A survey of observational problems of stellar evolution

Abstract

I shall start by giving an account of the remarkable work on moving star groups on which Dr Eggen has been engaged for the last 2 or 3 years. Dr Eggen cannot present the results of his researches himself to this Discussion, as he is at this moment in South Africa carrying out photoelectric observations on the Magellanic Clouds and on other objects. As is well known, Dr Eggen is distinguished for his photoelectric observations of the nearby open clusters such as the Hyades, work of which character has been the main observational basis for the entire theory of stellar evolution in its contemporary form. It is in these open clusters that one in able to isolate sets of stars with a common age and a common chemical constitution; that is to say sets of stars whose colour luminosity array, or H. -R. diagram, represents the locus of points corresponding to starts in the same state of evolution but differing in mass. Since the open clusters are not necessarily all of the same age, and are in fact adjudged to be of very different ages, the H. -R. diagrams of a number of open clusters afford the means of reconstructing the path, in a plane representing absolute magnitude and colour, of a star as it passes through a succession of stages of development. The very new is represented by the association h and X Persei, the comparatively new by open clusters such as Pleiades and Hyades, and so on to clusters which are held to be very old such as M67 (figure 1). The particular feature of Dr Eggen's recent investigations is that he has passed on from considering the open clusters such as Hyades to considering group of stars widely separated in the sky but possessing parallel motion. The Hyades themselves are distributed over a comparatively large area of the sky, that is to say about 15° square. Even from the trigonometrical parallaxes, the distances of the stars forming the cluster are quite well known and we can say at once that the cluster occupies a volume, more or less spherical, with a radius of about 10 parsecs. Only a fraction of the stars seen in this direction are members of the moving group. They are distinguished by the fact that their observed transverse motions, or proper motions, converge to a point in the sky, the radiant of the cluster. From the average of the radial velocities of the members we can construct the actual motion in kilometres per second of the group relative to the sun, which is probably within 1 or 2 km/s of the value 44 km/s, and then from the observed proper motion of each star and the star's angular distance from the radiant we can infer the parallax. Since proper motions can be found in well-observed stars with an accuracy of a few thousandths of a second of arc per year, and since the proper motions of Hyades members are about a tenth of a second of are per year, we can derive parallaxes of Hyades members with an error of only a few parts per cent from the proper motions, whereas trigonometrical parallaxes are much more uncertain than this as a rule.