Observational manifestations of gaseous baryon dark matter

The paper considers possible observational implications of the presence of dark matter in the Galaxy in the form of dense gas clouds—clumpuscules with masses M _c ～10^?3 M _⊙ and radii R _c～3×10¹³ cm. The existence of such clouds is implied by modern interpretations of extreme scattering events—variations in quasar radio fluxes due to refraction in dense plasma condensations in the Galactic halo. The rate of collisions between these clouds is shown to be rather high: from 1 to 10M _⊙ per year is ejected into the interstellar medium as a result of such collisions. The optical continuum and 21-cm emission from hot post-collision gas could be observable. Gas clouds composed of dark matter could be formed around O stars in an H II region with radius R～30 pc and emission measure EM?20 cm^?6 pc. They could also be observable in the H_α line. The evaporation of clumpuscules by external ionizing radiation could be a substantial source of matter for the interstellar medium. Assuming that the total mass of matter entering the interstellar medium over the Hubble time does not exceed the mass of luminous matter in the Galaxy, upper limits are found for the cloud radii (R _c<3.5×10¹² cm) and the contribution of clouds to the surface density of the Galaxy (<50M _⊙pc^?2). Dissipation of the kinetic energy of matter lost by clumpuscules could provide an efficient mechanism for heating gas in the Galactic halo.     

A giant avalanche on K2 mount,Karakorum

On September 16th, 1986, an ice avalanche from a hanging glacier near the K2 peak at 7800 m asl, Karakorum, triggered a massive avalanche of ice and snow. Ice and snow, impacting on the path, formed a dust cloud at the advancing tip. Grounding on the firn basin surface, ice and snow broke into fine powder and covered the whole basin. Fine powder of the dust cloud rose up to 500–600 m and drifted 4–5 km away. On the basis of field observations and measurements, topography and weather, conditions of the avalanche formation are analyzed. Judging by the data obtained, the avalanche was extremely large, its vertical descend being 2500 m, the maximum motion speed 124 m/s, volume of the avalanche mass 2 × 10⁵ m³ to 10⁷ m³, and impact pressure, as the avalanche grounded, 2.3 × 10⁶ Pa. It could have been one of the largest avalanches ever recorded, causing danger for mountaineering and expedition activities in this area.     

Infrared photometry of the unique object FG Sge in 1985–2001

Our analysis of many years of infrared photometry of the unique object FG Sge indicates that the dust envelope formed around the supergiant in August 1992 is spherically symmetrical and contains compact, dense dust clouds. The emission from the spherically symmetrical dust envelope is consistent with the observed radiation from the star at 3.5–5 µm, and the presence of the dust clouds can explain the radiation observed at 1.25–2.2 µm. The mean integrated flux from the dust envelope in 1992–2001 was ～(1.0±0.2)×10^?8 erg s^?1cm^?2. The variations of its optical depth in 1992–2001 were within 0.5–1.0. The maximum density of the dust envelope was recorded in the second half of 1993 and corresponded to mean optical depths as high as unity. Several times in the interval from 1992 to 2001, the dusty material of the envelope partially dissipated and was then replenished. For example, the optical depth of the dust cloud at λ=1.25 µm during the last brigthness minimum in the J band was τ_1.25≈4.3, which is much higher than the optical depth of the dust envelope of FG Sge. During maxima of the J brightness, the mean spectral energy distribution at 0.36–5 µm can be represented as a combination of radiation from a G0 supergiant that is attenuated by a dust envelope with a mean optical depth of 0.65±0.15 and emission from the spherically symmetrical dust envelope itself, with the temperature of the graphite grains being 750±150 K. At minima of the J brightness, only radiation from the dust envelope is observed at 1.65–5 µm, with the radiation from the supergiant barely detectable at 1.25 µm. As a result, the integrated flux during J minima is almost half that during J maxima. The mean mass of the spherically symmetrical dust envelope of FG Sge in 1992–2001 was (3 ± 1) × 10^?7M_⊙. This envelope’s mass varied by nearly a factor of two during 1992–2001, in the range (2 – 4) × 10^?7M_⊙. In Autumn 1992, the mass-loss rate from the supergiant exceeded 2 × 10^?7M_⊙/yr. The average rate at which matter was injected into the envelope during 1993–2001 was 10^?8M_⊙/yr. The mean rate of dissipation of the dust envelope was about 1 × 10^?8M_⊙/yr. During 1992–2001, the supergiant lost about 8.7 × 10^?7M_⊙. The parameters of the dust envelope were relatively constant from 1999 until the middle of 2001.     

