‘S’ is for ‘Spiral’, ‘AB’ is for … ‘Weakly Barred’

This glittering image shows the spiral galaxy IC 5332, which lies about 30 million light-years away in the constellation Sculptor, and has an almost face-on orientation to Earth. To explain what is meant by ‘face-on’, it is helpful to visualise a spiral galaxy as an (extremely) large disc. If the galaxy is oriented so that it appears circular and disc-shaped from our perspective here on Earth, then we can say that it is ‘face-on’. In contrast, if it is oriented so that it appears squashed and oval-shaped, then we would say that it is ‘edge-on’. The key thing is that the same galaxy would look extremely different from our perspective depending on whether it was face-on or edge-on as seen from Earth. Check out these previous Hubble Pictures of the Week for examples of another face-on spiral galaxy and an almost edge-on spiral galaxy. 

IC 5332 is designated as an SABc-type galaxy in the De Vaucouleurs system of galaxy classification. The ‘S’ is straightforward, identifying it as a spiral galaxy, which it clearly is, given the well-defined arms of bright stars and darker dust that curl outwards from the galaxy’s dense and bright core. The ‘AB’ is a little more complex. It means that the galaxy is weakly barred, which refers to the shape of the galaxy’s centre. The majority of spiral galaxies do not spiral out from a single point, but rather from an elongated bar-type structure. SAB galaxies — which are also known as intermediate spiral galaxies — do not have a clear bar-shape at their core, but also do not spiral out from a single point, instead falling somewhere in between. The lowercase ‘c’ describes how tightly wound the spiral arms are: ‘a’ would indicate very tightly wound, and ‘d’ very loosely wound. Thus, IC 5332 is quite an intermediate spiral galaxy on many fronts: weakly barred, with quite loosely wound arms, and almost completely face-on!

[Image Description: A close-in view of a spiral galaxy. It is seen face-on, showing its circular shape and tightly winding spiral arms. The galaxy glows brightly in the centre and dims to cool colours towards the edge. Dark, faint filaments of dust and brightly glowing, pink and orange bubbles of star formation mark the face of the galaxy.]

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