PROBLEMS OF MODERN SCIENCE 



recognised, which are characterised by their 

 members moving through space with equal 

 velocities on parallel lines. The Pleiades form 

 such a cluster, the motions of at least fifty of the 

 stars being equal and parallel. Another similar 



& Corona*. 



< h \ 8 Leonii 



Strius 



Fig. 6. URSA MAJOR CLUSTER 



group is that called the Ursa Major cluster, because 

 to it belong five of the stars in the Plough. It has 

 been found, however, that a number of stars in 

 other parts of the sky, such as Sirius, are also 

 members. The diagram (Fig. 6) gives some idea 

 of the relative positions of the stars composing 

 this cluster. 



The measures of the distances of the stars 

 when expressed in ordinary units are too great 

 60 



