290
[ch. xi
The Evolution of Binary Systems
On plotting spectral type against absolute magnitude, practically all of
these and similar stars are found to lie on the main sequence. W Crucis forms
an exception; the mean densities of its two components are given by Shapley*
as 1*3 x 10~ 6 and 31 x 10~®, and both components are of giant type. Its origin
may well be of the kind suggested at the end of § 165.
258. Aitken*f* has analysed the orbits of 119 spectroscopic binaries,
classified by period and eccentricity, with results shewn in the following
table :
Table XX. Spectroscopic Binaries classified by
Period and Eccentricity (Aitken).
Period of Orbit in Days
Total
0-5
5-10
10-20
20-50
50-150
Over 150
Number
Eccentricity
0 to 0-1
40
9
6
3
3
1
62
0-1 to 0-2
6
4
1
0
2
4
16
0-2 to 0-3
1
5
1
1
2
2
12
0-3 to 0-4
0
0
2
1
2
1
6
0-4 to 0-5
0
1
0
2
1
3
7
0'5 to 0-6
0
0
1
3
3
2
9
0-6 to 0-7
0
0
1
0
0
1
2
0-7 to 0-8
0
0
0
3
1
0
4
0-8 to 0-9
0
0
0
0
1
0
1
0-9 to 1-0
0
0
0
0
0
0
0
Total number
46
19
12
13
15
14
119
Average period
2-75
7-80
15-17
30-24
106-4
1035
Average
eccentricity
0-047
0147
0-202
0-437
0-371
0-328
He has analysed the orbits of 68 visual binaries in the same way, obtaining
the result shewn in Table XXI opposite, in which the periods are now measured
in years instead of days.
Both tables shew a marked increase of eccentricity with period, and the
phenomenon runs on from one table to the other, the eccentricities of the
visual binaries which have long periods being far higher than those of
the spectroscopic binaries whose periods are far shorter. The general uni
formity of this progression is clearly shewn in Table XXII, in which the
whole 187 orbits are divided, according to their periods, into seven groups
of approximately equal size by adding together suitable groups of columns of
the two preceding tables.
* Contributions from the Princeton University Observatory, No. 8 (1915).
t The Binary Stars , New York, 1918.
