Stellar Motions

STELLAR MOTIONS We learn about the motion of a star in two ways. Firstly, the change of position in the sky can be observed directly by making from time to time accurate measurements of the position with respect to surrounding stars. The motion thus detected is called proper motion. It is measured in seconds of arc per year ; if the distance of the star has also been measured, the result can be converted into miles per second. Secondly, by measurements of the positions of lines in the star's spectrum, the speed of approach or of recession can be found; this radial velocity is given imme diately in miles per second, whether the distance of the star is known or not. The two methods supplement one another, the first giving the motion transverse to the line of sight, and the second the motion in the line of sight; combining them we obtain the direction and speed of the whole motion.

Proper Motion.

The stars differ greatly in proper motion. For a naked-eye star a motion of about 3" per century may be taken as typical. The corresponding change of position would be measurable without difficulty in an interval of ten years. For some stars the displacement is less, either because their actual speeds are small, or because their motion is mainly in our line of sight, or because they are very distant. Other things being equal, the stars which are especially near to us will have large proper motions. Just as great apparent brightness may be due to nearness or to high intrinsic luminosity, so great proper motion may be due to nearness or to high actual speed; but, in general, great proper motion is a much more dependable indication of nearness than great apparent brightness, and it is used as a crite rion to pick out stars likely to have measurable parallaxes from the great majority which are too far away to be measured. Proper motions have been measured in tens of thousands ; and, moreover, there are methods of quickly detecting stars of large proper motion on a pair of plates of the same region taken at a suitable interval of time (see BLINK MICROSCOPE).

The star of greatest proper motion known up to the present is a ninth magnitude star discovered by Barnard, which travels I or per year (more than 30o times the typical speed mentioned above). Even at this speed it would take about 90o years to go

from end to end of Orion's belt. The following table gives particu lars of the stars of largest proper motion : The distance in light-years is given in the fourth column ; the transverse and radial speeds are given in km. per sec. in the fifth and sixth columns; the last column gives the intrinsic luminosity, the sun's luminosity being the unit. It will be noticed that these are all inconspicuous stars, faint both apparently and actually. Well-known stars which come further down the list are a Centauri and Arcturus, with annual proper motions 3".66 and 2".29 respectively. The motions cannot have caused any noticeable change in the appearance of the sky within historic times; but the change in, say, 250,000 years would be considerable, and it is doubtful if any of the familiar constellations except Orion would be recognizable. When the parallax is known, proper motion is converted into linear measure by the formula: The Solar Motion.—If we examine the proper motions in any small region of the sky, it is at once evident that they are not directed at random; an average motion in a certain direction is plainly shown. Comparing different regions of the sky and allowing for the different aspects of projection, it is found that the stars in the mean are moving in a direction towards the southern constellation Columba, not far from the south part of Canis Major. The same average motion is also shown by the spectroscopic radial velocities ; i.e., the stars in the Columba region have an average motion away from us, whilst those in the opposite part of the sky have an average motion towards us. The effect is so pronounced that a dozen stars taken at random will be sufficient to show the prevailing direction roughly (the first determination by Sir William Herschel was made from seven stars only) ; but in attempts to fix this important direction as accurately as possible many thousands of stars have been used.