Parameters for the dust envelope of the protoplanetary nebula IRAS 18062+2410

We have calculated a model for the dust envelope of the protoplanetary nebula IRAS 18062+2410 using observations from the ultraviolet to the far infrared. We assume that the envelope is spherically symmetrical and consists of identical silicate grains with a radius of 0.10 micron, and with the number density of the grains inversely proportional to the square of the distance. The optical depth of the envelope, whose inner boundary is 1.40×10^?3 pc from the center of the star, is 0.050 at 10 microns. At the inner envelope boundary, the temperature of the dust grains is 410 K and their density is 2.7×10^?7 cm^?3. Using calculations of stellar evolution at the stage following the exit from the asymptotic giant branch, we estimate the dust envelope’s expansion velocity to be 12 km/s. The mass-loss rate of the star preceding the ejection of the envelope was about 4.5×10^?6 M _⊙/yr. The observed excess of far-IR flux is not associated with the continuum radiation of the nebula, and may provide evidence for the presence of dust ejected by the star in earlier stages of its evolution.     

Interstellar organic matter in meteorites

Hydrogen which is highly enriched in deuterium is present in organic matter in a variety of meteorites including non-carbonaceous chondrites. The concentrations of this hydrogen are quite large. For example Renazzo contains 140 μmoles/g of the 10,000‰ δD hydrogen. The $\text{D}\text{H}$ ratios of hydrogen in the organic matter vary from 8 × 10^?5 to 170 × 10^?5 (δD ranges from ? 500‰ to 10,000‰) as compared to 16 × 10^?5 for terrestrial hydrogen and 2 × 10^?5 for cosmic hydrogen. The majority of the unequilibrated primitive meteorites contain hydrogen whose $\text{D}\text{H}$ ratios are greater than 30 × 10^?5. If the $\text{D}\text{H}$ ratios in these compounds were due to enrichment relative to cosmic hydrogen by isotope exchange reactions, it would require that these reactions take place below 150 K. In addition the organic compounds having $\text{D}\text{H}$ ratios above 50 × 10^?5 would require temperatures of formation of < 120 K. These types of deuterium enrichments must take place by ion-molecule reactions in interstellar clouds where both ionization and low temperatures exist. Astronomically observed $\text{D}\text{H}$ ratios in organic compounds in interstellar clouds are typically 180 × 10^?5 and range between about 40 × 10^?5 and 5000 × 10^?5. The $\text{D}\text{H}$ values we have determined are the lower limits for the organic compounds derived from interstellar molecules because all processes subsequent to their formation, including terrestrial contamination, decrease their $\text{D}\text{H}$ ratios.In contrast, the $\text{D}\text{H}$ ratios of hydrogen associated with hydrated silicates are relatively uniform for the meteorites we have analyzed with an average value of 14 × 10^?5; very similar to the terrestrial value. These phyllosilicates values suggest equilibration of H₂O with H₂ in the solar nebula at temperatures of about 200 K and higher.The ${}^{13}\text{C}{}^{12}\text{C}$ ratios of organic matter, irrespective its $\text{D}\text{H}$ ratio, lie well within those observed for the earth. If organic matter originated in the interstellar medium, our data would indicate that the ${}^{13}\text{C}{}^{12}\text{C}$ ratio of interstellar carbon five billion years ago was similar to the present terrestrial value.Our findings suggest that other interstellar material, representing various inputs from various stars, in addition to the organic matter is preserved and is present in the meteorites which contain the high $\text{D}\text{H}$ ratios. We feel that some elements existing in trace quantities which possess isotopic anomalies in the meteorites may very well be such materials.     

IR photometry and dust-shell models for two carbon stars

We present JHKLM photometry of the carbon stars ST And and T Lyn acquired in 2000–2010. Along with brightness variations due to pulsations, changes on timescales of 2000–3000 days are also observed. Our combined light curves can be satisfactorily represented with light elements derived from visual observations, but the maxima are delayed relative to the calculated times. A color-index analysis demonstrates that the dust shell of ST And is fairly weak, and is manifest only episodically, while the presence of hot dust was always detected for T Lyn. These results confirm models of spherically symmetric stellar dust shells based on mean-flux data, supplemented with observations in the intermediate IR from the IRAS and AKARI satellites. The visual optical depth of the relatively cool dust shell of ST And assuming a dust temperature at the inner edge of T ₁ = 510 K is very low: τ _V = 0.047. The dust shell of T Lyn is considerably hotter (T ₁ = 940 K), with τ _V = 0.95. We estimate the mass-loss rate to be 1.8 × 10⁻⁷ M _⊙/year for ST And and 3.7 × 10⁻⁷ M _⊙/year for T Lyn.     

