  We describe the physical and orbital properties of C/2011 W3. After surviving
perihelion, the comet underwent major changes (permanent loss of nuclear
condensation, formation of spine tail). The process of disintegration
culminated with an outburst on December 17.6 (T+1.6 d) and this delayed
response is inconsistent with the rubble pile model. Probable cause was thermal
stress from the heat pulse into the nucleus after perihelion, which could also
produce fragmentation of sungrazers far from the Sun. The spine tail was a
synchronic feature, made up of dust released at <30 m/s. Since the nucleus
would have been located on the synchrone, we computed the astrometric positions
of the missing nucleus as the coordinates of the points of intersection of the
spine tail's axis with lines of forced orbital-period variation, derived from
orbital solutions based on preperihelion astrometry from the ground. The
resulting osculating orbital period was 698+/-2 years, which proves that C/2011
W3 is the first major member of the predicted new, 21st-century cluster of
bright Kreutz-system sungrazers. The spine tail's tip contained dust 1-2 mm in
diameter. The bizarre appearance of the dust tail in images taken hours after
perihelion with coronagraphs on SOHO and STEREO is readily understood. The
disconnection of the comet's head from the preperihelion tail and the apparent
activity attenuation near perihelion are both caused by sublimation of all dust
at heliocentric distances smaller than ~1.8 solar radii. The tail's brightness
is strongly affected by forward scattering of sunlight by dust. The
longest-imaged grains had a radiation-pressure parameter beta ~ 0.6, probably
submicron-sized silicate grains. The place of C/2011 W3 within the hierarchy of
the Kreutz system and its genealogy via a 14th century parent suggest that it
is indirectly related to the celebrated sungrazer X/1106 C1.
