SPA 1996 Light Curves

Semi Regular and Irregular Variables

Beta Pegasi is an Irregular variable - there is no sign of any periodicity in its variations. Betelgeuse , Mu Cephei , Eta Geminorum , Alpha Herculis , R Lyrae , and Rho Persei are semi-regular variables. Although such variables do at times show signs of periodic variation, at other times their variations can appear more erratic. Typically, the observed brightness range will only be a few tenths of a magnitude. Note that allowances were made when constructing these light curves to allow for systematic differences between observers - it is not unusual for reddish stars such as these for one observer to always see the star several tenths of a magnitude brighter than does another observer !

As can be seen from these light curves, the periodicities are sometimes easy to see (as in Mu Cephei) and sometimes harder to see.

The variations in Mu Cephei do seem to fit in with the period of around 800 days that is often quoted, but the Betelgeuse light curve shows that minima in its brightness occured during 1989 and 1993 - the interval between these minima was about 4 years - somewhat less than the period of around 2100 days that is often quoted for Betelgeuse.

Gamma Cassiopeiae though normally around magnitude 2.2, brightened to mag 1.6 in 1936 and faded to mag 3.0 a few years later. As this light curve of monthly averages shows, any variations in recent years have been much smaller.

R Scuti shows alternate deep and shallow minima. As this light curve of individual brightness estimates shows, deep minima occured during April and September 1996. The shallow minima are harder to see - one occured in July. The next deep minimum will probably occur around June 1997.

Cepheid variables

Eta Aquilae , Delta Cephei and Zeta Geminorum are all Cepheid variables - the brightness variations of these stars are due to pulsations which affect their size and surface temperatures. All observations of these stars made during 1996 have been combined to produce these average light curves. Different types of pulsation produce differently shaped light curves - note that whereas minimum brightness occurs at around phase 0.7 for Eta Aquilae and Delta Cephei, it occurs at around phase 0.5 in Zeta Geminorum.

Eclipsing Binaries

RZ Cassiopeiae , Algol and Lambda Tauri are all eclipsing binaries. The two stars in each of these binary star systems are too close together for us to see individually, but we can see the total brightness of the system dip when the fainter star passes in front of the brighter one during primary eclipse. Based on earlier eclipses, predictions can be made for the times of future eclipses and observations can then be made to check whether the eclipses are occuring on schedule - i.e. is the primary eclipse centred on phase 0 ? Discrepancies between predictions and observations can reveal changes in the orbital period of the binary system. As these results (in which the phases were calculated using the elements in the General Catalogue of Variable Stars) show, the eclipses of RZ Cassiopeiae and Algol are both slightly late : RZ Cassiopeiae’s eclipse is centred on phase 0.02 (30-35 mins late) and Algol’s is centred on phase 0.01 (about 40 mins late).

Beta Lyrae and u Herculis are also eclipsing binaries. However, whereas systems such as Algol are constant in brightness between eclipses, these systems vary in brightness all of the time because gravitational forces have distorted both stars in each system making them egg shaped.

Beta Lyrae (1992)

Beta Lyrae (1995)

Beta Lyrae (1996)

These average light curves for Beta Lyrae for the years 1992, 1995 and 1996 show that its eclipses are not occuring at the predicted times - in fact the discrepancy is increasing from year to year.

Mira type variables

T Cephei

T Cephei , Omicron Ceti (Mira) , Chi Cygni , U Orionis , R Serpentis , R Trianguli and R Ursae Majoris are Mira type variables. As for Cepheid variables, the brightness variations are the result of pulsations. However, whereas the brightness variations in Cepheid variables typically have amplitudes of less than a magnitude and repeat every week or so, those of Mira type variables are much larger and slower - each cycle takes around a year give or take a couple of months and the brightness variations do not repeat exactly from one cycle to the next. Chi Cygni, for example, can occasionally reach 3rd magnitude, but in 1996 only reached 5th magnitude.

Maintained by Michael Oates Email: mike@ph.u-net.com Last modified 11th Aug 1997