The star cluster that wasn't: How did M73 fool generations of astronomers?

I love a good coincidence, and this story has two. One is pedestrian, the other cosmic.

It starts with me pondering Gaia data. Gaia is a European Space Agency observatory that is revolutionizing astronomy. Its mission is to accurately map the positions, colors, distances, and motions of over a billion stars — yes, a billion, about 1% of all the stars in our galaxy — and create a massive database of the results, essentially a 3D map of the Milky Way galaxy.

Such surveys are incredibly useful. N...