CHAPTER XI 
THE EVOLUTION OF BINARY SYSTEMS 
256. In the last chapter we found that binary systems of the /3 Lyrae type 
shew all the characteristics to be expected in stars which have just broken up 
by fission. 
These stars form one end of a continuous chain of binary systems. As we 
proceed along this chain the physical characteristics of the systems vary 
widely, substantial departures occurring from the characteristics shewn by 
systems of the /3 Lyrae type. In the present chapter we shall investigate 
what changes are likely to be produced in binary systems by the passage of 
time and the play of natural forces, with a view to examining to what extent 
the observed chain of binary formations can be interpreted as an evolutionary 
chain. 
Observational Material. 
257. The following table gives particulars of a few typical binary systems 
which shew the small separation, short period, low eccentricity of orbit and 
other features which must be regarded as the primary indication that a system 
has recently been formed by fission. 
The last column but one gives the radii of the two stars in terms of the 
radius of their relative orbit, and the final column gives the sums of these radii. 
If this sum were equal to unity the stars would be in contact. 
Table XIX. Newly-formed Binaries. 
Star 
Spectral 
Type 
Mass 
Eccentricity 
of Orbit 
Period 
in Days 
Radii in terms 
of Orbit 
Sum of 
Radii 
H.D. 1337 
0 8% 
36-3, 
33-8 
0 
3-52 
0-59, 
0-39 
0-98 
Y Puppis 
B 1 
19-2, 
17-9 
0-08 
1-45 
0-46, 
0-42 
0-88 
u Herculis 
B 3 
7-66, 
2-93 
0 05 
2-05 
0-31, 
0-37 
0-68 
TX Cassiop. 
B 3, Bb 
— 
2-93 
0-57, 
.0-30 
0-87 
0 Lvrae 
Bb 
0-018 
12-92 
0-68, 
0-27 
095 
RZ Centauri 
A 
0 
1-88 
0-49, 
0-24 
0-73 
WZ Cygni 
A 
0 
0-58 
0-46, 
0-38 
0-84 
S Antliae 
F 0 
— 
0-65 
0-50, 
0-39 
0-89 
RR Centauri 
F 
0 
0-30* 
0-50, 
0-50 
1-00 
W Ursae Maj. 
F8 
0-74, 
0-52 
0 
0-33 
0-37, 
0-37 
0-74 
S W Lacertae 
0 2 
— 
0-32 
0-42, 
0-46 
0-88 
W Crucis 
GO 
0 
198-5 
0-61, 
0-34 
0-95 
* J. Voute, Annalen v. d. Bosscha-Sterrenwacht , Lemberg (Java), n. (1927), 2e gedeelte. The 
elements of orbit are those calculated by Sliapley from an earlier light curve by Roberts, but in 
any case the light curve shews that the two components must be nearly or quite in contact.