VARIABLE COLOURS

Evening Post, Volume CXXIII, Issue 131, 4 June 1937, Page 4

VARIABLE COLOURS

IN BINARY STARS

The variable colours in six stars were described and discussed at the last meeting of the Astronomical Section of the Wellington Philosophical Society, Mr. A. G. C. Crust reading

a paper on the subject.

In four, of these stars, said Mr. Crust, astrophysicists. abroad, suspected the existence of companion stars, and their knowledge of the other two was very small. The Kelburn observations definitely proved that all were very close binary stars, such as could be detected only with the spectroscope or by the colour changes caused by eclipses. These colour changes were of three types. In a "total central" eclipse, the companion star passed centrally across the disc of its primary, causing a minute dimming of the light and a change to a bluer colour. Later, the companion passed behind the primary, giving a much greater diminution of the light and a reddening of the colour. For a small companion the changes were abrupt, and the light and colour remained nearly constant during totality. If the passage of the companion was not central, the change would not be so abrupt and totality would ■ not last so long. Of the six stars quoted, Gamma Serpentis. Pi Virginis, and Chi Eridani belonged to this class. In a "total grazing" eclipse, exemplified by Lambda Eridani, which was to be discussed in more detail on a later occasion, the companion' made a very gradual contact with the limb of the primary, and the eclipse was total for only a short time. The period of Lambda was three minutes less than one day, the companion revolved very close to its surface, and was eclipsed for three-quarters of an hour totally. Its orbit was inclined about 60 degrees to the line of sight and the colour changes at primary minimum occupied 5 hours in all. The system afforded an important geometrical proof of the relation between colour and diameter mentioned in the text-book of Russell, Dugan, and Stewart.

In a "partial eclipse," exemplified by Alpha Cancri and Rho Hydrae. the colour and magnitude showed gradual changes during the eclipses, the colour maxima and minima being V-shaped. It would be noted that in the magnitude curves, the brightness remained constant for a large fraction of the period, and declined during the eclipses, but. the colour alternately showed a change towards red and blue, marking primary and secondary minima on the magnitude curve. Alpha Cancri varied from mag. 4.3 to 5.1, but the rest of these stars varied only 0.1 or 0.2 mag. Such small changes were difficult to detect, but the striking colour changes could not be neglected, and sufficed to determine the periods with a high degree of certainty.