Description - This beautiful barred spiral is an excellent example of its type. The prominent inner ring is made of two tightly wrapped arms that wind from one end of the bar to the other. In this sense, it is clearly a pseudoring although it closes well enough to merit the r-subtype. In the Carnegie Atlas, the inner ring is regarded only as a ``near-ring'' and the galaxy is instead classified as the s-subtype. The two main outer arms form an incomplete outer pseudoring. The bar has prominent, slightly curved leading dust lanes that wrap around the central nuclear disk. The type of Sab is based on the knottiness and tightness of the inner ring spiral.

In addition to the features that define the type, NGC 1433 displays two short secondary spiral arcs lying off the leading ends of the bar. These remarkable features are very rarely seen in barred spirals (Buta 1984) and display a strong degree of symmetry in that each arc includes a bright superassociation. The virtually stellar association seen in the middle of the east arc is associated with a ring-shaped HII region (Crocker, Baugus, and Buta 1996).

The nucleus of NGC 1433 also includes a sharp-edged central disk of high surface brightness. In blue light, this central disk has a few patches that are star-forming regions and also includes a short, secondary bar. The secondary bar is misaligned with the primary bar by 62.

Multi-wavelength morphology - The pictures below show a near-infrared (H-band) image of NGC 1433 next to a B-band image. The H-band image shows that both inner and outer rings are still pseudorings in the old stellar population. However, the smoothness of the arms would merit a classification of Sa. Thus, the de Vaucouleurs classification of NGC 1433 is nearly invariant from B to H. The H-band classification is (R)SB( s)a.

The pictures also show a deprojected blue light image and a deprojected B-I color index map of NGC 1433. The deprojection uses a kinematic line of nodes position angle and a photometrically-determined inclination (Buta et al. 2001). The first image shows how the bright inner ring is intrinsically highly elongated along the bar, with an intrinsic axis ratio of 0.63. This degree of elongation turns out to be extreme. The average intrinsic elongation of SB inner rings is 0.81 0.06 (Buta 1995). The color index map shows how recent star formation in NGC 1433 is confined to the spiral pseudorings, the nuclear disk, as well as the secondary arcs. Star formation in the center is clearly ring-shaped, thus the central disk includes also a nuclear ring.

Interpretations - Buta (1986b) and Buta et al. (2001) interpret NGC 1433 as a classic resonance ring spiral. Based on a detailed rotation curve study, Buta et al. (2001) interpret the nuclear ring, inner ring, and outer ring in terms of inner Lindblad resonance (ILR), inner 4:1 ultraharmonic resonance (IUHR), and outer Lindblad resonance (OLR) respectively. The secondary spiral arcs represent a rare manifestation of an outer 4:1 resonance (OUHR). If the bar were weaker, these arcs would connect directly to the minor axis points of the inner ring. However, the bar is strong, which destabilizes periodic orbits in the gap region between the inner and outer rings, and makes the extra arms disconnect from the ring.