There are a few of these nebulæ which can be seen with the naked eye; one of these is the _Great Nebula of Orion_, and if you will look at Orion some night and draw an imaginary line a little below his belt and toward the east you will be able to find it. Another nebula that is bright enough to be seen with the naked eye is the _Great Nebula in Andromeda_. Both of these nebulæ are good tests for eyesight. Fig. 134 shows some of the different forms of nebulæ. [Illustration: FIG. 134.—DIFFERENT FORMS OF NEBULÆ.] =The Making of the Stars.=—To explain how our Sun and the planets were made, as well as all the other stars in the universe, two ideas have been worked out, and although these are quite unlike in many ways, yet both start out with the nebulæ. The first of these is called the _nebular hypothesis_ and the other and later one is called the _planetesimal hypothesis_. =The Nebular Hypothesis.=—We have found out what _nebula_ is believed to be and we should next understand exactly what the word _hypothesis_ means. An hypothesis is an idea worked out so that there is a fairly good chance of its being true. The _nebular hypothesis_, then, is an idea that has been worked out from what nebulæ are believed to be and what is known of the mighty forces of nature, and these when taken together seem to show that all the things in the sky—stars, planets, moons, comets and meteors—are made of nebular stuff. The nebular hypothesis says that when the nebular matter, or star stuff of which the planets were made, was separated from the parent nebula by centrifugal force, they were all whirled away in the same plane, turning on their own axes and traveling round the Sun in the same direction. =The Planetesimal Hypothesis.=—A later idea, is called the _planetesimal hypothesis_. _Planetesimal_ means little planet, and as we know already what hypothesis means, by coupling the two words together we may easily guess that it is an idea worked out which accounts for the making of solar systems out of little planets. The planetesimal hypothesis assumes that the sun, planets, and moons comprising the Solar System were formed from a star which was torn to pieces by another star which passed close by. The star was left in the form of a spiral nebula rather lumpy and with little particles of matter scattered all around. The meteors, or little planets, making up the nebula, attract each other, like all other bodies, and when they get close enough they are drawn together and dense masses of matter, or cores, are built up. The largest core is seen in the center of a spiral nebula, and as it becomes more compact it grows hotter and a Sun is made, while the other and smaller cores turn round it and attract little planets to them. And so the cores grow in size, and with more and more weight bearing toward their centers the gases are forced out and these make the air and water.