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 Rate of Period Change as a Diagnostic of Cepheid PropertiesThe rate of period change P˙ for a Cepheid is shown to be aparameter that is capable of indicating the instability-strip crossingmode for individual objects and, in conjunction with light amplitude,the likely location of the object within the instability strip. Theobserved rates of period change in over 200 Milky Way Cepheids aredemonstrated to be in general agreement with predictions from stellarevolutionary models, although the sample also displays features that areinconsistent with some published models and indicative of the importanceof additional factors not fully incorporated in models to date. Interferometric Observations of Cepheids. p-factor and center to limb darkening measurements.Cepheids distances are usually inferred from the Period-Luminosityrelationship, calibrated using the semi-empirical Baade-Wesselink (BW)method. Using this method, the distance is known to a multiplicativefactor, called the projection factor. Presently, this factor is computedusing numerical models - it has hitherto never been measured directly.Based on our new interferometric measurements obtained with the CHARAArray and the already published parallax, we present a geometricalmeasurement of the projection factor of a Cepheid, delta Cep. The valuewe determined, p=1.27±0.06, confirms the generally adopted valueof p=1.36 within 1.5 sigmas. Our value is in line with recenttheoretical predictions of \citet{Nardetto2004}. Moreover,center-to-limb variation (CLV) remains a possible slight source of biasfor the interferometric BW method. In order to address this problem, weare in the process of measuring the CLV of Polaris. Stellar pulsation and evolution: a stepping-stone to match reality.We discuss current status of evolutionary and pulsation predictions forintermediate-mass stars. In particular, we focus our attention on thedifferent physical mechanisms that might affect the current discrepancybetween evolutionary and pulsation estimates of Galactic and MagellanicCepheid masses. Theoretical findings and recent empirical evidenceindicate that the mass-loss may play a significan role in thislong-standing problem. Extended envelopes around Galactic Cepheids. II. Polaris and δ Cephei from near-infrared interferometry with CHARA/FLUORWe present the results of long-baseline interferometric observations ofthe classical Cepheids Polaris and δ Cep in the near infrared K'band (1.9-2.3 μm), using the FLUOR instrument of the CHARA Array.Following our previous detection of a circumstellar envelope (CSE)around ℓ Car (Kervella et al. 2006), we report similar detectionsaround Polaris and δ Cep. Owing to the large data set acquired onPolaris, in both the first and second lobes of visibility function, wehave detected the presence of a circum-stellar envelope (CSE), locatedat 2.4±0.1 stellar radii, accounting for 1.5±0.4% of thestellar flux in the K band. A similar model is applied to the δCep data, which shows improved agreement compared to a model withoutCSE. Finally, we find that the bias in estimating the angular diameterof δ Cep in the framework of the Baade-Wesselink method(Mérand et al 2005b) is of the order of 1% or less in the K band.A complete study of the influence of the CSE is proposed in thiscontext, showing that at the optimum baseline for angular diametervariation detection, the bias is of the order of the formal precision inthe determination of the δ Cep pulsation amplitude (1.6%). The Period Changes of PolarisThe evolutionary changes in pulsation period for the Cepheid Polaris arereinvestigated using archival observational material (radial velocities,photometry, and eye observations) over the interval 1844 to the present,including new photometry for the star obtained in 2003-2004. The star'spulsation period increased at a rate of 4.5 s yr-1 duringthat interval, with the exception of a brief hiatus between 1963 and1966, when it suddenly decreased, possibly as a result of a briefreduction in average stellar radius amounting to -0.055%. At roughly thesame time, the pulsation amplitude of Polaris underwent a marked change.Prior to 1963 the V amplitude was in excess of about 0.1 mag, possiblydecreasing at a rate of 0.019 mag century-1. Following thehiatus of 1963-1966, the pulsation amplitude underwent a sharp declineand now appears to be erratic on a cycle-to-cycle basis, always smallerthan 0.05 mag. The rapid rate of period increase for Polaris isconsistent with a first crossing of the Cepheid instability strip, whilethe hiatus of 1963-1966 and sudden decrease in pulsation amplitudethereafter suggest that the star may have left the instability strip forfirst crossers at that time, leaving it near the center of theinstability strip for Cepheids in higher crossing modes. Atlas and Catalog of Dark Clouds Based on Digitized Sky Survey IWe present a quantitative atlas and catalog of dark clouds derived byusing the optical database Digitized Sky Survey I''. Applying atraditional star-count technique