M. Arago on Double Stars, 35 



to warrant us even to hope for a plausible explanation. We 

 must look to lime and accurate observations, to inform us if the 

 green and blue stars be not suns which are already in the pro- 

 cess of waning ;* — if the different shades of these stars does 

 not indicate that their combustion is proceeding with different 

 degrees of intensity ; and, finally, if the tint, with the excess of 

 the most refrangible rays, which the lesser star often exhibits, be 

 not owing to the absorbent power of an atmosphere, which the 

 action of the star, usually much more brilliant than that which 

 it accompanies, may develope. In the study of phenomena, 

 wherein, we must, without doubt, take into marked consideration 

 the action which two suns, equally luminous, and of unknown 

 physical constitutions, exert upon each other, we have no. longer 



half minutes of diameter, and which is composed of a great number of stars, 

 all blue. The same astronomer speaks of a real nebulosity, that is to say, 

 a confused mass of radiant matter, the tint of which is also blue. Nothing 

 of this sort seems to have been observed on this side of the equator. 



* If the new star of 1572 possessed the physical constitution of the perma- 

 nent stars, the explanation of the blue colour, by the enfeebling of the com- 

 bustion, ought to be discarded. This star, which at the time of its sudden 

 appearance, on the llth of November 1572, so far surpassed the most brilliant 

 stars of the firmament for lustre, that it was seen with the naked eye in full 

 day light : it was then of a perfect whiteness. In January 1573, its light, con- 

 siderably enfeebled, had become i/elloio ; somewhat later it assumed the red- 

 dish colour of Mars, Aldebaran, or a of Orion : to the red, as reported by 

 the observers of the time, succeeded the livid white of Saturn, and this last 

 shade continued till the entire disappearance of the star. In all this there is , 

 no mention made of blue. The new star of 1604, in the same way, did not 

 exhibit this colour. It is therefore established, by two striking examples, that 

 a star may appear to come into existence, possessing the highest degree of in- 

 candescence, and then apparently diminish to its entire disappearance, without 

 ever becoming blue ! It is however to be remarked, that the disappearance of 

 the stars 1572 and 1604 having been observed with the naked eye, it might be 

 plausibly maintained, that the blue might have shown itself, but only when 

 being so far enfeebled, they could be found only in the class of telescopic stars. 

 And besides, there remains always this question — Are the new stars, and the 

 permanent ones of the same nature ? Perhaps the permanent stars, such as 

 our sun, only shine in virtue of a gaseous atmosphere which surrounds them, 

 and it is a property of a gas when combustion becomes feeble, that it should 

 then become blue. The absence of the principal prismatic shades, during the 

 different phases of the new and the changing stars, is a remarkable phenome- 

 non, whence important consequences regarding the velocity of the luminous 

 rays of the different colours may be deduced. This inquiry, however, must 

 be reserved for another occasion. 



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