PLATE I Messier 51
I. Messier 51
Messier 51 consists of two nebulae, N.G.C. 5194 and 5195, which form a double system analogous, in a sense, to a double star. N.G.C. 5194 is a typical, late type, open spiral, while 5195 has a nondescript, peculiar form suggesting affinities with elliptical types rather than with spirals.
The pair is so near that a few of the brightest (super-giant) stars can be seen in the spiral. These stars determine the distance-about two million light-years.
As nebulae are observed at greater and ever greater distances, the stars are soon lost and, eventually, the structures fade until, at the extreme limits of the telescope, the great stellar systems appear on the photographs as small, dim patches, barely distinguished from images of faint stars (see Plate VII).
II. Nebulae in the Virgo Cluster
III. The Coma Cluster
IV. The Corona Borealis Cluster
VI. Remote Clusters
VII. A Sample of the Universe
1. The Linear Law of Red-Shifts not interpreted as Velocity-Shifts
2. Departures from Linearity in the Law of Red-Shifts interpreted as Velocity-Shifts
3. The Law of Nebular Distribution on Alternative Interpretations of Red-Shifts