Guinan and senior N. Morgan studied Proxima Centauri using both
photometric and IUE data. At a distance of 4.3 LY, Proxima
Centauri (= 
Cen C; V645 Cen) is the nearest known star to the
Sun. This M5 V flare star is the faintest member of the 
Cen
triple star system (or moving group) and lies about 1400 AU nearer to
the Earth than its brighter G2V and K2V companions. Because of its
proximity and its membership in the triple system, Proxima has well
determined physical properties that include an age of 5-6 Gyr. In spite
of its old age, Proxima is a chromospherically active star with strong
Mg II h+k (280nm) emission as well as being a flare star. This star is
of great importance to magnetic dynamo theory because it is expected to
have a fully convective envelope. One quantity, not well determined yet
vital to understanding Proxima's magnetic behavior, is its rotation
period.
During May-August 1995, Proxima was observed about twice a week with
IUE. Low resolution LWP (200-320nm) spectra were obtained chiefly
to observe the chromospheric Mg II emission and use it to measure
Proxima's rotation period as active plage regions on the star's surface
rotate in and out of view. The IUE data have been analyzed and
the Mg II emission shows 
variations with a period of
days. This period is assumed to be the star's rotation
period. In addition, several flare events were observed and evidence
was found for rather fast changes (on a time-scale of weeks) in the
plage activity and distribution. Also, the analysis of the IUE
archival data indicates the probable presence of a long-term activity
cycle.
This study is supported from NASA grant NAG 5-2160 and NSF grant AST-9315365 which we gratefully acknowledge.