The fundamental plane and other scaling relations for galaxy groups and clusters

In this paper we study the relations between the main characteristics of groups and clusters of galaxies using the archival data of the SDSS and 2MASX catalogs. We have developed and implemented a new method of determining the size of galaxy systems and their effective radius which contains half of the galaxies and not half the luminosity, since the luminosity of the brightest galaxy in a group can account for over 50% of the total luminosity of the group. The derived parameters (log L_K, logR_e, and log σ₂₀₀) for 94 systems of galaxies (0.0038 < z < 0.09) determine the Fundamental Plane (FP), which, with a scatter of 0.15, is similar in form to the FP of galaxy clusters obtained by Schaeffer et al. (1993) and D’Onofrio et al. (2013) with other methods and for different bands. We show that the FP in the near-infrared region (NIR) for 94 galaxy systems has the form of L_K ∝ \(R_e^{0.70 \pm {{0.13}_\sigma }1.34 \pm 0.13}\), whereas in x-rays it has the form of—L_X ∝ \(R_e^{1.15 \pm {{0.39}_\sigma }2.56 \pm 0.40}\). The form of the FP for groups and clusters is consistent with the FP for early-type galaxies determined in the same way. The form of the FP for galaxy systems deviates from the shape that one would expect from virial predictions. Adding the mass-to-light ratio as a fourth independent parameter has little effect on this deviation, but decreases the scatter of the FP for a sample of rich galaxy clusters by 12%.     

Global structure of the local universe according to 2MRS survey

We report the results of a statistical analysis of the space distribution of galaxies of the 2MRS catalog, which contains redshifts of 43533 galaxies of the 2MASS all-sky IR survey. Because of the unique features of the 2MRS survey, such as its 90% sky coverage, galaxy selection in the IR, the complete incorporation of the old stellar population of galaxies, weakness of the dust extinction effects, and the smallness of the k- and e-corrections allowed us to determine the statistical properties of the global distribution of galaxies in the Local Universe. We took into account the main methodological factors that distort the theoretically expected relations compared to those actually observed. We construct the radial galaxy number counts N(R), SL(R, r) statistics, and the complete correlation function (conditional density) Γ(r) for volume-limited (VL) galaxy samples. The observed conditional density Γ(r) in the redshift space is independent of the luminosity of galaxies and has the form of a power-law function with exponent γ ≈ 1.0 over a large range scale-length spanning from 0.1 to 100 Mpc. We compare the statistical properties of the space distribution of galaxies of the 2MRS catalog with the corresponding properties of simulated catalogs: stochastic fractal distributions and galaxies of the Millennium catalog.     

Irregular dwarf galaxy KK 230: A relatively young galaxy at the periphery of the local group?

We present some results of the photometric analysis of the stellar population of the irregular dwarf galaxy KK 230 on the basis of the archive database of the Hubble space telescope. The color index-magnitude diagram for KK 230 gets to magnitude 27 ^m in the V and I bands, and it comprises stellar populations of various ages. The age of the youngest main-sequence stars is 3.2 × 10⁷ yr. These stars are distributed along the north-south direction in the picture plane, and this fact can be linked to the observed kinematics of the neutral gas in the galaxy. Older blue and red supergiants are no less than 1.6 × 10⁸ years old, and such an age implies that the star formation was episodic over the last several hundreds of millions of years. As judged from the position of the tip of the red giant branch, the distance modulus for KK 230 is m ? M = 26.5 ^m . The corresponding distance is D = 2 Mpc. Based on the average absolute magnitude M _I,RC and color index (V ? I)_I,RC of the red clump, we conclude that the majority of KK 230 stars have an age of no more than (2–3) × 10⁹ yr, their metallicity being Z ≈ 0.0004.     