IR photometry and models for the dust envelopes of two carbon stars

The results of JHKLM photometry of two carbon stars are presented: the irregular variable NQ Cas and the Mira star BD Vul. Data on the mean fluxes supplemented with mid-IR observations with the IRAS, AKARI, andWISE satellites are used to compute spherically symmetrical model dust envelopes for the stars, consisting of particles of amorphous carbon and silicon carbide. The optical depth in the visible for the comparatively cool dust envelope of BD Vul, with a dust temperature at its inner boundary T₁ = 610 K, is fairly low: τ_V = 0.13. The dust envelope of NQ Cas is appreciably hotter (T₁ = 1550 K), and has τ_V = 0.32. The estimated mass-loss rates are 1.5 × 10^?7M_⊙/yr for NQ Cas and 5.9 × 10^?7M_⊙/yr for BD Vul.     

Multicolor photometry of CH Cygni in 1978–1998

We present the results of long-term (1978–1998) infrared and optical observations of the unique symbiotic system CH Cygni. The system’s IR brightness and color variations are generally consistent with a model in which the source is surrounded by a dust envelope with variable optical depth. There was evidence for a hot source in the CH Cyg system during the entire period from 1978 to 1998, with the exception of several hundred days in 1987–1989. Over the observation period, there was tendency for the system to gradually redden at 0.36–5 µm, accompanied by a brightness decrease at 0.36–2.2µm and a brightness increase at 3.5 and 5 µm. The “activation” of the cool sources in 1986–1989 nearly coincided with the disappearance of radiation from the hot source. The dust envelope of CH Cyg is not spherically symmetrical, and its optical depth along the line of sight is substantially lower than its emission coefficient, the mean values being τ_ex(L)～0.06 and τ_em(L)～0.16. We confirm the presence of a 1800-to 2000-day period in both the optical and IR, both accounting for, and not accounting for, a linear trend. The spectral type of the cool star varied between M5III and M7III. The spectral type was M5III during the phase of maximum activity of the system’s hot source, while the spectral type was M7III when the star’s optical radiation was almost completely absent. The luminosity of the cool giant varied from (6300–9100)L _⊙; its radius varied by approximately 30%. The ratio of the luminosities of the dust envelope and the cool giant varied from 0.08 to 0.5; i.e., up to 50% of the cool star’s radiation could be absorbed in the envelope. The temperature of dust particles in the emitting envelope varied from 550 to 750 K; the radius of the envelope varied by more than a factor of 2. The expansion of the emitting dust envelope observed in 1979–1988 accelerated: its initial velocity (in 1979) was ～8 km/s, while the maximum velocity (in 1987–1989) was ～180 km/s. Beginning in 1988, the radiation radius of the dust envelope began to decrease, first at ～45 km/s and then (in 1996–1998) at ～3 km/s. From 1979 until 1996, the mass of the emitting dust envelope increased by approximately a factor of 27 (the masses in 1979 and 1988 were ～1.4×10^?7 M _⊙ and ～3.8×10^?6 M _⊙, respectively), after which (by 1999) it decreased by nearly a factor of 7. The mass-loss rate of the cool star increased in 1979–1989, reaching ～3.5×10^?6 ～3.5×10^?6 M _⊙/yr in 1988. Subsequently (up to the summer of 1999), the envelope itself began to lose mass at a rate exceeding that of the cool star. The largest input of matter to the envelope occurred after the phase of optical activity in 1978–1985. If the envelope’s gas-to-dust ratio is ～100, the mass of matter ejected in 1988 was ～4×10^?4 M _⊙.     

