We present the photometric and spectroscopic study of the very luminous Type IIn SN 2006gy for a time period spanning more than one year .
The evolution of multiband light curves , the pseudo-bolometric ( BVRI ) light curve and an extended spectral sequence are used to derive constraints on the origin and evolution of the SN .

A broad , bright ( { M _ { R } } \sim - 21.7 ) peak characterizes all monochromatic light curves .
Afterwards , a rapid luminosity fading ( \gamma _ { R } \sim 3.2 { mag ( 100 d ) ^ { -1 } } ) is followed by a phase of slow luminosity decline ( \gamma _ { R } \sim 0.4 { mag ( 100 d ) ^ { -1 } } ) between day \sim 170 and \sim 237 .
At late phases ( > 237 days ) , because of the large luminosity drop ( > 3 mag ) , only upper visibility limits are obtained in the B , R and I bands .
In the near-infrared , two K-band detections on days 411 and 510 open new issues about dust formation or IR echoes scenarios .

At all epochs the spectra are characterized by the absence of broad P-Cygni profiles and a multicomponent H \alpha profile , which are the typical signatures of type IIn SNe . H \alpha velocities of FWHM \approx 3200 { km s ^ { -1 } } and FHWM \approx 9000 { km s ^ { -1 } } are measured around maximum phase for the intermediate and high velocity components , respectively , and they evolve slowly with time .

After maximum , spectroscopic and photometric similarities are found between SN 2006gy and bright , interaction-dominated SNe ( e.g .
SN 1997cy , SN 1999E and SN 2002ic ) .
This suggests that ejecta-CSM interaction plays a key role in SN 2006gy about 6 to 8 months after maximum , sustaining the late-time-light curve .
Alternatively , the late luminosity may be related to the radioactive decay of { \sim 3 M _ { \odot } } of { { } ^ { 56 } Ni } .

Models of the light curve in the first 170 days suggest that the progenitor was a compact star ( { R \sim 6 - 8 \cdot 10 ^ { 12 } cm } , { M _ { ej } \sim 5 - 14 M _ { \odot } } ) , and that the SN ejecta collided with massive ( { 6 - 10 M _ { \odot } } ) , opaque clumps of previously ejected material .
These clumps do not completely obscure the SN photosphere , so that at its peak the luminosity is due both to the decay of { { } ^ { 56 } Ni } and to interaction with CSM .
A supermassive star is not required to explain the observational data , nor is an extra-ordinarily large explosion energy .