Observation of Variable Stars.

A recent circular issued by the Astronomical Observatory makes a plea for co-operation in observing variable stars. The number of variable stars of long period is now so great that the observation of many of them has been neglected. Although the investigation of the curves which represent the variation in the light of variables of small range can be more efficiently conducted by precise photometric observation, valuable work can be done on the long period stars by persons without extensive apparatus and experience.

The system of observation employed, known as Argelander's method, is essentially as follows. A series of stars of constant brightness is selected, as near as possible to the variable to be observed, in such a way that the brightest is somewhat brighter than the variable at its maximum intensity, and the faintest somewhat fainter than the variable at minimum. Between these extremes the stars differ one from another in brilliance by about half a magnitude, and they are designated by letters in the order of brilliance. This sequence of stars of known magnitude enables the brightness of the variable star to be closely determined at any time, by comparison with the two constant stars, one brighter and one fainter, which are then nearest it in magnitude.

The advantages which would be secured by the proposed co-operation are many. It is very much to be desired that all such variables should be observed in the same way, so that all may be reduced to a uniform scale of magnitudes. Observations on all the stars cannot be made to advantage at one station, but it is desirable that the records should be compiled in unified form. Moreover, many observers might in this way be secured who would not undertake independent work, whether from inexperience or lack of incentive.

To facilitate the work with bright variables, which may be investigated with the naked eye or with only a small glass, photographic enlargements have been made of portions of the Bonn Durch-musterung charts. A region three degrees square, surrounding each variable, has been three times enlarged, giving a map on the standard scale of one minute of arc to the millimetre. On these enlargements the designations of the stars in sequence have been marked. Copies will be furnished to observers at cost, or free of expense to persons of experience who are ready to co-operate in the work. Complete charts with the proper magnitudes for the stars represented can soon be furnished for seventy two variables. If the plan meets with success it is hoped that it may be extended to the investigation of other variable stars of long period.