<P> How these clusters are formed is not yet known, but their formation might well be related to that of globular clusters . Why M31 has such clusters, while the Milky Way has not, is not yet known . It is also unknown if any other galaxy contains this kind of clusters, but it would be very unlikely that M31 is the sole galaxy with extended clusters . </P> <P> Another type of cluster are faint fuzzies which so far have only been found in lenticular galaxies like NGC 1023 and NGC 3384 . They are characterized by their large size compared to globular clusters and a ringlike distribution around the centers of their host galaxies . As the latter they seem to be old objects . </P> <P> Stellar clusters are important in many areas of astronomy . Because the stars were all born at roughly the same time, the different properties of all the stars in a cluster are a function only of mass, and so stellar evolution theories rely on observations of open and globular clusters . </P> <P> Clusters are also a crucial step in determining the distance scale of the universe . A few of the nearest clusters are close enough for their distances to be measured using parallax . A Hertzsprung--Russell Diagram can be plotted for these clusters which has absolute values known on the luminosity axis . Then, when similar diagram is plotted for a cluster whose distance is not known, the position of the main sequence can be compared to that of the first cluster and the distance estimated . This process is known as main - sequence fitting . Reddening and stellar populations must be accounted for when using this method . </P>

Star clusters are extremely useful to find what two quantities of a star