to 1043 plates contained in thedatabase, we produced an AV map covering the entire region inthe galactic latitude range |b| ≤ 40°. The map was drawn at twodifferent angular resolutions of 6' and 18', and is shown in detail in aseries of figures in this paper. Based on the AV map, weidentified 2448 dark clouds and 2841 clumps located inside them. Somephysical parameters, such as the position, extent, and opticalextinction, were measured for each of the clouds and clumps. We alsosearched for counterparts among already known dark clouds in theliterature. The catalog of dark clouds presented in this paper lists thecloud parameters as well as the counterparts. Non-LTE Line-Formation and Abundances of Sulfur and Zinc in F, G, and K StarsExtensive statistical-equilibrium calculations on S I and Zn I were carried out, in orderto investigate how the non-LTE effect plays a role in the determinationof their abundances in F-K stars. Having checked on the spectra ofrepresentative F-type stars (Polaris, Procyon, α Per) and the Sunthat our non-LTE corrections yield a reasonable consistency between theabundances derived from different lines, we tried an extensive non-LTEreanalysis of published equivalent-width data of S I and Zn I lines for metal-poorhalo/disk stars. According to our calculations, S I 9212/9228/9237 lines suffer significant negative non-LTEcorrections (lesssim 0.2‑0.3 dex), while LTE is practically validfor S I 8683/8694 lines. As far as the verymetal-poor regime is concerned, a marked discordance is observed betweenthe [S/Fe] values from these two abundance indicators, in the sense thatthe former attains a nearly flat plateau (or even a slight downwardbending) while the latter shows an ever-increasing trend with a furtherlowering of metallicity. The characteristics of [Zn/Fe] reported fromrecent LTE studies (i.e., an evident/slight increase of [Zn/Fe] with adecrease of [Fe/H] for very metal-poor/disk stars) were almostconfirmed, since the non-LTE corrections for the Zn I 4722/4810 and 6362 lines (tending to be positive andgradually increasing toward lower [Fe/H]) are quantitatively of lesssignificance (lesssim 0.1 dex). Polaris, the nearest Cepheid in the Galaxy: atmosphere parameters, reddening and chemical compositionWe present the results of our analysis of spectroscopic observations ofα UMi (Polaris) obtained in 1994 (five spectra) and 2001-2004 (30spectra). The mean atmospheric parameters we derived are as follows:Teff= 5968 K, ΔTeff= 50 K; logg= 2.2;Δlogg= 0.1; Vt= 4.35 km s-1,ΔVt= 0.1 km s-1 in 1994 and Teff=6015 K, ΔTeff= 170 K; logg= 2.2, Δlogg= 0.3;Vt= 4.35 km s-1, ΔVt= 0.9 kms-1 in 2001-2004. A comparison of our Teff withthose obtained from (B-V)0 data during the last 60 yr showsboth sporadic and regular changes with a period of 25-30 yr, which isclose to the orbital period, 29.9-30.5 yr. We also determined the colourexcess E(B-V) = 0.034 mag, AV= 0.102 mag, and chemicalcomposition for 30 elements to verify the hypothesis that the starcrosses the Cepheid instability strip for the first time. With[Fe/H]=+0.07 dex, Polaris shows [C/H]=-0.17 dex, [N/H]=+0.42 dex and[O/H]=-0.00 dex, which corresponds to the third (or fifth) crossing. Onthe basis of the distance, radius and surface gravity values, Polaris isprobably a first-overtone pulsator. Polaris is located near the blueedges of the Cepheid instability strip for the fundamental mode and thefirst overtone, and an abrupt decrease of the pulsational amplitude andits recent increase could be due to its binarity. Predicting accurate stellar angular diameters by the near-infrared surface brightness techniqueI report on the capabilities of the near-infrared (near-IR) surfacebrightness technique to predict reliable stellar angular diameters asaccurate as <~2 per cent using standard broad-band Johnson photometryin the colour range -0.1 <= (V-K)O<= 3.7 includingstars of A, F, G, K spectral type. This empirical approach is fast toapply and leads to estimated photometric diameters in very goodagreement with recent high-precision interferometric diametermeasurements available for non-variable dwarfs and giants, as well asfor Cepheid variables. Then I compare semi-empirical diameters predictedby model-dependent photometric and spectrophotometric (SP) methods withnear-IR surface brightness diameters adopted as empirical referencecalibrators. The overall agreement between all these methods is withinapproximately +/-5 per cent, confirming previous works. However, on thesame scale of accuracy, there is also evidence for systematic shiftspresumably as a result of an incorrect representation of the stellareffective temperature in the model-dependent results. I also comparemeasurements of spectroscopic radii with near-IR surface brightnessradii of Cepheids with known distances. Spectroscopic radii are found tobe affected by a scatter as significant as >~9 per cent, which is atleast three times greater than the formal error currently claimed by thespectroscopic technique. In contrast, pulsation radii predicted by theperiod-radius (PR) relation