M51 — Whirlpool Galaxy

About

A grand-design spiral galaxy in Canes Venatici, roughly 23 to 31 million light-years away depending on the method. Lord Rosse's 1845 drawings were the first to show spiral structure in a nebula. Its arms are shaped by the passing companion NGC 5195.

Imaging tips

• About 11′ long, so 2,200–3,670 mm on APS-C spans a third to half of the frame; NGC 5195 is only 4.4′ away, in the same field.
• The tidal stream is very faint: one team recommends at least 4 hours from the darkest sky you can reach, and shared multi-site data pushed theirs far beyond that.
• A faint Hα cloud lies just north of the pair; the same team needed 15–30 minute Hα subs, because short ones left it buried in sensor noise.
• A 13-hour RGB-only image, 130 subs of 120 s per channel, was processed to bring out the star streams around the companion.
• Best from March to May.