NGC 309: Two and a Half Milky Ways Wide and Still Spinning Like a Pinwheel
NGC 309 is a grand-design intermediate spiral galaxy in Cetus, classified SAB(r)c, meaning a weak central bar wrapped in an inner ring from which loosely wound arms unwind across a broad, star-forming disk
The galaxy lies at a distance of 257 million light-years, derived from its motion relative to the cosmic microwave background, and spans about 3.2 arcminutes on the sky at an apparent magnitude of 13.4. At that distance the disk measures roughly 236,000 light-years across, well over twice the diameter of the Milky Way and placing NGC 309 among the largest spiral galaxies known
The image shows a compact yellow nucleus surrounded by a tightly coiled inner ring threaded with pink HII regions, the signature of massive star formation lighting up hydrogen along the spiral pattern. Outside the ring, two dominant blue arms sweep outward and gradually fragment into a lacework of star clusters and fainter arm segments, with dust lanes tracing the inner edges of the spiral. The field around it is scattered with distant background galaxies, many of them edge-on spirals and ellipticals far beyond NGC 309 itself
A catalogue quirk comes with this galaxy: it was listed by Holmberg in 1937 as the primary member of a double system, Holm 27, but its supposed companion has since been shown to be a foreground star. NGC 309 is, in fact, an isolated giant, which makes its size all the more interesting because it built this enormous disk without an obvious partner to stir it up
Large late-type spirals like this one are laboratories for how spiral structure is sustained on such scales. The weak bar and inner ring funnel gas toward the centre while the density waves in the outer disk keep compressing gas into new generations of hot blue stars. NGC 309 has hosted five recorded supernovae, a reminder that a disk this active is constantly recycling its material. Discovered by Wilhelm Tempel in 1876, it remains one of the most impressive face-on spirals in the southern autumn sky
Imaged in LRGB on the ASA Astrosysteme AZ 1500, Camera Moravian C5A 150M, at Observatorio El Sauce, Chile
Image Acquisition and Processing: Mike Selby