On November 11, 1572, on his way home, Tycho was surprised by a "new" star in Cassiopeia which was shining at about the brightness of Jupiter and which had not been seen in this place before. He had found the famous Supernova of 1572 which he observed for 16 months until March, 1574, and described it in his book, "Stella Nova."
Consequently, Tycho turned to professional astronomy and with the help of Danish King Frederick II, established his observatories "Uranienburg" (1580) and "Sternenburg" (or Stjerneborg, 1584) on the island Hven, where he performed astrometric measures which were the best in pre-telescopic times. Results were the discovery that comets are situated in space beyond the Moon, of Lunar variation and acurate planetary pathes in the sky. Of praticular importance were his considerably acurate planetary observations, notably of Mars; these data were used by Kepler to deduce the laws of planetary motion (Kepler's laws).
Tycho also invented a new (the "Tychonic") world system, opposing Copernicus because of the lack of observable stellar parallaxes: In his system, the planets Mercury, Venus, Mars, Jupiter and Saturn should orbit the Sun, but the Sun together with the Moon should revolve around an Earth at rest. Moreover, he was engaged in astrology and alchimy.
Tycho left Denmark after the death of King Frederick in 1597, and in 1600, went to Prague to accept a post as Emperial Mathematician and Astronomer at the court of Rudolf II. He died on October 24, 1601 at Benatky near Prague.
Honors include naming famous and conspicuous Moon crater Tycho (43.4S, 11.1W, 102 km diameter, in 1935) and Mars crater Tycho Brahe (49.8S, 213.9W, 106 km diameter, in 1973). Asteroid (1677) Tycho Brahe was discovered on September 6, 1940 by Y. Vaisala at Turku observatory and provisionally designated 1940 RO; other pre-discovery and later independent observations were designated 1928 SP, 1935 FL, 1952 QN1, 1952 SD1 and A916 UA.
Tycho's observing place, Stjerneborg on the island Hven, now Swedish territory, has been reconstructed and can be visited as the Tycho Brahe Museum, see e.g. the description in De Vorkin (2003).
His great star catalogue, of epoch 1601, was published after his death only, by Johannes Kepler in 1602-1603, and contained 777 stars. A later edition was enlartged to 1005 stars. Of these, Tycho described six as nebulous, of which only one, the Praesepe Cluster (M44), corresponds to a real object, while the others are merely asterisms, or stars erroneously seen as nebulous due to unknown reasons.
The list of Tycho's nebulous objects follows:
(1) No. 175 Her 20. The highest [star in Her] (Neb) Asterism? (2) No. 576 Cnc 1. Nebula in the breast [of Cnc], Praesepe (Neb) M44 (3) No. 717 Cap 4. Cloudiness above and preceding the horn Ast. of [of Cap] (Mag. 6) Zeta 1/2 (4) No. 718 Cap 5. Cloudiness at the western base of the Star triangle on the forehead (Neb.) Pi Cap (5) No. 719 Cap 6. Cloudiness to the East (Neb.) Omicron Cap (6) No. 721 Cap 8. Cloudiness preceding the forehead (Neb.) Sigma CapNumbers are those in Bailly (1843), here taken from Kenneth Glyn Jones (1968).
It is surprising that this great observer failed to detect the two most prominent nebulae, the Orion Nebula (M42) and the Andromeda "Nebula" (M31). For the latter, this resulted in speculations about a possible variability in brightness, e.g. by Bullialdus (1667) and Le Gentil (1759).