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the giant leap that gaia spectroscopy will lead us through can be sized by four basic considerations : ( @xmath4 ) gaia will record multi - epoch spectra for a magnitude complete sample of stars @xmath5 larger than any whole - sky existing database ( e.g. hd survey , progressing michigan project , etc . ) ; ( @xmath6 ) for each target , an average of 67 epoch spectra will be recorded over the five year mission lifetime ; ( @xmath7 ) the wavelength and flux calibrated spectra will be available in digital format to the community ; ( @xmath8 ) the foreseeable spectral dispersion ( 0.75  / pix are currently baselined ) is significantly higher that those of other whole - sky surveys .    a review of gaia spectroscopy has already been presented by munari ( 1999a , hereafter m99a ) . we will consider here mainly updates to the content of m99a reflecting advancements in some areas over the last couple of years . therefore , to cope with the generous but limited amount of space available to this review in its printed format , basic physics and overall considerations developed in m99a will not be discussed here again . technical aspects connected to spacecraft optical and mechanical assembly , telemetry budgets , modus operandi , limiting magnitudes etc . , are covered in the esa s _ gaia concept and technology study report _ ( esa - sci-2000 - 4 ) and in abridged format in perryman et al . ( 2001 ) . gaia spectra will cover the 8490 - 8750   wavelength range , centered on the near - ir ca  ii triplet and head of the paschen series . the extention to 8750   ( the redder ca  ii line laying at 8662.141  ) allows observation of remarkable n  i # 1 and 8 multiplets in hot stars and particularly strong fe  i , mg  i and ti  i lines in cool stars . gaia wavelength range is the only spectral window in the near - ir which is not crunched by telluric absorptions ( cf . figure  1 of m99a ) , allowing uncontaminated ground - based preparatory and follow - up observations . ca  ii triplet is by far the strongest line feature in the red / near - ir spectra of cool stars ( cf . fig  1 of munari & castelli 2000 ; jaschek & jaschek 1995 ) , being core - saturated even in metal - poor halo stars , thus allowing derivation of radial velocities on epoch - spectra of even the faintest and more metal deprived gaia spectral targets . cool stars will vastly dominate among the latter ( with o and b stars barely traceable ) . gaia wavelength range ( restricted to @xmath9 by optical and telemetry constraints ) is placed toward peak emission in cool stars and lower interstellar extinction , with obvious benefits for the number of detectable targets . the number of gaia spectral targets ( approaching 10@xmath10 in current estimates ) , will require fully automatic data treatment and analysis . line intensities and ratios may be useful in pre - flight ground - based preparatory work and/or quick - look classification and re - direction along the reduction pipeline of actual gaia data . however , it is clear that proper exploitation of gaia spectra will required a smart spectral synthesis approach . even if currently available synthetic models of stellar atmosphere ( marcs , atlas , phoenix , etc . ) and nebular regions ( cloudy , etc . ) will be probably quite improved by the time gaia data will be available ( and new families of models will probably be developed too ) , nevertheless they play a fundamental role right now in the current infancy of gaia spectroscopy , by offering precious guidelines , ways to improve basic underlying physics ( for ex . atomic constants ) and unlimited databases for simulations . most of gaia performances will depend on the eventually adopted spectral dispersion . an example of how lowering the resolution affects spectral appearance of a k0  iii star  a typical gaia target  is given in figure  1 . on one side the race for fainter limiting magnitudes and smallest demand on telemetry push for mid to low spectral dispersions . on the other side , getting the best astrophysical return and the highest accuracy of radial velocities most decidedly ask for high dispersions . the best compromise will have to balance between them .         