NOTES ON NEBULAE. 129 



work of the sky, and this illusion can only be dispelled by 

 enabling him to witness the actual contrast in the way I 

 have described. The central portions of the nebula are, 

 however, so brilliant and so wonderfully marked with 

 interesting detail, that even a small instrument will suffice 

 to reveal much of its beauties. 



In the centre of the nebula is the star known to astro- 

 nomers as Theta Orionis, the most prominent star of 

 the sword handle. To the eye this looks like an ordinary 

 star, but the telescope speedily dispels that notion. Theta 

 Orionis is found to consist of four, or rather six, stars 

 all so close together that the unaided eye fails to 

 distinguish them separately. A structure so complex 

 gives to this star quite a special, indeed a unique, 

 interest, wholly apart from the marvellous nebula of 

 which it is the focus. We must dwell a little on the 

 peculiarities of this star. We are familiar with stars 

 which are called double ; there are indeed some ten thou- 

 sand objects so designated known to astronomers and 

 duly registered in catalogues. Some of these are no 

 doubt only casual doubles. It happens that two stars lie 

 nearly in the same line of vision : they are thus found to 

 be very close together in the heavens. Such objects are 

 merely said to be optically double, and so far as their 

 physical nature is concerned they are of comparatively 

 little interest, though their practical utility in facilitating 

 the discovery of the distance of the nearer of the pair 

 ought not to be overlooked. It will, however, often hap- 

 pen that two stars are not merely apparently near each 

 other on the sky, but are actually quite close together, in 

 comparison, that is, to the immense distance at which they 



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