  Clustering scientific publications in an important problem in bibliometric
research. We demonstrate how two software tools, CitNetExplorer and VOSviewer,
can be used to cluster publications and to analyze the resulting clustering
solutions. CitNetExplorer is used to cluster a large set of publications in the
field of astronomy and astrophysics. The publications are clustered based on
direct citation relations. CitNetExplorer and VOSviewer are used together to
analyze the resulting clustering solutions. Both tools use visualizations to
support the analysis of the clustering solutions, with CitNetExplorer focusing
on the analysis at the level of individual publications and VOSviewer focusing
on the analysis at an aggregate level. The demonstration provided in this paper
shows how a clustering of publications can be created and analyzed using freely
available software tools. Using the approach presented in this paper,
bibliometricians are able to carry out sophisticated cluster analyses without
the need to have a deep knowledge of clustering techniques and without
requiring advanced computer skills.
