But, as I have said, the estimate of Sirius's distance used in these calculations is the smallest that can be accepted. Good authorities regard the distance as being not less than 100,000,000,000,000 miles; in which case the star's brilliancy must be as much as 228 times greater than that of the sun! And yet even Sirius is probably not the greatest sun belonging to the visible universe. There can be little doubt that Canopus, in the southern hemisphere, is a grander sun than Sirius. To our eyes, Canopus is only about half as bright as Sirius, and it ranks as the second star in the heavens in the order of brightness. But while Sirius's distance is measurable, that of Canopus is so unthinkably immense that astronomers can get no grip upon it. If it were only twice as remote as Sirius, it would be equal to two of the latter, but in all probability its distance is much greater than that. And possibly even Canopus is not the greatest gem in the coronet of creation. Sirius, as we saw when talking of Procyon (see Chapter I), is a double star. For many years after Bessel had declared his belief that the Dog-Star was subjected to the attraction of an invisible companion, telescopes failed to reveal the accompanying star.[D] Finally, in 1862, a new telescope that Alvan Clark had just finished and was testing, brought the hidden star into view. The suggestion that it may shine by reflected light from Sirius has been made. In that case it must, of course, be a planet, but a planet of such stupendous magnitude that the imagination can scarcely grasp it; a planet probably as large as our sun, perhaps larger; a planet equal in size to more than a million earths! But, as was remarked of the faint stars in Alpha Capricornis, it is probable that the hypothesis of reflected light is not the true one. More probably the companion of Sirius shines with light of its own, though its excessive faintness in comparison with its bulk indicates that its condition must be very different from that of an ordinary star. [D] The following extract from a letter by Bessel to Humboldt, written in 1844 (see "Cosmos," vol. iii, p. 186), is interesting, in view of the discoveries made since then: "At all events I continue in the belief that Procyon and Sirius are true double stars, consisting of a visible and an invisible star. No reason exists for considering luminosity an essential property of these bodies. The fact that numberless stars are visible is evidently no proof against the existence of an equally incalculable number of invisible ones. The physical difficulty of a change in the proper motion is satisfactorily set aside by the hypothesis of dark stars."