Fig. 55.

Chief stars of Orion, with Aldebaran.
(After Proctor.)

Now first let me tell you that these last stars do not, so far as we know, lie in the nebula, but are scattered about in the heavens between us and it, perhaps millions of miles nearer our earth. But with the bright star in the centre it is different, for the spectroscope tells us that the mist passes over it, so that it is either behind or in the nebula. Moreover, this star is very interesting, for it is not really one star, but six arranged in a group (see Fig. 56). You can see four distinctly through my telescope, forming a trapezium or four-sided figure, and more powerful instruments show two smaller ones. So θ Orionis, or the Trapezium of Orion, is a multiple star, probably lying in the midst of the nebula.

Fig. 56.

The trapezium, θ Orionis, in the nebula of Orion. (Herschel.)

The next question is, What is the mist itself composed of? For a long time telescopes could give us no answer. At last one night Lord Rosse, looking through his giant telescope at the densest part of the nebula, saw myriads of minute stars which had never been seen before. "Then," you will say, "it is after all only a cluster of stars too small for our telescopes to distinguish." Wait a bit; it is always dangerous to draw hasty conclusions from single observations. What Lord Rosse said was true as to that particular part of the nebula, but not the whole truth even there, and not at all true of other parts, as the spectroscope tells us.

For though the light of nebulæ, or luminous mists, is so faint that a spectrum can only be got by most delicate operations, yet Dr. Huggins has succeeded in examining several. Among these is the nebula of Orion, and we now know that when the light of the mist is spread out it gives, not a continuous band of colour such as would be given by stars, but faint coloured lines on a dark ground (see Fig. 57). Such lines as these we have already learnt are always given by gases, and the particular bright lines thrown by Orion's nebula answer to those given by nitrogen and hydrogen, and some other unknown gases. So we learn at last that the true mist of the nebula is formed of glowing gas, while parts have probably a great number of minute stars in them.

Fig. 57.
Nebula-spectrum.