The fraction of star-forming to quiescent dwarf galaxies varies from almost infinity in the field to zero in the centers of rich galaxy clusters. What is causing this pronounced morphology-density relation? What do quiescent dwarf galaxies look like when studied in detail, and what conclusions can be drawn about their formation mechanism? Here we

We present the result of a study on the expansion properties and internal kinematics of round/elliptical planetary nebulae of the Milky Way disk, the halo, and of the globular cluster M 15. The purpose of this study is to considerably enlarge the small sample of nebulae with precisely determined expansion properties (Schönberner et al. \cite{SJSPCA

Context. K-means is a clustering algorithm that has been used to classify large datasets in astronomical databases. It is an unsupervised method, able to cope very different types of problems. Aims: We check whether a variant of the algorithm called single pass k-means can be used as a fast alternative to the traditional k-means. Methods: The

Telescopes on the North Atlantic Islands of La Palma and Tenerife, in the Canary Islands Archipelago have recorded optical extinction for approximately 30 years whilst making routine astronomical observations. In La Palma, these measurements come from stellar measurements, while in Tenerife, these measurements come from solar observations. We have

Quasars are considered to be the most luminous subclass among Active Galactic Nuclei. Although they appear to be distributed everywhere in the Universe, only a small fraction of them happen to be part of a fortuitous alignment with a galaxy or a galaxy cluster that lies along the line-of-sight, which results in multiple images of the background