in k on the left and corresponding spectral type for luminosity class v on the right ) for moderately metal poor stars ( [ z / z@xmath11=@xmath120.5 ) . all spectra are on the same ordinate scale , only displaced in their zero - points.,width=548 ]    the spectra presented in this review carry a 0.25   / pix dispersion , at the high end of the 0.25@xmath121.0   / pix range currently considered , thus allowing the reader to guess the highest possible gaia performancies . in the following , effective temperatures and surface gravities for mkk spectral types are adopted from strayzis ( 1992 ) . a temperature sequence spanning the mkk classification scheme is presented in figure  2 . m to f spectral types are governed by the ca  ii triplet , hotter ones by he  i , n  i and the hydrogen paschen series . a rich forest of metal lines populates the gaia wavelength range ( cf . figure  3 in m99a ) , which is dominated by ca  ii , fe  i , ti  i atomic lines and cn molecular transitions . relevant absorptions are also due to mg  i , si  i , cr  i , n  i , co  i , ni  i , mn  i , s  i as well as tio , with other elements and molecules contributing weaker spectral signatures . such a harvest makes spectral classification over the @xmath13250    gaia range nearly as much easy as it is for the @xmath141000    classical mkk range ( which extends from 3900 to 4900   ) . only o and b stars perform less good , which is however of no concern given their barely traceable fraction among gaia targets . diagnostic line ratios useful for spectral classification purposes can be easily derived on gaia spectra . two examples of line ratios are illustrated in figure  3 . near - ir ca  ii over paschen lines are highly effective in classifying late b , a and f stars , as it if for ca  ii h and k over balmer lines in optical spectra . ti 8674.7/fei 8675.4 works very well in g , k and m stars , with the following expression giving the fitting curve in figure  3 ( right panel ) : @xmath15 a @xmath164% error in the line ratio ( typical for the observations in figure  3 ) corresponds to just @xmath17 65 k. it worth to mention that also gaia photometry will ( obviously ) estimate the temperature of target stars ( however with contamination from interstellar reddening , if present ) , providing independent data to be compared with spectroscopic findings . figure  5 illustrates line behavior with luminosity class . with lowering surface gravity ( increasing luminosity , decreasing pressure ) the intensity of absorption lines goes up , as much as in classical optical spectra and required by physics . the equivalent width of ca  ii lines shows a pronounced positive luminosity effect ( collisional de - population of excited state less effective with decreasing pressure ) . width of paschen lines presents a negative luminosity effect ( being pressure broadened , similarly to balmer lines ) as given in figure  4 . it is again worth noticing that surface gravities will be also quite effectively measured by gaia combining astrometric distances with photometry ( cf . sect . 2.1 in munari 1999b , hereafter m99b ) . as for temperature / spectral type , it is easy to derive diagnostic line ratios highly sensitive to surface gravity / luminosity class . the steep behavior of ( si  i 8728.0 + fe  i 8729.1)/(mg  i 8736.0 ) in g5 stars is illustrated in figure  4 , with fitting curve given by :    @xmath18    a @xmath163% error ( typical for figure  4 data ) corresponds to just @xmath19=0.11        = 5000  k ( @xmath2g8 spectral type ) and [ z / z@xmath11=@xmath120.5 ( synthetic spectra from munari & castelli 2000 ) . _ center : _ gravity effects on the profile of paschen 14 , 8598 for @xmath20=9750  k ( @xmath2a0 spectral type ) and [ z / z@xmath11=@xmath120.5 ( synthetic spectra from zwitter et al . 2001 ) . _ right : _ dependence upon gravity of the ratio ( sii  8728.0 + fei  8729.1)/mgi  8736.0 ( boschi and munari 2001 ) on g5 real spectra ( from munari & tomasella 1999).,width=548 ]              the intensity of absorption lines obviously correlates with the chemical abundances . figure  6 presents spectra of two g5  iii stars of widely differing metallicities ( [ fe / h]=@xmath122.65 and @xmath120.02 ) .    even at the lowest metallicities ( like those found in the halo and globular clusters ) , ca  ii lines remain core - saturated while nearly all other metallic lines have gone , allowing accurate radial velocity measurements and fine rotational velocity estimates .    