We present the results of our study of starforming regions in the lenticular galaxy NGC 4324. During a complex analysis of multiwavelength observational data -- the narrow-band emission-line images obtained with the 2.5-m telescope at the Caucasus Mountain Observatory of the Sternberg Astronomical Institute of the Moscow State University and the archival images in the broad bands of the SDSS, GALEX and WISE surveys -- we have detected young starforming complexes (clumps) located in the inner ring of the lenticular galaxy NGC 4324, and we have established a regular pattern of their distribution along the ring, which, nevertheless, changes with time (with age of starforming regions). We suggest several possible evolutionary paths of the lenticular galaxy NGC 4324, of which the accretion of gas-rich satellites or giant clouds (the so-called minor merging) is the most probable one.

We present a probabilistic model built for the optical cross-match between the SRG/eROSITA X-ray sources and photometric data from the DESI Legacy Imaging Surveys. The model relies both on positional and photometric information on optical objects nearby X-ray sources and allows performing selection with precision and recall $${\approx}94\%$$ (for $$F_{\textrm{X,0.5-2}}>10^{-14}$$ erg s $${}^{-1}$$ cm $${}^{-2}$$ ). With this model, we calibrated positional error of the SRG/eROSITA sources detected in the Lockman Hole: $$\sigma_{\textrm{corr}}=0.87\sqrt{\sigma_{\textrm{det}}^{2.53}+1.12^{2}}$$ . The model will become a part of the SRGz system for data analysis of the X-ray data obtained from the all-sky SRG/eROSITA survey.

We analyze the reasons for imaging in the $$V$$ and $$I$$ filters to measure the distances to galaxies by the TRGB method. We show the possibility of using the TRGB method if Hubble Space Telescope (HST) images only in one F814W ( $$I$$ ) filter are available. Based on archival HST images, we have carried out stellar photometry for 12 galaxies: NGC 1058, NGC 1365, NGC 1448, NGC 3079, NGC 3810, NGC 4321, NGC 4328, NGC 4485, NGC 4490, NGC 4651, NGC 6951, and NGC 7331, and measured their distances by the TRGB method. For many of the galaxies from this list such measurements have been made for the first time. A comparison of our results for galaxies with known TRGB measurements has shown good agreement.

For both periods of the double-mode classical Cepheid AS Cas we have constructed $$O-C$$ diagrams spanning a time interval of 132 years. The $$O-C$$ diagrams have the shape of parabolas, which has allowed us for the first time to determine the quadratic light elements and to calculate the rates of evolutionary changes in their periods: $$dP_{0}/dt=+0.065({\pm}{0.010})$$ s yr $${}^{-1}$$ for the fundamental mode and $$dP_{1}/dt=+0.163({\pm}{0.006})$$ s yr $${}^{-1}$$ for the first overtone, in agreement with the theoretical calculations for the third crossing of the instability strip. The pulsation stability test proposed by Lombard and Koen has confirmed that the changes in the periods are real.

Based on spectrophotometric observations, we have studied the variable activity of the symbiotic star CH Cyg from 2008 to 2018. The activity of the star was accompanied by a periodic increase of the flux in continuum and spectral lines. In the period of its activity, in both 2015 and 2018, absorption components appeared in the profiles of emission lines. The shape and radial velocity of the absorption components changed within a day. The maximum radial velocity of the absorption components was about $$-2000$$ km s $${}^{-1}$$ . By modeling the observed spectral energy distribution, we have established that in the active state the luminosity and radius of the warm pseudo-photosphere of the symbiotic star increased by dozens of times, reaching $$L_{\textrm{WD}}=88.14L_{\odot}$$ and $$R_{\textrm{WD}}=6.77R_{\odot}$$ , respectively, at maximum in 2018, and the source of activity of the symbiotic star is accretion onto a white dwarf from a red giant of spectral type M8III. The accretion rate at the maximum luminosity of the pseudo-photosphere has been estimated to be $$\dot{M}_{\textrm{acc}}\sim 4.7\times 10^{-8}M_{\odot}$$ yr $${}^{-1}$$ . We have found a correlation between the TiO 6144 and 7125 molecular bands and the $$R$$ brightness of the star, which suggests that a period of 750 days may be the pulsation period of the cool component, and, hence, we have concluded that the pulsations of the cool component at periastron or at a phase close to the periastron can provide an additional inflow of matter accreting onto the white dwarf. Thus, we have concluded that a variable mass accretion rate onto the white dwarf is responsible for the photometric variability of the symbiotic star. We show that 2015 and 2018 were the most active years for this star.