according to the Cepheid period result aresignificantly less dispersed, indicating a quite small scatter as aresult of the finite width of the Cepheid instability strip, as expectedfrom pulsation theory. The resulting low level of noise stronglyconfirms our previous claims that the pulsation parallaxes are the mostaccurate empirical distances presently available for Galactic andextragalactic Cepheids. Der Polarstern - ein ungewohnlicher Cepheide.Not Available Analysis of the Na, Mg, Al, and Si Abundances in the Atmospheres of Red Giants of Different Spectral SubgroupsWe analyze the Na, Mg, Al, and Si abundances in the atmospheres of morethan 40 stars, includingred giants of different spectral subgroups(normal red giants, mild and classical barium stars) and severalsupergiants. All these elements exhibit abundance excesses, with theoverabundance increasing with the star’s luminosity. Thedependence of the overabundances for each of these elements on theluminosity (or log g) is the same for all the spectral subgroups,testifying to a common origin: they are all products of hydrogen burningin the NeNa and MgAl cycles that have been dredged up from the stellarinteriors to the outer atmospheric layers by convection that graduallydevelops during the star’s evolution from the main sequence to thered-giant stage. The sodium abundances derived for several stars arelower than for other stars with similar atmospheric parameters. The agesand kinematic characteristics of these two groups of stars suggest thatthey probably belong to different stellar generations. High-Mass Triple Systems: The Classical Cepheid Y CarinaeWe have obtained a Hubble Space Telescope STIS ultraviolethigh-dispersion echelle-mode spectrum of the binary companion of thedouble-mode classical Cepheid Y Car. The velocity measured for the hotcompanion from this spectrum is very different from reasonablepredictions for binary motion, implying that the companion is itself ashort-period binary. The measured velocity changed by 7 kms-1 during the 4 days between two segments of theobservation, confirming this interpretation. We summarize binary''Cepheids that are in fact members of a triple system and find that atleast 44% are triples. The summary of information on Cepheids withorbits makes it likely that the fraction is underestimated.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc. Mean Angular Diameters and Angular Diameter Amplitudes of Bright CepheidsWe predict mean angular diameters and amplitudes of angular diametervariations for all monoperiodic PopulationI Cepheids brighter than=8.0 mag. The catalog is intended to aid selecting mostpromising Cepheid targets for future interferometric observations. CHARM2: An updated Catalog of High Angular Resolution MeasurementsWe present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773 Improved Baade-Wesselink surface brightness relationsRecent, and older accurate, data on (limb-darkened) angular diameters iscompiled for 221 stars, as well as BVRIJK[12][25] magnitudes for thoseobjects, when available. Nine stars (all M-giants or supergiants)showing excess in the [12-25] colour are excluded from the analysis asthis may indicate the presence of dust influencing the optical andnear-infrared colours as well. Based on this large sample,Baade-Wesselink surface brightness (SB) relations are presented fordwarfs, giants, supergiants and dwarfs in the optical and near-infrared.M-giants are found to follow different SB relations from non-M-giants,in particular in V versus V-R. The preferred relation for non-M-giantsis compared to the earlier relation by Fouqué and Gieren (basedon 10 stars) and Nordgren et al. (based on 57 stars). Increasing thesample size does not lead to a lower rms value. It is shown that theresiduals do not correlate with metallicity at a significant level. Thefinally adopted observed angular diameters are compared to thosepredicted by Cohen et al. for 45 stars in common, and there isreasonable overall, and good agreement when θ < 6 mas.Finally, I comment on the common practice in the literature to average,and then fix, the zero-point of the V versus V-K, V versus V-R and Kversus J-K relations, and then rederive the slopes. Such a commonzero-point at zero colour is not expected from model atmospheres for theV-R colour and depends on gravity. Relations derived in this way may bebiased. Period-luminosity relations for Galactic Cepheid variables with independent distance measurementsIn this paper, we derive the period-luminosity (PL) relation forGalactic Cepheids with recent independent distance measurements fromopen cluster, Barnes-Evans surface brightness, interferometry and HubbleSpace Telescope astrometry techniques. Our PL relation confirms theresults from recent works, which showed that the Galactic Cepheidsfollow a different PL relation to their Large Magellanic Cloud (LMC)counterparts. Our results also show that the slope of the Galactic PLrelation is inconsistent with the LMC slope with more than 95 per centconfidence level. We apply this Galactic PL relation