the possibility to perform chemical analysis on the recorded spectra will depend on the spectral dispersion that will be eventually adopted for gaia . figure  1 is illuminating in this sense . at 0.25  / pix hundreds of absorptions lines blossom over the whole gaia range , tracing tens of different elements which _ individual _ chemical abundances can be derived as routinely done with high resolution optical spectra secured with ground - based telescopes . at 0.5  / pix , lines from different elements merge into blends and individual chemical abundances can be derived ( with a limited accuracy ) probably only for fe  i , ti  i and mg  i ( tests are underway ) . at even lower dispersions , chemical analysis looks hopeless , with absorption lines washing into an unfeatured continuum . it is also worth to mention that gaia narrow band photometry ( m99b ; v.vansevicius , m.grenon , these proceedings ) should estimate metallicity ( a weighted average of individual chemical abundances ) from color indexes with a sensivity comparable to best performing , existing ground - based photometric systems ( moro & munari 2000 ) . projected rotational velocity ( @xmath21 ) is another intrinsic property of stars that gaia can measure with relevant confidence , provided the spectral dispersion is high enough . the impact of axial rotation on stellar evolution is only recently being appreciated and modeled ( cf . a.maeder , this volume ; maeder & meynet 2001 and references therein ) . a sequence of rotationally broadened f0  iii spectra is presented in figure  7 . ca  ii and paschen lines perform equally well up to very high @xmath22 , offering bright prospects for measuring rotational velocities both in cool and hot stars , respectively . other , weaker lines are useless at @xmath2340  km  sec@xmath0 , being washed out in the adjacent continuum . rotational velocities of g - k - m stars ( constituting the largest fraction of gaia targets ) are usually confined to @xmath2415  km  sec@xmath0 ( glebocki et al . 2000 ) . to effectively discriminate at so low values , a properly high spectral resolution is necessary . 0.5  / pix spectra would not allow gaia to distinguish @xmath2515  km  sec@xmath0 stars from non - rotating ones , i.e. rotational velocities will be undetermined for the majority of gaia targets . instead , 0.25  / pix spectra can detect and measure rotational velocities to an accuracy of @xmath265  km  sec@xmath0 , thus providing decent sensitivity to rotation for _ all _ gaia targets . the rotational broadening of narrow lines may be expressed in term of the total width at half intensity ( in  ) as @xmath27 munari et al . ( 2001a , hereafter m01a ) have investigated in detail gaia radial velocity performances as function of spectral resolution and signal - to - noise ratio , by obtaining 782 real spectra and using them as inputs for 6700 automatic cross - correlation runs . m01a have explored the dispersions 0.25 , 0.5 , 1 and 2  / pix ( bracketing the 0.75  / pix currently baselined ) over s / n ranging from 12 to 110 , carefully maintaining the condition fwhm ( psf ) = 2 pixels . m01a have investigated late - f to early - m stars ( constituting the vast majority of gaia targets ) , slowly rotating ( @xmath28 = 4 km  sec@xmath0 , as for field stars at these spectral types ) , of solar metallicity ( @xmath29[fe / h]@xmath30 = @xmath120.07 ) and not binary ( target stars selected among iau standard radial velocity stars ) . the results are accurately described by : @xmath31 where @xmath32 is the cross - correlation standard error ( in km sec@xmath0 ) and @xmath33 is the spectral dispersion ( in  / pix ) . inserting 0.25 , 0.5 , 1 , 2 for @xmath33 in the above expression gives the four fitting curves in figure  8 . the spectral dispersion is the dominant factor governing the accuracy of radial velocities , with s / n being less important . spectral mis - match affects the results only at the highest s / n . m01a findings suggests that mission - averaged gaia radial velocities on non - variable , single stars can match the @xmath20.5 km  sec@xmath0 mean accuracy of tangential motions at @xmath3 mag , provided the observations are performed at a dispersion not less than 0.5  / pix . binary and/or fast rotating and/or pulsating and/or surface spotted stars will require higher accuracies ( thus higher spectral dispersions ) in order to disentangle perturbing effects from baricentric motion .    the flattening of rv performances at the highest s / n in figure  8 and eq.