Such, then, is the mechanism of the first phase in the history of a spiral nebula according to the Planetesimal Hypothesis.
Whereas, next to the star itself, the spiral nebula is the type object which occurs most frequently.
In the centre of a spiral nebula is seen a mass both more luminous and more resolvable than the rest.
The spiral nebula is evidently the standard type, and the condensing nebula must conform to it.
If we accept this theory, the origin of the spiral nebula becomes intelligible.
In any case, we have here a good suggestion of the origin of the spiral nebula and of its further development.
The most interesting specimen of a spiral nebula is situated in Canes Venatici.
Around and around they would whirl in a spiral nebula, till one got a head start on a race for home and mother.
The hypothesis, therefore, that our universe is a spiral nebula has much to be said for it.
It is even suggested that the spiral nebula 162 originated by disruption of one star by a swift-moving passing star.