to find thedistance to NGC 4258. Our result of μo= 29.49 +/- 0.06 mag(random error) agrees at the ~1.4σ level with the geometricaldistance of μgeo= 29.28 +/- 0.15 mag from water masermeasurements. Polaris: a closer "first crosser".Not Available A Spectral Atlas of F and G StarsWe present an atlas of a group of bright stars in the range of spectralclasses F--G and luminosity classes I--V. The spectra were obtainedwith spectral resolution R  15,000 within spectral region4500--6620 Å. Typical spectra of stars with different metallicity[Fe/H] are included. We also show the digital version of the spectraldata in FITS format. The Indo-US Library of Coudé Feed Stellar SpectraWe have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http. A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF SpectrometerWe present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA). Astrometry of the 1572 supernova (B Cassiopeiae)Contemporary European measurements of the Milky Way supernova of 1572 (BCas) have been analyzed to compute a modern position for the star in1572. This work adds to that done by previous 20th-century authors byusing a new procedure and previously unused astrometry by 16th-centuryEuropean observers. Comparison is made with earlier analyses and tomodern positional data on the supernova remnant (3C 10). Polaris with New PrecisionWe have observed Polaris with the Wide-Field Infrared Explorer (WIRE)star camera. We present a light curve with a typical uncertainty ofapproximately 0.0001 mag in the binned data. There are two surprises inthe observations of this low amplitude (approximately 0.02 mag) overtoneCepheid pulsator. First, the amplitude of variation decreases and thenincreases during a month of observation. This may be the analog to theBlazhko effect in RR Lyr stars, but is virtually unknown in classicalCepheids. Second, we can put a tight upper limit on the amplitude of anyfrequencies other than the first overtone. Near the period of Polaris (4days) a resonance has been identified in the Fourier parameters of thelight curves of other first overtone pulsators which is thought toresult from excitation of the first and fourth overtone. However, wefind no sign of the fourth overtone in the light curve. Comparisons withpulsation behavior in other stars and also implications will bediscussed. Financial assistance has been provided by Chandra X-rayCenter NASA Contract NAS8-039073 (to NRE) and NASA grant NAG5-9318 (toDB and HP). Exhaustive Monitoring of Polaris in Night Sky Live CONCAM DataPolaris is automatically monitored every clear night throughout the yearby every northern CONtinuous CAMera (CONCAM) in the Night Sky Liveglobal network. For clear nights, the coverage of Polaris isunprecedented, typically returning over 100 observations per station pernight for every clear night. Currently, Polaris in under nearlycontinuous yearly observation by the CONCAM stations on Haleakala, MaunaKea, Mt. Wilson, Kitt Peak, Canary Islands, and Wise Observatory inIsrael. Analyzing the automatically recorded photometry from Polaris isvaluable to understanding this unusual and evolving Cepheid. Preliminaryanalyses of differential CONCAM photometry are presented for Polaris(Alpha Ursae Minoris) over the time scales from minutes to years. NightSkyLive.net: Bringing the Night Sky into Your ClassroomShow your class a full live night sky with a single click. The Night SkyLive project now has 10 fisheye CONtinuous CAMeras (CONCAMs) deployedaround the world that send live images of the night sky back tohttp://NightSkyLive.net every few minutes. Any classroom that has accessto a web browser can see the current night sky, live, horizon tohorizon, similar in depth to what the human eye can see, and annotated,above a major observatory somewhere in the world. Additionally, archivedimages and automatically generated movies show how the night skyappeared over the past night and the past year, and how it will likelyappear above your student's heads tonight, all through the night. Stars,planets, and constellations are automatically labelled. In addition tolive products, canned on-line tutorials for beginning students usearchived NSL images to explain concepts such as diurnal motion and anddemonstrate the transience of variable stars. Projects for more advancedundergraduates include using the automatically generated photometryfiles to follow the light curves of well known stars such as Polaris,Betelgeuse, and Alpha Centauri. Cepheid distances from infrared long-baseline interferometry. III. Calibration of the surface brightness-color relationsThe recent VINCI/VLTI observations presented in Paper I have nearlydoubled the total number of available angular diameter measurements ofCepheids. Taking advantage of the significantly larger color rangecovered by these observations, we derive in the present paper highprecision calibrations of the surface brightness-color relations usingexclusively Cepheid observations. These empirical laws make it possibleto determine the distance to Cepheids through a Baade-Wesselink