A Text-Book of Astronomy

by George C. Comstock

The Sun's Nearest Neighbors

No.	Star.	Magnitude.	R. A.	Dec.	Parallax.	Distance.
1	α Centauri	0.7	14.5h.	-60°	0.75"	0.27
2	Ll. 21,185	6.8	11.0	+37	0.45	0.46
3	61 Cygni	5.0	21.0	+38	0.40	0.51
4	η Herculis	3.6	16.7	+39	0.40	0.51
5	Sirius	-1.4	6.7	-17	0.37	0.56
6	Σ 2,398	8.2	18.7	+59	0.35	0.58
7	Procyon	0.5	7.6	+5	0.34	0.60
8	γ Draconis	4.8	17.5	+55	0.30	0.68
9	Gr. 34	7.9	0.2	+43	0.29	0.71
10	Lac. 9,352	7.5	23.0	-36	0.28	0.74
11	σ Draconis	4.8	19.5	+69	0.25	0.82
12	A. O. 17,415-6	9.0	17.6	+68	0.25	0.82
13	η Cassiopeiæ	3.4	0.7	+57	0.25	0.82
14	Altair	1.0	19.8	+9	0.21	0.97
15	ϵ Indi	5.2	21.9	-57	0.20	1.03
16	Gr. 1,618	6.7	10.1	+50	0.20	1.03
17	10 Ursæ Majoris	4.2	8.9	+42	0.20	1.03
18	Castor	1.5	7.5	+32	0.20	1.03
19	Ll. 21,258	8.5	11.0	+44	0.20	1.03
20	ο2 Eridani	4.5	4.2	-8	0.19	1.08
21	A. O. 11,677	9.0	11.2	+66	0.19	1.08
22	Ll. 18,115	8.0	9.1	+53	0.18	1.14
23	B. D. 36°, 3,883	7.1	20.0	+36	0.18	1.14
24	Gr. 1,618	6.5	10.1	+50	0.17	1.21
25	β Cassiopeiæ	2.3	0.1	+59	0.16	1.28
26	70 Ophiuchi	4.4	18.0	+2	0.16	1.28
27	Σ 1,516	6.5	11.2	+74	0.15	1.38
28	Gr. 1,830	6.6	11.8	+39	0.15	1.38
29	μ Cassiopeiæ	5.4	1.0	+54	0.14	1.47
30	ϑ Eridani	4.4	3.5	-10	0.14	1.47
31	ι Ursæ Majoris	3.2	8.9	+48	0.13	1.58
32	β Hydri	2.9	0.3	-78	0.1	1.58
33	Fomalhaut	1.0	22.9	-30	0.13	1.58
34	Br. 3,077	6.0	23.1	+57	0.13	1.58
35	ϑ Cygni	2.5	20.8	+33	0.12	1.71
36	β Comæ	4.5	13.1	+28	0.11	1.87
37	ψ5 Aurigæ	8.8	6.6	+44	0.11	1.87
38	π Herculis	3.3	17.2	+37	0.11	1.87
39	Aldebaran	1.1	4.5	+16	0.10	2.06
40	Capella	0.1	5.1	+46	0.10	2.06
41	B. D. 35°, 4,003	9.2	20.1	+35	0.10	2.06
42	Gr. 1,646	6.3	10.3	+49	0.10	2.06
43	γ Cygni	2.3	20.3	+40	0.10	2.06
44	Regulus	1.2	10.0	+12	0.10	2.06
45	Vega	0.2	18.6	+39	0.10	2.06

in which the numbers in the first column are those placed adjacent to the stars in the diagram to identify them.

Fig. 122.—Stellar neighbors of the sun.

190. Light years.—The radius of the inner circle in [Fig. 122], 1,000,000 times the earth's distance from the sun, is a convenient unit in which to express the stellar distances, and in the preceding table the distances of the stars from the sun are expressed in terms of this unit. To express them in miles the numbers in the table must be multiplied by 93,000,000,000,000. The nearest star, α Centauri, is 25,000,000,000,000 miles away. But there is another unit in more common use—i. e., the distance traveled over by light in the period of one year. We have already found ([§ 141]) that it requires light 8m. 18s. to come from the sun to the earth, and it is a simple matter to find from this datum that in a year light moves over a space equal to 63,368 radii of the earth's orbit. This distance is called a light year, and the distance of the same star, α Centauri, expressed in terms of this unit, is 4.26 years—i. e., it takes light that long to come from the star to the earth.

In [Fig. 122] the stellar magnitudes of the stars are indicated by the size of the dots—the bigger the dot the brighter the star—and a mere inspection of the figure will serve to show that within a radius of 30 light years from the sun bright stars and faint ones are mixed up together, and that, so far as distance is concerned, the sun is only a member of this swarm of stars, whose distances apart, each from its nearest neighbor, are of the same order of magnitude as those which separate the sun from the three or four stars nearest it.

[Fig. 122] is not to be supposed complete. Doubtless other stars will be found whose distance from the sun is less than 2,000,000 radii of the earth's orbit, but it is not probable that they will ever suffice to more than double or perhaps treble the number here shown. The vast majority of the stars lie far beyond the limits of the figure.

191. Proper motions.—It is evident that these stars are too far apart for their mutual attractions to have much influence one upon another, and that we have here a case in which, according to [§ 34], each star is free to keep unchanged its state of rest or motion with unvarying velocity along a straight line. Their very name, fixed stars, implies that they are at rest, and so astronomers long believed. Hipparchus (125 B. C.) and Ptolemy (130 A. D.) observed and recorded many allineations among the stars, in order to give to future generations a means of settling this very question of a possible motion of the stars and a resulting change in their relative positions upon the sky. For example, they found at the beginning of the Christian era that the four stars, Capella, ϑ Persei, α and β Arietis, stood in a straight line—i. e., upon a great circle of the sky. Verify this by direct reference to the sky, and see how nearly these stars have kept the same position for nearly twenty centuries. Three of them may be identified from the star maps, and the fourth, ϑ Persei, is a third-magnitude star between Capella and the other two.