second was the Great Comet of November 1577, which Tycho also measured for parallax from his new observatory at Uraniborg (built between 1576 and 1580 partly on the strength of his 1572 reputation). The 1577 comet’s measured parallax also placed it well above the Moon, demonstrating that comets — which Aristotelian doctrine had unambiguously treated as sublunar atmospheric phenomena — were in fact celestial objects moving through the supposedly fixed crystal spheres. The two observations together — a new star that should not have been there, and a comet moving through space that should have stopped it — comprehensively dismantled the medieval Aristotelian cosmology on observational rather than theoretical grounds.

The further development of European astronomy from this point — Kepler’s laws (1609-1619), Galileo’s telescopic observations (1610s), the Newtonian synthesis (1687) — built on the foundation of an observationally-determined cosmology that Tycho’s 1572 and 1577 measurements had made institutionally necessary.

The afterlife

The supernova of 1572 — now catalogued as SN 1572 in the modern astrophysical nomenclature — is one of the eight historically-recorded naked-eye supernovae in our galaxy. Modern radio observation of the supernova remnant (the diffuse expanding shell of gas that the original explosion produced) places it approximately 8,000 light-years from Earth in the direction of Cassiopeia, classifies it as a Type Ia supernova (a thermonuclear detonation of a carbon-oxygen white dwarf in a binary system), and dates the explosion itself to approximately 6,000 BC — meaning the light that reached the Danish sky on 11 November 1572 had left its source approximately at the time of the founding of the Mesopotamian Sumerian cities. The remnant continues to expand at approximately 9,000 kilometres per second; it is approximately seven light-years across in current observations.

Tycho himself died at Prague in October 1601, twenty-nine years after the supernova he had measured at Herrevad Abbey. His tombstone in the Týn Church bears, among the standard genealogical details, the year 1572 as the moment of his entry into substantive scientific reputation. The pamphlet De Nova Stella is still in print.