Spatiotemporal dynamics of land cover and their impacts on potential dust source regions in the Tarim Basin,NW China

Human-driven dynamics of land cover types in the Tarim Basin are able to affect potential dust source regions and provide particles for dust storms. Analyses about dynamics of potential dust source regions are useful for understanding the effects of human activities on the fragile ecosystem in the extremely arid zone and also provide scientific evidence for the rational land development in the future. This paper therefore selected the Tarim Basin, NW China, as a representative study area to reveal spatiotemporal dynamics of land cover and their impacts on potential dust source regions. The results showed that farmland, desert and forest increased by 28.63, 0.64 and 29.27%, while grassland decreased by 10.29% during 1990–2010. The largest reclamation, grassland loss and desertification were 639.17 × 10³, 2350.42 × 10³ and 1605.86 × 10³ ha during 1995–2000. The relationship between reclamation and grassland loss was a positive correlation, while a highly positive correlation was 0.993 between the desertification and grassland loss at different stages. The most serious dust source region was the desertification during 1990–2010 (1614.58 thousand ha), and the serious region was stable desert (40,631.21 thousand ha). The area of the medium and low dust source region was 499.08 × 10³ and 2667.27 × 10³ ha. Dramatic reclamation resulted in the desertification by destroying natural vegetation and breaking the balance of water allocation in various regions.     

Physical properties of molecular clouds as functions of their Galactocentric distance from CS and C<Superscript>34</Superscript>S observations

Twenty-eight CS molecular clouds toward HII regions with Galactocentric distances from ～ 4 to 20 kpc have been studied based on observations obtained in the J=2→1 lines of CS and C³⁴S on the 20-meter radio telescope of the Onsala Space Observatory (Sweden) in March 2001. All 28 clouds have been mapped with an angular resolution of ～40″. The peak intensity in the C³⁴S line has been measured for 20 objects. An LTE analysis has been performed and the parameters of the molecular cloud cores derived. The core sizes are d_A=0.3–4.8 pc, with a median value of ～1.6 pc. The mean hydrogen densities in the cloud cores are n_H2=3.5×10²–3.7 × 10⁴ cm^?3, with a median value of ～7.2×10³ cm^?3. The value of n_H2 ends to decrease with increasing Galactocentric distance of the cloud. The masses of most clouds are 10²?6×10³M_⊙, with the most probable value being M_CS～10³M_⊙. The data follow the dependence M_CS ∝d _A ^(2.4–3.2) . As a rule, the cloud masses are lower than the virial masses for M_CS<10³M_⊙.     

The nonthermal radio emission of WR 140 in a model with colliding two-phase winds

We present a model in which the nonthermal radio emission of binary systems containing Wolf-Rayet and O components is due to collisions between clouds belonging to dense phases of the wind of each star. The relativistic electrons are generated during the propagation of fast shock waves through the clouds and their subsequent de-excitation. The initial injection of superthermal particles is due to photoionization of the de-excited cold gas by hard radiation from the shock front. Therefore, the injection takes place in cloud regions fairly far from the front. Further, the superthermal electrons are accelerated by the betatron mechanism to relativistic energies during the isobaric compression of the cloud material, when most of the gas radiates its energy. Collisions between the clouds can occur far beyond the contact boundary between the rarefied wind components. Thus, the model avoids the problem of strong low-frequency absorption of the radiation.     

Thermal expansion of gehlenite, Ca2Al[AlSiO7], and the related aluminates LnCaAl[Al2O7] with Ln = Tb, Sm

The thermal expansion of gehlenite, Ca₂Al[AlSiO₇], (up to T=830 K), TbCaAl[Al₂O₇] (up to T=1100 K) and SmCaAl[Al₂O₇] (up to T=1024 K) has been determined. All compounds are of the melilite structure type with space group Thermal expansion data were obtained from in situ X-ray powder diffraction experiments in-house and at HASYLAB at the Deutsches Elektronen Synchrotron (DESY) in Hamburg (Germany). The thermal expansion coefficients for gehlenite were found to be: α₁=7.2(4)×10⁻⁶×K⁻¹+3.6(7)×10⁻⁹ΔT×K⁻² and α₃=15.0(1)×10⁻⁶×K⁻¹. For TbCaAl[Al₂O₇] the respective values are: α₁=7.0(2)×10⁻⁶×K⁻¹+2.0(2)×10⁻⁹ΔT×K⁻² and α₃=8.5(2)×10⁻⁶×K⁻¹+2.0(3)×10⁻⁹ΔT×K⁻², and the thermal expansion coefficients for SmCaAl[Al₂O₇] are: α₁=6.9(2)×10⁻⁶×K⁻¹+1.7(2)×10⁻⁹ΔT×K⁻² and α₃=9.344(5)×10⁻⁶×K⁻¹. The expansion mechanisms of the three compounds are explained in terms of structural trends obtained from Rietveld refinements of the crystal structures of the compounds against the powder diffraction patterns. No structural phase transitions have been observed. While gehlenite behaves like a ‘proper’ layer structure, the aluminates show increased framework structure behavior. This is most probably explained by stronger coulombic interactions between the tetrahedral conformation and the layer-bridging cations due to the coupled substitution (Ca²⁺+Si⁴⁺)–(Ln ³⁺+Al³⁺) in the melilite-type structure. This article has been mistakenly published twice. The first and original version of it is available at .     

