If it is remarked that Sirius is fifty millions of millions of miles
away from us, it is not at once obvious that he is one of our very near
neighbours; but this is equal to 8 light-years, not twice the distance
of α Centauri, our next-door neighbour among the stars. Some faint
stars in the south must also be counted as very close to us: such are
a little star in Cetus, τ Ceti, only 10 light-years away; ε Indi, 11½;
and two in the River Eridanus, ε and δ Eridani, 10½ and 18 respectively.
But the most interesting among these near neighbours of the south is
a little yellow star in Pictor, too faint to be seen without a good
binocular or a telescope, and bearing the very modern name of CZ
5ʰ.243. This stands for Cordoba Zones 5.243 hours, and means that it
was catalogued at Cordoba Observatory in South America and its position
fixed in the fifth hour of Right Ascension. After this it was observed
by Mr. Innes at the Cape, and he was startled to find (like Hipparchus
comparing his work with that of Timocharis) that its position no longer
agreed with that found at Cordoba. “Can this be motion?” he asked, and
found that the star had indeed a larger visible motion across the sky
than any other, not even excepting the famous “runaway star” in Ursa
Major. Apparent motion, however, depends upon distance as well as real
speed, and when the distance of CZ 5ʰ.243 had been calculated it was
found that its real speed, amazing as it is, yet falls slightly short
of that of the northern star. The rates are 163 and 174 miles per
second.
A few other stars are known to move at speeds approaching 100 miles
a second, and one was announced in December 1913 to have a velocity
of 200 miles a second, but runaway stars are rare. They do not seem
to show any preference for special parts of the sky or special
stellar types, and it is impossible to say what causes them to rush
with such headlong haste through space, or what is their goal. The
average rate for a star is about 13 miles a second, but, as we have
seen, it differs with different types, the average speed increasing
progressively from the blue to the red classes.
Another remarkable fact lately discovered is that the different types
of stars are not indiscriminately scattered through space. Our own sun
seems to be surrounded, to a distance of about 100 light-years, by
suns more or less like himself, while the greater number of the red
Antarian stars lie at a much greater distance from us. Most distant
of all, considered as a class, are the bluest of the Orion stars; for
though the distances of individuals vary greatly, the average distance
of these stars from us is more than 500 light-years. Yet they are
often very bright, so this is another proof of their great intrinsic
brilliancy.