(4.1 ) results from the spectral type mismatch between template and program stars in m01a study ( while metallicities and rotational velocities were instead pretty similar and therefore not influent ) . the large but however limited number of real spectra ( 782 ) and the extreme paucity of iau rv standards sharing the same spectral classification , forced spectral type mismatch in m01a investigation . extensive simulations on huge numbers of synthetic spectra have been performed by zwitter ( 2001 ) to get rid of mismatches unavoidable with real spectra . his results confirm eq . ( 4.1 ) and figure  8 behavior at low and mid s / n . as expected , they do not show the upward curvature at the highest s / n induced in figure  8 by the spectral mismatch ( see also d. katz , these proceedings , for additional results on gaia radial velocities ) . m01a and zwitter ( 2001 ) investigations argue in favor of increasing the spectral dispersion to get more accurate radial velocities , for a fixed photon budget ( like in gaia fixed exposure time observations ) . from the results with real spectra of figure  8 , with current parameters for gaia spectrograph , a 1 km  sec@xmath0 accuracy on epoch rvs of a k0 star is reached at @xmath113.0 mag with 0.25   / pix spectra , at @xmath110.3 mag for 0.5   / pix , and @xmath16.7 mag for 1   / pix dispersion . the reason for this is quite obvious looking at figure  1 : at high dispersions _ all _ pixels carry rv information , even at low s / n , while at low dispersion only _ a few _ pixels carry rv information , no matter how high is the s / n . photometry and model fitting . _ lower right panel : _ orbital and physical parameters as derived from modeling of radial velocity and photometric data.,width=548 ]      eclipsing stars represent the most astrophysically relevant type of binary stars gaia will spectroscopically observe during its mission . they are a prime tool to derive fundamental stellar parameters like mass and radius , or the temperature scale . moreover , their use as accurate , geometric distance indicators is rapidly growing ( current best distances to lmc , dwarf spheroidals , globular clusters ) . their study is by no means a simple task as evidenced by the fact that stellar parameters have been derived with an accuracy of 1% or better for less than a hundred objects . scaling the hipparcos results ( 0.8% of the 118218 stars surveyed turned out to be eclipsing ) , @xmath34 of all @xmath35 mag gaia targets should be eclipsing binaries , with a average g7 spectral type . it may be estimated that about 25% of the eclipsing binaries will be double - lined in gaia spectral observations ( cf . carquillat et al . 1982 ) , thus @xmath36 of all @xmath35 mag gaia targets . even if for only 5% of them it should be possible to derive orbits and stellar parameters at 1% precision , this still would be @xmath37 what humanity have so far collected from devoted ground - based efforts during the whole last century ( cf . andersen 1991 ) . that a 1% accuracy in orbital and physical parameters of eclipsing binaries is feasible within gaia has been demonstrated by munari et al . ( 2001b , hereafter m01b ) . 0.25  / pix spectra over the gaia range have been secured from the ground for 15 eclipsing binaries ( mostly unstudied in literature and distributed among detached , semidetached , contact and intrinsic variable types ) and combined with hipparcos @xmath38 , @xmath39 , @xmath40 photometry to properly simulate gaia data harvest . example results from m01b for v505  per are given in figure  9 . semi - major axis , masses , surface gravities , effective temperatures , inclination , eccentricity , baricentric velocity , epoch and orbital period are in the 1% or better accuracy regime . radii of individual components ( and therefore individual bolometric magnitudes ) however depends on how well the branches of eclipses are mapped . in the case of hipparcos observations of v505  per _ only three points _ cover the principal eclipse . even if model solution can fit these three points to a formal accuracy of @xmath21% ( cf . lower - right panel of figure  9 ) , nevertheless the massive undersampling casts many doubts on the true accuracy of derived radii . in fact , high quality and massive ground - based observations of v505  per by marschall et al . ( 1997 ) fully confirm gaia - like solution of v505  per as reported in figure  9 for all orbital and physical parameters but individual radii . if the sum of them is r@xmath41 + r@xmath42 = 2.54 r@xmath43 for the gaia - like solution and 2.55 for marschall et al . , the individual values are 1.29 