Stellar population of the blue compact dwarf galaxy SBS 1415+437

The stellar population of the blue compact dwarf galaxy SBS 1415+437 is investigated using the archive database of the Hubble space telescope. The color index-magnitude diagram for stars reaches a magnitude of 29 ^m in the V and I bands. It comprises young main-sequence stars, blue and red supergiants, and the old population of red giant branch and asymptotic giant branch. The tip of the red giant branch αTRGB) was used to calculate the distance modulus, which turned out to be m ? M = 30.65 ± 0.08 ^m . The corresponding distance to the galaxy is D = 13.5 ± 1.0 Mpc. The youngest stars are distributed irregularly near the bright H II region in the southwest part of SBS 1415+437. The old population occupies a larger area, it is distributed more evenly and forms the galactic halo. The spatial distribution of young stars shows that the star formation in the galaxy spread in the direction from northeast to southwest over the last 5 × 10⁷ yr with an average rate of 60 km/s. The TRGB of SBS 1415+437 was found to be appreciably shifted to the blue range: (V ? I) _TRGB ≈ 1.30. The galaxy age turns out to be not smaller than the age of Galactic globular clusters (about 10¹⁰ yr), provided that the galaxy originally had a very low metallicity (our photometric estimate is [Fe/H] = ?2.4). If the metallicity of SBS 1415+437 changed almost not at all in the course of evolution and was equal to [Fe/H] = ?1.3 (as estimated from the emission lines of ionized gas), the galaxy age is no more than 2 × 10⁹ yr.     

OB stars in the Tycho-2 and 2MASS catalogues

The Tycho-2 proper motions and five-band Tycho-2 and 2MASS photometry for approximately 2.5 million common stars have been used to select OB stars and to determine the extinction and photometric distance for each of them. We have selected 37 485 stars and calculated their reddenings based on their positions in the two-color V _T -H, J-Ks diagrams relative to the zero-age main sequence and the theoretical reddening line for B5 stars. Tests confirm that the selected stars belong to the spectral types O-B with a small admixture of later types. We calculate the extinction coefficient R and its variations with Galactic longitude based on the positions of the selected stars in the two-color B _T -V _T , V _T -Ks diagram. The interstellar extinction for each star is calculated as the product of the reddening found and the coefficient R. The extinction and its variations with Galactic longitude agree well with the extinction based on the model by Arenou et al. (1992). Calibration of the relation between the absolute magnitude and reduced proper motion V _T − + 5 + 5 log μ for Hipparcos stars has allowed us to calculate the absolute magnitudes and photometric distances for the selected stars. The distances found agree with those derived from the Hipparcos parallaxes within 500 pc. The distribution of the stars and the extinction variations with distance found show that the selected stars form an almost complete sample of stars with spectral types earlier than B5 within about 750 pc of the Sun. The sample includes many noticeably reddened stars in the first and second Galactic quadrants that are absent from the Hipparcos and Tycho Spectral Types Catalogues. This slightly changes the pattern of the distribution of OB stars compared to the classical pattern based on Hipparcos. Original Russian Text ? G.A. Goncharov, 2008, published in Pis’ma v Astronomicheskiĭ Zhurnal, 2008, Vol. 34, No. 1, pp. 10–20.     

The fallacy of Oppenheimer Snyder collapse: no general relativistic collapse at all,no black hole,no physical singularity

By applying Birkhoff’s theorem to the problem of the general relativistic collapse of a uniform density dust, we directly show that the density of the dust ρ=0 even when its proper number density n would be assumed to be finite! The physical reason behind this exact result can be traced back to the observation of Arnowitt et al. (Phys. Rev. Lett. 4: 375, 1960) that the gravitational mass of a neutral point particle is zero: m=0. And since, a dust is a mere collection of neutral point particles, unlike a continuous hydrodynamic fluid, its density ρ=mn=0. It is nonetheless found that for k=?1, a homogeneous dust can collapse and expand special relativistically in the fashion of a Milne universe. Thus, in reality, general relativistic homogeneous dust collapse does not lead to the formation of any black hole in conformity of many previous studies (Logunov et al., Phys. Part. Nucl. 37: 317, 2006; Kiselev et al., Theor. Math. Phys. 164: 972, 2010; Mitra, J. Math. Phys. 50: 042502, 2009a; Suggett, J. Phys. A 12: 375 1979b). Interestingly, this result is in agreement with the intuition of Oppenheimer and Snyder (Phys. Rev. 56: 456, 1939) too:“Physically such a singularity would mean that the expressions used for the energy-momentum tensor does not take into account some essential physical fact which would really smooth the singularity out. Further, a star in its early stages of development would not possess a singular density or pressure, it is impossible for a singularity to develop in a finite time.”     

Results of Magnetic-Field Measurements with the 6-m Telescope. V. Observations in 2011

The paper presents results of measurements of magnetic-field longitudinal components B _e , radial velocities V _r , and projections of the rotation velocity in the line of sight v _e sin i for 74 objects, mainly main-sequence chemically peculiar stars and standard stars. Observations were carried out in 2011 at the 6-m BTA telescope using the Main Stellar Spectrograph (MSS) with a Zeeman analyzer. Seven new magnetic stars were discovered: HD38129, HD47152, HD50341, HD63347, HD188501, HD191287, and HD260858. Three more stars were suspected to have magnetic fields. Observations of magnetic standard stars and non-magnetic stars confirm the absence of any systematic errors capable of introducing distortions into the B _e longitudinal-field measurement results. The paper gives comments on the research results for each of the 74 stars.     