Parameters of warm molecular clouds from methyl acetylene observations

The results of a survey of 63 Galactic star-forming regions in the 6_K–5_K and 5 _K –4_K methyl acetylene lines at 102.5 and 85.5 GHz are presented. Fourty-three sources were detected at 102.5 GHz, and twenty-five at 85.5 GHz. Emission was detected toward molecular clouds with kinetic temperatures of 20–60 K (so-called “warm clouds”). The CH₃CCH abundances in these clouds are about several ×10^?9. Five sources (NGC 2264, G30.8-0.1, G34.26+0.15, DR 21(OH), S140) were mapped using the maximum-entropy method. The sizes of the mapped clouds fall in the range 0.1–1.7 pc, and the clouds have virial masses of 90–6200 M_⊙ and densities between 6×10⁴ and 6×10⁵ cm^?3. The CH₃CCH sources coincide spatially with the CO and CS sources. Chemical-evolution simulations show that the typical methyl acetylene abundances in the observed clouds correspond to ages of ≈6×10⁴ years.     

Study of the star-forming region L379 IRS3 in CH<Subscript>3</Subscript>OH and CS

A molecular cloud and high-velocity outflow associated with the star-forming region L379 IRS3 have been mapped in the 6_?1-5₀E methanol and CS (3-2) lines using the 12-meter Kitt Peak telescope. The estimated CS column density and abundance in the molecular cloud are 8×10¹⁴ cm^?2 and 4×10^?9, respectively. LVG modeling of the methanol emission constrains the gas density in the cloud to (1–4)×10⁵ cm^?3 and the gas kinetic temperature to 20–45 K. The upper limit on the density of the high-velocity gas is 10⁵ cm^?3.     

Shock tracers in the molecular cloud G1.6-0.025

Observations of the molecular cloud G1.6-0.025 in the 2_K-1_K and J₀-J_?1E series and 5_?1-4₀E line of CH₃OH, the (2-1) and (3-2) lines of SiO, and the 7_?7-6_?6 line of HNCO are described. Maps of the previously observed extended cloud with V_lsr～50 km/s and high-velocity clump with V_lsr～160 km/s, as well as a newly detected clump with V_lsr～0 km/s, have been obtained. The extended cloud and high-velocity clump have a nonuniform structure. The linewidths associated with all the objects are between 20 and 35 km/s, as is typical of clouds of the Galactic center. In some directions, emission at velocities from 40 to 160 km/s and from ?10 to +75 km/s is observed at the clump boundaries, testifying to a connection between the extended cloud and the high-velocity clump and clump at V_lsr～0 km/s. Compact maser sources are probaby contributing appreciably to the emission of the extended cloud in the 5_?1-4₀E CH₃OH line. Non-LTE modeling of the methanol emission shows that the extended cloud and high-velocity clump have a relatively low hydrogen density (<10⁴ cm^?3). The specific column density of methanol in the extended cloud exceeds 6×10⁸ cm^?3s, and is 4×10⁸?6×10⁹ cm^?3s in the high-velocity clump. The kinetic temperatures of the extended cloud and high-velocity clump are estimated to be <80 K and 150–200 K, respectively. Possible mechanisms that can explain the link between the extended cloud with V_lsr～50 km/s and the clumps with V_lsr～0 km/s and ～160 km/s are briefly discussed.     

The dust envelope of R CrB in June 2001: Detection of a compact IR source

We present the results of a joint analysis of JHK interferometric and UBVJHKLM photometric observations of RCrB acquired in June 2001. The baseline for the IOTA interferometer was 21.18 m. During the observations, the star was in its bright state in the V band and near its maximum brightness in the L band. Our analysis reveals an IR source that is considerably smaller than the extended dust envelope discovered earlier. We identify this compact IR source with the emission from a group of dust clouds. The linear scale (diameter) of the IR source was d _in,c ≈ 13.5D_* (its angular diameter is θ_in,c≈6.4 mas). About 7% of the star’s radiation was obscured by this group of clouds, which contributed ～14% of the total IR excess of R CrB and ～22% of the K-band flux. The color temperature of the compact source was only ～300 K higher than the color temperature of the extended dust envelope. The inner boundary of the extended dust envelope had a diameter of d _in,e ≈ 90D_* (θ_in,e≈43 mas).     

