XI
IRREGULAR AND DOUBTFUL VARIABLES
Besides these well-marked classes of long-period, short-period, and Algol variables, there are some stars which seem to vary spasmodically, remaining sometimes for months or even for years without any change; and there are others whose variability is suspected but has never been confirmed. Useful work might be done by amateurs in trying to decide the status of these doubtful stars.
It must be borne in mind that red stars are notoriously difficult objects. Two observers comparing one at the same time with the same star will often come to opposite conclusions, showing how difficult it is to compare stars which differ much in colour. Whenever possible, red comparison stars should be selected to avoid this uncertainty.
The second star on the list below has a very peculiar type of variation, unlike any other except a northern star, R Coronae, and a third recently discovered. The spectra of RY Sagittarii and R Coronae are also peculiar and resemble one another.
The following are bright southern variables suspected to vary, or known to vary without recognised laws:
Southern Irregular and Suspected Variables
| Name. | Maximum Magnitude. | Minimum Magnitude. | Remarks. | ||
|---|---|---|---|---|---|
| Z Sculptoris | 6 | 8 | Period unknown. | ||
| RY Sagittarii | 6.5 | Fainter than 11.5 | Usually steady, but subject to a sudden drop at irregular intervals. Worth watching. | ||
| RT Capricorni | 6.5 | About 8 | Period probably irregular. Very few observations published. | ||
| S Phoenicis |  | 7.2 or 6.8 | 8.7 or 8.4 |  | Observers differ. Period irregular. |
| W Ceti | 6.5 | 12 | Period perhaps 366 days | ||
| T Ceti | 5.4 | 6.9 | Irregular. | ||
| S Leporis | 6.5 | 7.5 | Irregular. | ||
| U Hydrae | 4.5 | 6.3 | Irregular. Red “carbon star.” | ||
| θ Apodis | 5,5 | 6.6 | Period probably irregular. | ||
| R Apodis | 5.5 | 6.2 | Suspected. | ||
| T Indi | 7.2 | 8.9 | No regular period found | ||
XII
STAR-CLUSTERS
Endlessly varied is the grouping of the stars. We find solitary stars, twin stars, large stars with small companions, bright stars with faint companions, stars alike in colour and type, stars in strongly contrasted colours, one young, the other aged, stars with invisible companions, couples close enough to touch one another, and couples so distant that the satellite takes more than a century to revolve round its primary. One or even both components of a bright pair may be themselves divisible into a closer pair, sometimes into a whole group of stars.
More beautiful and wonderful still are the clusters of stars of various tints and magnitudes, where scores, even hundreds, are gathered together in what to the eye is a tiny patch, though it may in fact take light many years to cross from one side of it to the other.