Spectroscopic study of the peculiar galaxy UGC 5119

We present the results of our study of the stellar kinematics in the elliptical galaxy UGC 5119, which has previously been suspected to be a polar-ring galaxy. We have detected a rapidly rotating disk in the central region (r ≤ 3.2 kpc) of the galaxy’s main body and found a radial velocity gradient along its minor axis (in the putative ring). We conclude that UGC 5119 is a medium-luminosity elliptical galaxy with a rapidly rotating disk component and a stellar (probably polar) ring. We have calculated the Lick indices of the Hβ, Mggb, Fe 5270, and Fe 5335 absorption lines and compared them with evolutionary synthesis models. Differences in the [Mg/Fe] ratios, metallicities, and ages of the stars have been found: the young stellar population with a solar [Mg/Fe] ratio and a high metallicity dominates in the circumnuclear region (r ≤ 1 kpc), while the old one with a low metal abundance dominates in the ring.     

Evolution of Intergalactic Gas in the Neighborhood of Dwarf Galaxies and Its Manifestations in the HI 21 cm Line

Low-mass galaxies are known to have played the crucial role in the hydrogen reionization in the Universe. In this paper we investigate the contribution of soft x-ray radiation (E ~ 0.1–1 keV) from dwarf galaxies to hydrogen ionization during the initial reionization stages. The only possible sources of this radiation in the process of star formation in dwarf galaxies during the epochs preceding the hydrogen reionization epoch are hot intermediate-mass stars (M ~ 5–8 M_⊙) that entered the asymptotic giant branch (AGB) stage and massive x-ray binaries. We analyze the evolution of the intergalactic gas in the neighborhood of a dwarf galaxy with a total mass of 6 × 10⁸M_⊙ formed at the redshift of z ~ 15 and having constant star-formation rate of 0.01–0.1 M_⊙ yr^?1 over a starburst with a duration of up to 100 Myr. We show that the radiation from AGB stars heats intergalactic gas to above 100 K and ensures its ionization x_e ? 0.03 within about 4–10 kpc from the galaxy in the case of a star-formation rate of star formation 0.03–0.1 M_⊙ yr^?1, and that after the end of the starburst this region remains quasi-stationary over the following 200–300 Myr, i.e., until z ~ 7.5. Formation of x-ray binaries form in dwarf galaxies at z ~ 15 results in a 2–3 and 5–6 times greater size of the ionized and heated region compared to the case where ionization is produced by AGB stars exclusively, if computed with the “x-ray luminosity–star-formation rate” dependence (L_X ~ f_XSFR) factor f_X = 0.1 and f_X ~ 1, respectively. For f_X ? 0.03 the effect of x-ray binaries is smaller that that of AGB star population. Lyα emission, heating, and ionization of the intergalactic gas in the neighborhood of dwarf galaxies result in the excitation of the 21 cm HI line. We found that during the period of the starburst end at z ~11.5–12.5 the brightness temperature in the neighborhood of galaxies is 15–25 mK and the region where the brightness temperature remains close to its maximum has a size of about 12–30 kpc. Hence the epoch of the starburst end is most favorable for 21 cm HI line observations of dwarf galaxies, because at that time the size of the region of maximum brightness temperature is the greatest over the entire evolution of the dwarf galaxy. In the case of the sizes corresponding to almost 0.’1 for z ~ 12 regions with maximum emission can be detected with the Square Kilometre Array, which is currently under construction.     

FG Sge at the stage of dust shell ejection

New photometric observations of the variable star FG Sge, a rapidly evolving planetary nebula nucleus, were performed in 2003–2008. On 230 nights, we obtained 86 UBV and 155 BV RI (or R _c , I _c ) magnitude estimates. The maximum amplitude of the V-band light variations was >8 ^m . Six deep minima and four high maxima were observed. Analysis of the light curve has shown that the pulsation period of the star remained constant since 1991 and was P = 115 days. We have studied the wavelength dependence of the extinction at various phases of the light curve. The blueing of the B-V color at deep minima is interpreted as the result of light scattering in the circumstellar dust shell of the star formed by preceding dust ejections since 1992. Our spectroscopic observations performed on nine nights in 2003–2007 with the 125-cm telescope at the Crimean Station of the Sternberg Institute have confirmed the previously detected intensity variations of the Swan bands and the sodium doublet with brightness. It is noted that the Swan bands originate in the upper atmosphere, the star’s extended envelope, while the sodium doublet originates mainly in the circumstellar shell of FG Sge. We suggest that the star is currently located in the temperature-luminosity diagram at the turning point of the horizontal track of cooling in the direction of hot stars—evolution caused by the last helium shell flash at the planetary nebula stage.     