typetechnique. The least dispersed relations are based on visible-infraredcolors, for instance FV(V-K) = -0.1336 ± 0.0008 (V-K)+ 3.9530 ± 0.0006}. The convergence of the Cepheid (this work)and dwarf star (Kervella et al. \cite{kervella04c}) visible-infraredsurface brightness-color relations is strikingly good. The astrophysicaldispersion of these relations appears to be very small, and below thepresent detection sensitivity.Table \ref{table_measurements1} is only available in electronic form athttp://www.edpsciences.org 400 years of Uranometria.Not Available Ultrahigh-Energy Cosmic RaysNot Available The Spectral Correlation Function of Molecular Clouds: A Statistical Test for Theoretical ModelsWe compute the spectral correlation function (SCF) of 13COJ=1-0 maps of molecular cloud complexes. The SCF is a power law overapproximately an order of magnitude in spatial separation in every map.The power-law slope of the SCF, α, its normalization,S0(1 pc), and the spectral line width averaged over the wholemap, σv, are computed for all the observational maps.The values of α, S0(1 pc), and σv arecombined to obtain empirical correlations to be used as tests fortheoretical models of molecular clouds. Synthetic spectral maps arecomputed from different theoretical models, including solutions of themagnetohydrodynamic (MHD) equations with different values of the rmsMach number of the flow and stochastic models with different powerspectra of the velocity field. In order to compute the radiativetransfer from the MHD models, it is necessary to assign the models aphysical scale and a physical density. When these assignments are madeaccording to Larson-type relations, the best fit to the observationalcorrelations is obtained. Unphysical stochastic models are instead ruledout by the empirical correlations. MHD models with equipartition ofmagnetic and kinetic energy of turbulence do not reproduce theobservational data when their average magnetic field is orientedapproximately parallel to the line of sight. The Universality of Turbulence in the Molecular Interstellar Medium and Its Exploitation as a Distance EstimatorThe turbulent energy spectrum of molecular clouds in a variety ofenvironments is measured via principal component analysis (PCA) ofspectral line imaging observations at millimeter wavelengths. Molecularclouds with known distances have been previously shown to accuratelyobey a universal scale dependence of turbulent velocity dispersion overspatial scales of 1-50 pc, via both standard object-based analysis and,more recently, PCA. The PCA-based spectrum is accurately obeyed suchthat it may be used as a distance estimator for molecular clouds with~30% accuracy, where the error budget is contributed to strongly byinput H II region distances used for the calibration. The use of13CO spectral line data for distance estimation is examinedand compared to the distance calibration established for 12COobservations. We show that distances estimated using 13CO arein good agreement with those obtained using 12CO, with apossible ~10% distance overestimation for 13CO relative tothe 12CO calibration. Several molecular clouds with knowndistances are subjected to PCA, and we demonstrate that the universalspectrum is closely respected by all clouds; PCA-based distancesestimated under the assumption of exact adherence to the universalspectrum are derived and are shown to be in excellent agreement withoptically estimated distances. We examine the possibility that the PCAdistance estimation method may be used to solve the kinematic distanceambiguity in the inner Galaxy. We establish how PCA may be used todiagnose severe blending of near/far emission and, in cases of little orno blending, to accurately establish the near or far distance. The innerGalaxy results provide initial support for the global validity of theuniversal PCA spectrum previously demonstrated for the outer Galaxyonly. In conjunction with the accurate velocity information provided bymillimeter wavelength spectral line data, PCA can provide usefulinformation for studies of Galactic structure and kinematics. Period Changes in Galactic Classical Cepheids. Slow Evolution of Long-Period CepheidsWe compared period changes derived from O-C diagrams for 63 classicalCepheids from our Galaxy with model calculations. We found that forCepheids with log P > 1.0 the observed changes are smaller thanpredicted values, except variable SZ Cas. However some of the firstovertone Cepheids, particularly EU Tau and Polaris, change its periodmuch faster than it follows from theory. Summary of the known data onperiod changes in Cepheids from the Galaxy and from the MagellanicClouds (previous papers) leads to conclusion that none of the 999Cepheids is undergoing the first crossing of the instability strip. Alsothe observed period changes for long-period Cepheids are a few timesslower than predicted by the models. These results imply that muchlarger fraction of helium is burned in the Cepheid stage than it ispredicted by models.
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