A combination of Hubble Space Telescope ( HST ) WFPC2 and NICMOS images are used to investigate the gas/dust and stellar structure inside the central 300 pc of the nearby active galaxy NGC 4303 .

The NICMOS H-band ( F160W ) image reveals a bright core and a nuclear elongated bar-like structure of 250 pc in diameter .
The bar is centered on the bright core , and its major axis is oriented in proyection along the spin axis of the nuclear gaseous rotating disk recently detected ( Colina & Arribas 1999 ) .

The V - H ( F606W - F160W ) image reveals a complex gas/dust distribution with a two-arm spiral structure of about 225 pc in radius .
The southwestern arm is traced by young star-forming knots while the northeastern arm is detected by the presence of dust lanes .
These spirals do not have a smooth structure but rather they are made of smaller flocculent spirals or filament-like structures .

The magnitudes and colors of the star-forming knots are typical of clusters of young stars with masses of 0.5 to 1 \times 10 ^ { 5 } M _ { \odot } , and ages of 5 to 25 million years .

The overall structure of the nuclear spirals as well as the size , number and masses of the star-forming knots are explained in the context of a massive gaseous nuclear disk subject to self-gravitational instabilities and to the gravitational field created by the nuclear bar .
According to the model , the gaseous disk has a mass of about 5 \times 10 ^ { 7 } M _ { \odot } inside a radius of 400 pc , the bar has a radius of 150 pc and a pattern speed of \sim 0.5 Myr ^ { -1 } , and the average mass accretion rate into the core ( R < 8 pc ) is \sim 0.01 M _ { \odot } yr ^ { -1 } for about 80 Myr .