Transition between general relativity and quantum gravity using quark and strange quark matter with some kinematical test

Contemporary piece of writing devotes to the investigation of plane symmetric cosmological model with quark and strange quark matter in the deformations of the Einstein’s theory of General Relativity (GR). At small or large scales (ultraviolet or infrared gravity), deformations of the Einstein’s theory could provide a better handling of cosmic acceleration without magnetism (along with singularities). In particular, a proper deformation of GR in the ultraviolet regime could play the role of describing the transition between GR and quantum gravity. As a matter of fact, although with a different purpose in mind, it was Einstein himself who proposed in the 30’s the reformulation of GR by taking the field of orthonormal frames or tetrads as the dynamical variable instead of the metric tensor (Einstein, Phys. Math. Kl 217, 401, 1928). As per the observation, pressure and energy density of the model approaches the bag constant in negative and positive ways at \(t\rightarrow \infty \), i.e. \(p\rightarrow -B_c \) and \(\rho \rightarrow B_c \), the negative pressure due to the Dark Energy (DE) in the context of accelerated expansion of the universe. So the strange quark matter gives an idea of existence of dark energy in the universe and supports the observations of the SNe-I (Riess et al., Astron. J. 116,1009, 1998; Perlmutter et al., Astrophys. J. 517, 565, 1999). Also these results agree with the study of Aktas and Aygun (Chinese J. Phys. 55, 71, 2017) and Sahoo et al. (New. Astron. 60, 80, 2018).     

Bulk motions of flat galaxies on 100-Mpc scales from new data

We use a new expanded and partially modified sample of 1501 thin edge-on spiral galaxies from the RFGC catalog to analyze the non-Hubble bulk motions of galaxies on the basis of a generalized multiparameter Tully-Fisher relation. The results obtained have confirmed and refined our previous conclusions (Parnovsky et al. 2001), in particular, the statistical significance of the quadrupole and octupole components of the galaxy bulk velocity field. The quadrupole component, which is probably produced by tidal forces from overdense regions, leads to a difference in the recession velocities of galaxies on scales of 8000–10000 km s^?1 up to 6% of their Hubble velocity. On Local Supercluster scales (3000 km s^?1), its contribution increases to about 20%. Including the octupole components in the model causes the dipole component to decrease to the 1σ level. In contrast, in the dipole model, the galaxy bulk velocity relative to the frame of reference of the cosmic microwave background is 310±75 km s^?1 toward the apex with l=311° and b=12°. We also consider a sample of 1493 galaxies that was drawn using a more stringent galaxy selection criterion. The difference between the results of our data analysis for this sample and for the sample of 1501 galaxies is primarily attributable to a decrease in the dipole velocity component (290±75 km s^?1 toward the apex with l=310° and b=12°) and a decrease in σ by about 2%.     

A photometric study of faint galaxies in the field of GRB 000926

We present our B, V, R_c, and I_c observations of a \(3'.6 \times 3'\) field centered on the host galaxy of GRB 000926 (α_2000.0=17^h04^m11^s, \(\delta _{2000.0} = + 51^ \circ 47'9\mathop .\limits^{''} 8\)). The observations were carried out on the 6-m Special Astrophysical Observatory telescope using the SCORPIO instrument. The catalog of galaxies detected in this field includes 264 objects for which the signal-to-noise ratio is larger than 5 in each photometric band. The following limiting magnitudes in the catalog correspond to this limitation: 26.6 (B), 25.7 (V), 25.8 (R), and 24.5 (I). The differential galaxy counts are in good agreement with previously published CCD observations of deep fields. We estimated the photometric redshifts for all of the cataloged objects and studied the color variations of the galaxies with z. For luminous spiral galaxies with M(B)z～1.     