and 1.26 for the latter and 1.40 and 1.14 r@xmath43 for m01b . numerical experiments however show that by just doubling the number of points covering the principal eclipses in the hipparcos lightcurve would fix the result to a much higher degree of confidence and close to the marschall et al . findings . it is therefore of great relevance to the broadest stellar astrophysics that a minimum number of _ identical _ photometric bands is replicated in all the three viewing fields of gaia , so to achieve maximum density of points in lightcurve mapping of eclipsing binaries ( with obvious benefit to all other types of variable objects observed by gaia ) . the high quality of the results on v505  per shown in figure  9 ( typical of other cases investigated in m01b ) is however definitively depending upon the adopted 0.25 / pix spectral dispersion . at such a dispersion , lines from both components are easily resolved and measured , while at the coarser 0.75  / pix dispersion currently baselined for gaia the lines would merge into unresolved profiles that could even hide the binary nature of the object . radial velocity mapping of pulsating stars add precious information to the effort of understanding their nature ( cf . bono et al . non - radial pulsators manifest marked variations of line profiles that can be observed and modeled if the spectral resolution is high enough . spectra of template radially pulsating stars ( rr  lyr , @xmath44  cep , @xmath45  cet ) are presented in m99a . the wealth of strong absorption lines , in particular ca  ii , assures that accurate radial velocities can be obtained at even the lowest metallicities ( rr  lyr itself has [ z / z@xmath11=@xmath121.37 ) . pulsation curves of representative cases are given in figure  10 . shapes and details of such curves ( e.g. the glitch in w  crt at phase 0.67 , or bg  lac at phase 0.52 ) , tell a lot about interior physics of these stars . it may be easily anticipated that gaia spectroscopic monitoring of thousands of them ( compared to the very few cases investigated from the ground ) will loudly impact on our understanding of stellar radial pulsations , provided the accuracy of epoch radial velocities will be good enough . semi - amplitudes for lyrids are of the order of @xmath46rv@xmath240 km  sec@xmath0 , @xmath46rv@xmath215 for cepheids and @xmath46rv@xmath27 for miras . so low amplitudes need high spectral resolution to be properly mapped : at 0.25  / pix even glitches in the pulsation curves will be detected , while at 0.75  / pix the pulsation curves of the majority of miras will be unobservable . the 8480@xmath128750   gaia wavelength range offers fine detection capabilities and diagnostic potential toward stellar peculiarities . spectra of a few representative peculiar stars are presented in figure  12 ( for other types and examples see m99a ) and briefly commented hereafter . be stars show strong and variable paschen and he  i emission lines , which profile trace conditions within the circumstellar disk . hot mass - losing stars ( p - cyg type ) present paschen and he  i lines with the characteristic emission / absorption profile that can be modeled to derive the mass - loss rate . very wide and bright ca  ii profiles ( and usually weaker paschen ) trace the fast expansion of novae ejecta ( fwhm=1280 km  sec@xmath0 in nova cyg 2001 # 1 ) , with sub - structures correlated to and tracing the dishomogeneities and clumps in the dispersing material . either hot and cool pre - zams objects well perform over the gaia range . herbig ae / be stars display both caii and paschen lines in strong emission , usually emerging from an absorption core . in cooler pre - zams objects ca  ii lines play the game , from the exceptional intensity in t  tau ( cf . the spectrum in m99a ) to the weak emission components in fu  ori . active atmosphere / spot stars reveal their nature by a complex cool absorption spectrum and structured emission profiles . major bright and active areas over the stellar surface correlates with individual , radial velocity displaced ( by axial rotation ) components in the spectrum , as in the by  dra spectrum in figure  12 where emission cores in ca  ii separated by 74 km  sec@xmath0 are paralleled by fe  i double absorption lines separated by an identical 74 km  sec@xmath0 . spectra of interacting binary stars offer a fine and intriguing display over the gaia spectral range . symbiotic stars usually present strong paschen , ca  ii and weaker he  i emission lines , while in some other case the naked absorption spectrum of the cool giant can be observed without contamination from the hot companion . the same ca  ii , paschen and he  i lines plus strong n  i ( multiplet # 1 and # 8) dominate spectra of the x - ray transient xte j0421 + 560 in figure  12 , with complex line profiles indicating kinematically decoupled emitting regions . wide , strong and multi - component , orbital - phase variable ca  ii profiles stand out in the spectra of the recurrent nova rs  oph .    