The Whirlpool Galaxy: found in 1773, first spiral ever seen

The Whirlpool Galaxy (M51) was discovered on October 13, 1773, by French astronomer Charles Messier, who saw it through a 10-cm refractor in Canes Venatici and logged it as the 51st entry in his list of fuzzy objects that comet hunters should ignore. He described it as a “very faint nebula, without stars”.

What makes M51 historic is not its discovery but what came after: it was the first object in which spiral structure was ever seen. Understanding it took about a century and a half:

• 1773: Messier records a faint glow while observing a comet.
• 1781: his friend Pierre Méchain finds the smaller companion, now NGC 5195. Messier’s catalogue notes “It is double, each has a bright center”, and his personal copy holds a small sketch of the pair, one of the rare drawings of his objects to survive.
• Spring 1845: Lord Rosse, using his giant telescope, makes out curving arms and paints them; M51 is sometimes called “Rosse’s Question Mark”.
• 1920s: Edwin Hubble’s Cepheid measurements show that spiral nebulae lie far outside the Milky Way. Until then many astronomers thought objects like M51 were part of our own galaxy.

Those arms are probably no accident. Many astronomers think the Whirlpool’s grand-design spiral is sharpened by its encounter with NGC 5195, which Hubble images show passing behind it.

How far away is it?

Its distance is less certain than most sources suggest. NASA gives 31 million light-years, the SEDS Messier database adopts 27 million, and older photometric estimates reached 37 million. M51 also stays busy: amateur astronomers discovered supernovae in it in 1994 and 2005, and another erupted in 2011.