Spectroscopic and photometric characteristics of stars in the field of the cluster vdB 130

The observational data for 24 stars toward the young cluster vdB 130 are analyzed. The spectroscopic and photometric BV RIJHK observations have been carried out with the following telescopes: 6-m at the Special Astrophysical Observatory of the Russian Academy of Sciences, 60-cm at the Southern Station of the Moscow State University, and 2.5-m at the Caucasus Observatory of the Sternberg Astronomical Institute of theMoscow State University. Nine stars previously selected as cluster members have been found to belong to different subtypes of type B. A minimum color excess toward the cluster, E(B ? V) = 0.9 mag, has been revealed for the vdB 130 stars lying outside the molecular cloud. Maximum color excesses, E(B ? V) = 1.3?1.4 mag, have been found in the spectra of cluster stars 1r and 5r observed in dust blobs. Inside the cluster R _v is shown to differ from the standard one. The overwhelming majority of the remaining investigated stars belong to late types and have minor color excesses (≤0.3) typical of close distances.     

GALEX catalogs of UV sources: statistical properties and sample science applications: hot white dwarfs in the Milky Way

We describe the content and properties of UV source catalogs from GALEX’s All-Sky Imaging Survey (AIS, 5σ depth ≈19.9(FUV)/20.8(NUV) mag, in the AB system) and Medium-depth Imaging Survey (MIS, 5σ depth ≈22.6(FUV)/22.7(NUV) mag), constructed by Bianchi L., et al.: Mon. Not. R. Astron. Soc. (2010, in press). The catalogs contain 65.3/12.6 million (AIS/MIS) unique UV sources with photometric error in NUV less than 0.5 mag, over 21?435(AIS)/1579(MIS) square degrees. Matched optical data from GSC-II provide additional B, R, I photometry for the brightest sources, and SDSS provides u g r i z photometry over 7325(AIS)/1103(MIS) square degrees overlap areas. We discuss statistical properties that are relevant for understanding sample selection biases and completeness, in potential science applications of these catalogs. The FUV (1344–1786 Å) and NUV (1771–2831 Å) photometry uniquely enable selection of the hottest stellar objects, in particular hot white dwarfs (WD), which are elusive at optical wavelengths because of their hot temperatures and faint luminosities. From the GALEX-SDSS matched sources we selected ～40?000 Milky Way (MW) stars hotter than about 18?000 K (FUV-NUV?相似文献   

Magnetic-field dependence of chemical anomalies in CP stars

The dependence of the degree of anomaly of parameter Z of Geneva photometry (Z0 = Z _CP ?Z _norm.) on the average surface magnetic field Bs is analyzed. The Z0 value is proportional to the degree of anomaly of chemical composition. It was found that Bs → 0 corresponds Z0 → ?0.010÷ ?0.015, i.e., part of CP stars are virtually devoid of magnetic field, but exhibit chemical anomalies. This effect may be due to selection whereby only objects with strong chemical anomalies are classified as CP stars, thereby producing a deficit of stars with relatively weak anomalies. Moreover, CP stars have other sources of stabilization of their atmospheres besides the magnetic field, e.g., slow rotation. Formulas relating Z0 to Bs are derived.     

One-meter Schmidt telescope of the Byurakan Astrophysical Observatory: New capabilities

In 2013–2015 the Laboratory of spectroscopy and photometry of extragalactic objects (LS-PEO) of the Special Astrophysical Observatory together with Armenian specialists upgraded the 1-m Schmidt telescope of the Byurakan Astrophysical Observatory of the National Academy of Sciences of Armenia. We completely redesigned the control system of the telescope: we replaced the actuating mechanisms, developed telescope control software, and made the guiding system. We reworked and prepared a 4k × 4k Apogee (USA) liquid-cooled CCD with RON ~ 11.1 e^?, a pixel size of 0.″868, and field of view of about 1□°, and in October 2015 mounted it in the focus of the telescope. The detector is equipped with a turret bearing 20 intermediate-band filters (FWHM = 250 Å) uniformly covering the 4000–9000 Å wavelength range, five broadband filters (u, g, r, i, z SDSS), and three narrow-band filters (5000 Å, 6560 Å and 6760 Å, FWHM = 100 Å). During the first year of test operation of the 1-m telescope we performed pilot observations within the framework of three programs: search for young stellar objects, AGNevolution, and stellar composition of galaxy disks.We confirmed the possibility of efficiently selecting of young objects using observations performed in narrow-band Hα and [SII] filters and the intermediate-band 7500 Å filter. Three-hours long exposures with SDSS g-, r-, and i-band filters allow us to reach the surface brightness level of 28^m/□″ when investigating the stellar content of galaxy disks for a sample of nine galaxies. We used observations performed with the 1-m telescope in five broadband (SDSS u, g, r, i, and z) and 15 intermediate-band filters (4000–7500 Å) to construct a sample of quasar candidates with 0.5 < z < 5 (330 objects) in about one-sq. degree SA68 field complete down to R_AB = 23^m. Spectroscopic observations of 29 objects (19.^m5 < R < 22^m) carried out at the 6-m telescope of the Special Astrophysical Observatory of the Russian Academy of Sciences confirmed the quasar nature of 28 objects.     

