Since 1990, Guinan, with collaborators D. Dorren (Edinburgh, Scot.), M. Güdel(PSI, Switzerland), L. DeWarf, G. McCook and S. Messina (Catania, Italy), has been engaged in the Sun in Time project. This is a program of coordinated, multiwavelength observations of solar-type stars. These stars have been selected as proxies for the Sun at stages throughout its main-sequence lifetime; they are single, main-sequence G0 - G5 stars of ages from 70 Myr (representing the ZAMS Sun) to stars that have reached the Terminal Age Main-Sequence (TAMS) with an age of 10 Gyr. With the exception of a few, nearby, TAMS G0-5 IV stars, all of the stars have similar physical properties such as: mass, radius, temperature, and depth of convective-zone. They differ only in age, rotation and level of magnetic activity.
During 1995, observations of the G2 V, K2 V and M5 V stars of the
Cen ABC star system were made by Guinan with the IUE satellite.
These observations were carried out about twice per week over an three
month interval from May to August 1995, to obtain the rotation periods
of the three stars(
Cen A: G2 V; 
Cen B: K2 V; and
Cen C = Proxima Cen: M5 V) which form this nearby system. The
chief aim is to obtain the rotation periods of the component stars by
using the chromospheric and transition emission line features, arising
from active stellar active sites, as markers for measuring rotation.
The emission lines of O I (1305Å), C II (1335Å), C IV (1550Å), Si
II (1809Å) and Mg II (2800Å) were chiefly used. Some of the data
has been reduced and analyzed and the preliminary results indicate a
rotation period of 
days for Proxima. For 
Cen B, the
rotation period is about 
days. The period for 
Cen
A is more difficult to determine because the chromospheric and TR UV
line emissions are relatively weak, but there seems to be some evidence
of a periodicity of about 
days. This period determination for
Cen A, is tentative while the rotational periods for the
Cen B and C appear well determined. This work is done in
collaboration with astronomy students Erich Jay (
Cen A) and
Nicholas Morgan (Proxima Cen). Visiting scientist, S. Messina (Univ. of
Catania) is working with Guinan on the analysis of the observations of
Cen B and with the overall interpretation of the results.
DeWarf and Dorren are involved with the modeling and interpretation of
the results. This work is partially supported by grants from the NASA
IUE programs.