finally , the interstellar medium can manifest itself in gaia spectra . figure  11 show the spectra of three similar hot stars affected by different amount of reddening , where absorption by the diffuse interstellar band at 8620   clearly stands out . the equivalent width of the latter correlates surprisingly well with reddening , offering interesting opportunities for gaia diagnostic of the interstellar medium ( munari 2000 ) : @xmath47 efforts are currently underway ( cf . moro and zwitter 1999 ) to investigate the dependence of the slope coefficient ( 2.69 ) upon the intrinsic and tri - dimensional properties of the galactic interstellar medium ( @xmath48 ) . 99 agnolin p. 2000 , degree thesis , dep . of astronomy , univ . of padova andersen j. 1991 , a&a review 3 , 91 bono g. , caputo f. , cassisi s. , castellani , v. , marconi m. 1997 , apj 477 , 346 boschi f. 2000 , degree thesis , dep . of physics , univ . of milan boschi f. , munari u. 2001 , aj , to be submitted carquillat j.m . , nadal r. , ginestet n. , pedoussaut a. 1982 , a&a 115 , 23 castelli f. , munari u. , 2001 , a&a 366 , 1003 glebocki r. , gnacinski p. , stawikowski a. 2000 , acta astron . 50 , 509 jaschek c. , jaschek m. 1995 , the behaviour of chemical elements in stars , cambridge uiv . press maeder a. , meynet g. 2001 , a&a 373 , 555 marrese p.m. 2000 , degree thesis , dep . of astronomy , univ . of padova marschall , l.a . , stefanik , r.p . , lacy , c.h . , torres , g. , williams , d.b . , and agerer , f. 1997 , aj 114 , 793 moro d. , zwitter t. 2000 , in proc . of  molecules in space and in the laboratory " , i.porceddu and s.aiello ed.s , soc . 171 moro d. , munari u. 2000 , a&as 147 , 361 munari u. 1999a , in proc . of gaia " esa - estec workshop , v.strayzis ed . 8 , 73 ( * m99a * ) munari u. 1999b , in proc . of  gaia " esa - estec workshop , v.strayzis ed . , astron . 8 , 123 ( * m99b * ) munari u. 2000 , in proc . of  molecules in space and in the laboratory " , i.porceddu and s.aiello ed.s , soc . it . 67 , pag . 179 ( astro - ph/0010271 ) munari u. , tomasella l. , 1999 , a&as 137 , 521 munari u. , castelli f. , 2000 , a&as 141 , 141 munari u. , agnolin p. , tomasella l. , 2001a , balt.astron . , vol . 10 , november 2001 issue , in press ( astro - ph/0105167 , * m01a * ) munari u. , tomov t. , zwitter t. , milone e.f . , kallrath j. , marrese p.m. , boschi f. , pra a. , tomasella l. , moro d. 2001b , a&a , in press ( astro - ph/0105121 , * m01b * ) munari u. , tomasella l. , marrese p.m. , boschi f. , zwitter t. , moro d. , 2001c , a&a , to be submitted perryman , m.a.c . , de boer , k.s . , gilmore , g. , hoeg , e. , lattanzi , m.g . , lindegren , l. , luri , x. , mignard , f. , pace , o. , de zeeuw , p.t . 2001 , a&a 369 , 339 straiys v. 1992 , multicolor stellar photometry , pachart pub . house , tucson zwitter t. 2001 , a&a , to be submitted zwitter t , munari u. , castelli f. , 2001 , a&a , in preparation
gaia spectroscopic and radial velocity performancies are reviewed on the base of ground - based test observations and simulations . 
 the prospects for accurate analysis of stellar atmospheres ( temperature , gravity , chemical abundances , rotation , peculiarities ) and precise radial velocities ( single stars , binaries , pulsating stars ) are colorful provided the spectral dispersion is high enough . 
 a higher dispersions also favors a given precision of radial velocities to be reached at fainter magnitudes : for example , with current parameters for gaia spectrograph , a 1 km  sec@xmath0 accuracy on epoch rvs of a k0 star is reached at @xmath113.0 mag with 0.25   / pix dispersion spectra , at @xmath110.3 mag for 0.5   / pix , and @xmath16.7 mag for 1   / pix . 
 gaia radial velocities for single stars can match the @xmath20.5 km  sec@xmath0 mean accuracy of tangential motions at @xmath3 mag , provided the observations are performed at a dispersion not less than 0.5  / pix .