Einstein–Maxwell Field Equation in Isotropic Coordinates: An Application to Modeling Superdense Star

We present a charged analogue of Pant et al. (2010, Astrophys. Space Sci., 330, 353) solution of the general relativistic field equations in isotropic coordinates by using simple form of electric intensity E that involve charge parameter K. Our solution is well behaved in all respects for all values of X lying in the range 0 <X≤ 0.11, K lying in the range 4 <K≤ 6.2 and Schwarzschild compactness parameter u lying in the range 0 <u≤ 0.247. Since our solution is well behaved for wide ranges of the parameters, we can model many different types of ultra-cold compact stars like quark stars and neutron stars. We have shown that corresponding to X = 0.077 and K = 6.13 for which u = 0.2051 and by assuming surface density ρ _b =4.6888×10¹⁴ g cm ^?3 the mass and radius are found to be 1.509M _⊙, 10.906 km respectively which match with the observed values of mass 1.51M _⊙ and radius 10.90 km of the quark star XTE J1739-217. The well behaved class of relativistic stellar models obtained in this work might have astrophysical significance in the study of more realistic internal structures of compact stars.     

A Catalog of Isolated Galaxy Pairs Limited to Absolute Magnitude -18.5 Drawn from HyperLEDA Database

The present paper is devoted to the construction of a catalog of isolated galaxy pairs extracted from the HyperLEDA extragalactic database. The radial velocities of the galaxies in the pairs are in the range [3000, 16000] km s^?1. In order to get an unbiased pair catalog as complete as possible, we have limited the absolute magnitude of the galaxies to M ≤ ?18.5. The criteria used to define the isolated galaxy pairs are the following: 1) velocity criterion: radial velocity difference between the pair members ΔV < 500 kms^?1; 2) interdistance criterion: projected distance between the members r_p < 1 Mpc; 3) reciprocity criterion: each member is the closest galaxy to the other one, which excludes multiplets; 4) isolation criterion: we define a pair as isolated if the ratio ρ = r₃/r_p of the projected distance of the pair to its closest galaxy (this one having a velocity difference lower than 500 km s^?1 with respect to the pair) and the members projected interdistance r_p is larger than 2.5.We have searched for these closest galaxies first in HyperLEDA M-limited source catalog, then in the full one.We have managed not to suppress the small number of pairs having close-by but faint dwarf galaxy companions. The galaxy pair catalog lists the value of ρ for each isolated pair. This method allows the user of the catalog to select any isolation level (beyond the chosen limit ρ > 2.5). Our final catalog contains 13 114 galaxy pairs, of which 57% are fairly isolated withρ > 5, and 30% are highly isolated with ρ ≥ 10.     

<Emphasis Type="Italic">GALEX</Emphasis> and star formation

Wide-field far-UV (FUV, 1344–1786 Å) and near-UV (NUV, 1771–2831 Å) imaging from GALEX provides a deep, comprehensive view of the young stellar populations in hundreds of nearby galaxies, shedding new light on the process of star formation (SF) in different environments, and on the interplay between dust and SF. GALEX’s FUV-NUV color is extremely sensitive to stellar populations of ages up to a few hundred Myrs, unambiguously probing their presence and enabling age-dating and stellar mass estimate, together with the characterization of interstellar dust extinction. The deep sensitivity, combined with the wide field-of-view, made possible in particular the discovery and characterization of star formation in extremely low-density, diffuse gas environments such as outer galaxy disks, tidal tails, low-surface-brightness galaxies (LSB) and dwarf Irregular galaxies, and of rejuvenation episodes in early-type galaxies. Such results provide several missing links for interpreting galaxy classes in an evolutionary context, extend our knowledge of the star-formation process to previously unexplored conditions, constrain models of galaxy disk formation, and clarify the mutual role of dust and star formation. We review a variety of star-forming environments studied by GALEX, and provide some model analysis tools useful for interpretation of GALEX measurements, and potentially as basic science planning tools for next-generation UV instruments.