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.