GALAXIES
The Heron Galaxy
— Arp 84 · NGC 5394 & NGC 5395 —
Processed by Nicolas Rolland, Paris. 9.7 hours gathered by Roboscope at Apollo 11 Observatory, Fregenal de la Sierra, Spain.
Two spiral galaxies in Canes Venatici, 160 million light-years away, close enough that each has started to pull the other apart. NGC 5395 is the larger, about 140,000 light-years across; NGC 5394 is roughly 90,000. Between them runs a bridge of stars drawn out of their discs by the encounter, and that bridge is the reason this image exists.
Two galaxies mid-encounter
Halton Arp listed the pair in his 1966 Atlas of Peculiar Galaxies, which collected galaxies whose shapes could not be explained on their own. This one earns its place: the smaller galaxy appears to reach towards the larger, and the arms of both are visibly bent out of the plane they should occupy. The system is nicknamed the Heron for the outline the two make together — a body and a raised neck — though whether you see the bird depends on how far you are willing to go along with it.
The bridge, and why it is hard
The tidal bridge is the faintest thing in this frame and the most interesting. Unlike a nebula, it emits nothing that a filter can isolate: it is starlight, spread thin, at the same wavelengths as the sky behind it and the outer halos of both galaxies. There is no way to separate it optically, only enough signal or not enough. Here there was not much — the bridge sits barely above the background, and getting it to show without inventing structure around it was the whole difficulty of the image.
Nine hours, and what they allowed
The data comes from a Planewave CDK17 at f/6.8 with an FLI Kepler 4040, at the Apollo 11 Observatory in Extremadura — 432 mm of aperture sampling at 0.314 arcseconds per pixel, which is fine enough to hold the structure of the bridge rather than just its presence. What it did not have was time. The run gave 9.7 hours in total and 3.4 of those in luminance, the layer that carries faint structure, and that ratio set the ceiling: the bridge in this image is what 3.4 hours of luminance contained under that sky, through that telescope. The rest was down to processing, working inside that limit.
