The Asp in Autumn
ON CLEAR APRIL EVENINGS, the splashy summer constellations slide toward the western horizon, and deep sky enthusiasts are itching to dive into the galaxy-rich fields of Virgo, Leo, and Coma Berenices. These constellations, along with Canes Venatici and Ursa Major, headline 32 of the 40 Messier galaxies. But farther south and often ignored is the winding constellation of Hydra, the Water Snake, which uncoils across 100° of the sky and contains its own bonanza of galaxies.
At a distance of 165 million light-years, the Hydra I Cluster is one of the nearest rich galaxy clusters beyond the Virgo Cluster. Physically, the two clusters display a striking resemblance. Hydra I is three times as distant as the Virgo cluster, but its apparent size is /3 as large. So their linear dimensions are nearly the same. And each cluster contains 50 galaxies within two magnitudes of their dominant members, M87 in Virgo and NGC 3311 in Hydra, both supergiant ellipticals with comparable absolute magnitudes.
Redshift surveys have demonstrated that the cluster is remarkably isolated in space, with