We present the results of our analysis of the optimal flight to the trans-Neptunian object (90377) Sedna at the launch in 2029. Sedna is a distant space object with a perihelion and aphelion $${\sim}74$$ and 1000 AU, respectively, suggesting that this celestial body may belong to the Oort cloud. The orbital period of Sedna is more than 11 000 years. In our study we have considered a direct flight to Sedna and a flight with gravity assists of Venus, the Earth, and Jupiter under a restriction on the flight duration and the total $$\Delta V$$ . As the main scenario for the flight to Sedna for the launch in 2029 we have chosen the Earth–Venus–Earth–Earth–Jupiter flight, which provides $$\Delta V$$ cost no more than 4.64 km s $${}^{-1}$$ under flight time restrictions of 30 years. We have also investigated the possibility of extending the scenario for the flight to Sedna by including a close flyby of one or more asteroids. As an example, we consider in detail the scenario with the flyby of the main-belt asteroids (20) Massalia and (5080) Oya.

The emission from the Crab nebula exhibits a significant gamma-ray variability. In this paper we have analyzed this variability in terms of periodicity. Using the pulsar ephemeris constructed from radio data, we have obtained the light curve of the Crab nebula with the suppressed contribution of the pulsar in the energy range 100–300 MeV from August 2008 to March 2020. Using analysis of variance, we have found evidence for a periodicity of flares in the nebula with a period of 49 weeks. The statistical significance of the signal is 0.02 $$(2.1\sigma)$$ if only the highest peak in the periodogram is taken into account and 0.003 $$(2.7\sigma)$$ if two peaks, the highest one and the peak at twice the period (98 weeks) that can be caused by the same signal, are taken into account. If the periodicity found is real, then one should expect new periods of enhanced activity of the nebula in August 2021 and 2022.

The possibility of the existence of radiation belts near a planet possessing an intrinsic magnetic field and located in a stellar plasma flow is considered using Mercury as an example. An analysis is performed both using Størmer’s theory of charged particle motion, in which not the specific trajectories of individual particles, but the regions of allowed motion in an axisymmetric field are considered, and by analyzing the trajectories. The existence of trapped radiation near Mercury is shown to be possible, and the criteria for the formation of a stable population of trapped particles are formulated.

A novel method is proposed for the reconstruction of the early bolometric light curve for supernovae IIP with the available set of H $$\alpha$$ emission spectra on the first day after the shock breakout. The method uses the effect of radiative acceleration of the preshock circumstellar gas that is manifested in broad H $$\alpha$$ wings. The efficiency of the method is demonstrated in the case of SN 2013fs for which the spectra were taken between 6 and 10 h after the shock breakout. An important feature of this method is that it does not require any photometry, distance, and extinction.

Only a handful of quadruple systems with two accurate inner visual orbits are known. Architecture of two such systems is studied here to determine period ratios, mutual orbit orientation, and other parameters; updated orbital elements and their errors are derived. Gliese 225.1 (HIP 28442) is composed of three K-type and one M-type dwarfs and has inner orbital periods of 67.2+-0.2 and 23.4+-0.5 yr. Its inner orbits have small mutual inclination and are likely coplanar with the outer orbit of ~2 kyr period. The quadruple system FIN 332 (HIP 92037) consists of four early A type stars with similar masses and magnitudes. Both its inner orbits with periods of 27.6+-0.2 and 39.8+-0.4 yr have large eccentricities (0.82 and 0.84). Their orientation in the sky is remarkably similar. In contrast, the outer orbit with a period of ~5 kyr has a large relative inclination to the inner orbits. Dynamics and formation of these quadruple systems are briefly discussed.