Narrow Results

Globular clusters are gravitationally bound stellar systems containing on the order of 10⁵ stars. Due to the high stellar densities in the cores of these clusters, close encounters and even physical collisions between stars are inevitable. These dynamical interactions can produce exotic types of single and binary stars that are extremely rare in the galactic field, but which may be important to the dynamical evolution of their host clusters. A common feature of these dynamically-formed stellar populations is that many of their members are relatively hot, and thus bright in the far-ultraviolet (FUV) waveband. In this short review, we describe how space-based FUV observations are being used to find and study these populations.

In the nonlinear Navier–Stokes viscous flow dynamics, physical damping is mathematically accomplished by a braking term in the momentum equation, corresponding to a heating term in the energy equation, both responsible of the conversion of mechanical energy into heat. In such two terms, it is essential the role of the viscous stress tensor, relative to contiguous macroscopic moving flow components, depending on the macroscopic viscosity coefficient ν. A working formulation for ν can always be found analytically, tuning some arbitrary parameters in the current known formulations, according to the geometry, morphology and physics of the flow. Instead, in this paper, we write an alternative hybrid formulation for ν, where molecular parameters are also included. Our expression for ν has a more physical interpretation of the internal damping in dilute gases because the macroscopic viscosity is related to the small scale molecular dissipation, not strictly dependent on the flow morphology, as well as it is free of any arbitrary parameter. Results for some basic 2D tests are shown in the smoothed particle hydrodynamics (SPH) framework. An application to the 3D accretion disc modeling for low mass cataclysmic variables is also discussed. Consequences of the macroscopic viscosity coefficient reformulation in a more strictly physical terms on the thermal conductivity coefficient for dilute gases are also discussed.

High energy transients make up a diverse and exotic class of objects, from terrestrial lightning to $\gamma$-ray bursts at cosmological distances. In this review, we provide a detailed look at some of the more exciting transients observed over the last few years by Swift and other high energy missions.

The observations of Betelgeuse since Christmas 2011 were started either to study the star and to contribute to observational astronomy from urban contexts, with high level of light pollution, using the naked eye. Since then 811 observations to November 9, 2021 have been made and communicated to the AAVSO international database. Antares and also other interesting long-period variable stars like VY CMa, RS Oph (recurrent nova), V766 Cen and several novae and SN2014J are part of the 2.5 K database of visual observations, obtained with the Argelander method applied to naked eye or telescopic observations. It is an invitation for the young generations, to contribute to these studies with careful and feasible observations, which require a reduction for airmass extinction of the variable star and of the comparison stars, when they are not on the same narrow field of view. The possibility to deduce the airmass by reading the notes (containing the location of the observation) of all the AAVSO-SGQ catalogue is finally presented as a point of force of my observations.

High energy transients make up a diverse and exotic class of objects, from terrestrial lightning to γ-ray bursts at cosmological distances. In this review, we provide a detailed look at some of the more exciting transients observed over the last few years by Swift and other high energy missions.

The theory of General Relativity deals with very accurate measurements that show significant divergences from Newtonian predictions only with speed near to the velocity of light. The photometry of the radiation from collapsing star’s shells like novae and supernovae is a starting point for relativistic cosmic phenomena. The visual observations described in this paper provided the needed photometrical and timing accuracy to follow these phenomena. More than 1200 observations of variable stars, included the type 1a SN2014J, Nova DEL 2013, Nova CEN 2013 and Nova SGR 2015 no. 2 have been sent to the AAVSO by the author, with SGQ code, during the period 1998-2015, and contributed also to IAU and HST observational campaigns; they have been analyzed to evaluate the photometric accuracy, in the context of the International Year of Light 2015.