Formation of planetary systems and brown dwarfs around single stars

The conditions for the formation of planets and brown dwarfs around single main-sequence stars are considered in two scenarios. The formation of planets and brown dwarfs requires that the initial specific angular momentum of a solar-mass protostar be (0.32)×10¹⁸ cm²/s. The accreted matter of the protostar envelope forms a compact gas ring (disk) around the young star. If the viscosity of the matter in this ring (disk) is small, increasing its mass above a certain limit results in gravitational instability and the formation of a brown dwarf. If the viscosity of the gas is sufficiently large, the bulk of the protostar envelope material will be accreted by the young star, and the gas disk will grow considerably to the size of a protoplanetary dust disk due to the conservation of angular momentum. The formation of dust in the cool part of the extended disk and its subsequent collisional coalescence ultimately results in the formation of solar-type planetary systems.     

IR photometry and a model of a dust shell in V2108 Ophiuchi

We present the results of JHKLM photometry of the oxygen Mira variable V2108 Oph acquired in 2000–2004. The period of brightness variations is refined (570 ± 3 days), and light and color curves in the near-IR are presented. The mean fluxes, color temperatures, and sizes for two blackbodies representing the combined radiation of the star and dust shell at minimum and maximum brightness are estimated. Additional IRAS data were used to compute a model with a spherically symmetric dust shell of silicate grains; the best-fit model has a radius for its inner boundary of 2.4 × 10¹⁴ cm, a dust temperature at this boundary of 1150 K, an optical depth of the shell at 0.55 μm of 16.8, and implies a distance to the star of 980 pc. We estimate the mass-loss rate for V2108 Oph to be 1.2 × 10⁻⁵ M _⊙/yr. Original Russian Text ? M.B. Bogdanov, O.G. Taranova, V.I. Shenavrin, 2006, published in Astronomicheskiĭ Zhurnal, 2006, Vol. 83, No. 5, pp. 437–442.     

The stellar epoch in the evolution of the Galaxy

We consider the astrophysical evolution of the Galaxy over large time scales, from early stages (an age of ～10⁸ yrs) to the end of traditional stellar evolution (～10¹¹ yrs). Despite the fact that the basic parameters of our stellar system (such as its size, mass, and general structure) have varied little over this time, variations in the characteristics of stars (their total luminosity, color, mass function, and chemical composition) are rather substantial. The interaction of the Galaxy with other stellar systems becomes an important factor in its evolution 100–1000 Gyr after its origin; however, we take the Galaxy to be isolated. In the model considered, the basic stages of Galactic evolution are as follows. The Galaxy forms as the result of the contraction (collapse) of a protogalactic cloud. The beginning of the Milky Way’s life—the relaxation period, which lasts about 1–2 Gyr—is characterized by active star formation and final structurization. The luminosity and colors of the Galaxy are correlated to the star formation rate (SFR). The young Galaxy intensely radiates high-energy photons, which are mostly absorbed by dust and re-emitted at IR wavelengths. In the subsequent period of steady-state evolution, the gas content in the Galactic disk gradually decreases; accordingly, the SFR decreases, reaching 3–5M _⊙/yr at the present epoch and decreasing to 0.03M _⊙/yr by an age of 100 Gyr. Essentially all other basic parameters of the Galaxy vary little. Later, the decrease in the SFR accelerates, since the evolution of stars with masses exceeding 0.4M _⊙ (i.e., those able to lose matter and renew the supply of interstellar gas) comes to an end. The Galaxy enters a period of “dying”, and becomes fainter and redder. The variation of its chemical composition is manifested most appreciably in a dramatic enrichment of the interstellar gas in iron. The final “stellar epoch” in the life of the Galaxy is completed ～10¹³ yrs after its formation, when the evolution of the least massive stars comes to an end. By this time, the supplies of interstellar and intergalactic gas are exhausted, the remaining stars become dark, compact remnants, there is no further formation of new stars, and the Galactic disk no longer radiates. Eventually, infrequent outbursts originating from collisions of stellar remnants in the densest central regions of the Galaxy will remain the only source of emission.