Page:Popular Science Monthly Volume 58.djvu/28

20 is the so-called Trapezium, already described. That these four stars form a system by themselves cannot be doubted. The darkness of the nebula immediately around them suggests that they were formed at the expense of the nebulous mass.

Great interest has recently been excited in the spiral form of certain nebulæ. The great spiral nebula M. 51 in Canes Venatici has long been known. We reproduce a photograph of this object and another. It is found by recent studies at the Lick Observatory that a spiral form can be detected in a great number of these objects by careful examination.



Another striking feature of numerous nebulæ is their varied and fantastic forms, of which we give a number of examples. The ’Triphid nebula' is a noted one in this respect.

The great nebula of Andromeda is second only to that of Orion. It also is plainly visible to the naked eye and can be more readily recognized as a nebula than can the other. It has frequently been mistaken for a comet. Seen through a telescope of high power, its aspect is singular, as if a concealed light were seen shining through horn or semi-transparent glass. It is somewhat elliptical in form, as will be seen from a photograph by Sir William Roberts, F.R.S., which we reproduce (page 19).