Orientation effects are believed to be important in sources which harbor relativistic jets in their nuclei. BL Lac type objects are interpreted as a type of this class of objects whose galaxy counterpart is a bright elliptical galaxy. This interpretation requires every bright elliptical galaxy to have a BL Lac type nucleus beamed usually away from

Astronomy in the Canary Islands has grown from its historical beginnings at the site-testing expedition of C. Piazzi Smyth in 1856. Simony, Edgecomb, and Mascart followed in his footsteps. The Observatorio Astronomico del Teide was provisionally founded in 1959 to study the astronomical qualities of the Canary Islands' sky. It led to the present

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A summary is presented of the physical considerations governing line formation in molecular clouds with kinetic temperatures in the range from 5 to 100 K and hydrogen number densities in the range from 100 to one million per cubic centimeter. An exposition of the classical Sobolev formulation due to Goldreich and Kwan (1973) is followed by results

This publication is a compilation of photographic and cartographic comparisons of large, well-known geographical features on planets and moons in the solar system. Included are structures caused by impacts, volcanism, tectonics, and other natural forces. Each feature is discussed individually and then those of similar origin are compared at the

The decay of a sunspot region was studied using high resolution white-light pictures. The photometric and geometric parameters of a light-bridge showing granular structure were determined. No difference is found comparing it with the quiet photosphere.