1955AJ 60 . . 247B 1955 August stars werecountedandthesurfacebrightnessesmeasuredphotoelectrically.Thecount-brightnessratiothusobtained was foundtobesignificantlyhigherforthecompanionthanAndromedaGalaxy.Thecount-brightnessratio the companionagreeswiththatforglobularclusters,whereasratioAndromedaGalaxy the solarneighborhood.ThisindicatesthatbulkoflightAndromedaGalaxyiscontributednotbyextreme galactic systemsisofobviouscosmologicalinter- Population IIstarsbutlikelybyoldIstars. est. Thereis,however,verylittlerelatedobser- vational material.Cplor-magnitudediagrams down tozeroabsolutemagnitudeorfaintercan be obtainedwithpresentinstrumentsforonly the nearestsystemssuchasMagellanic able characteristicsmaybementioned.Oneis us informationonthosetypesofstarswhich the spectralenergydistribution,whichcangive Clouds. Formoredistantsystems,threeobserv- provide mostofthelightsystem.Asecond ratio, whichcangiveuscluesregardingthose observable characteristicisthemass-luminosity system. Athirdobservablecharacteristic,the faint starswhichprovidemostofthemass paper. count-brightness ratio,istobediscussedinthis definite limit,dividedbythebrightnessof absolute magnitude.Ifthesysteminquestionis system. Thebrightnessismeasuredhereinterms the numberofresolvedstars,brighterthana reasonably homogeneousinstellarcontent,the chosen tobetheluminosityofastarzero of visualluminosities,andtheluminosityunitis ously donotneedtocovertheentiresystembut counts andthebrightnessmeasurementsobvi- may berestrictedtoselectedfieldsinthesystem. lations. FigureIshowstheluminosityfunction it isquitesensitivetodifferencesinstellarpopu- sis ofthesolarneighborhood(Bok1937)and for PopulationIaccordingtovanRhijn’sanaly- luminosity functionofPopulationIIaccording cluster Messier3.Inthisfigurethetwofunc- to Bandage’s(1954)analysisoftheglobular Suppose, forexample,thatwecanresolveand vertical lineinFigure1;then clearlyaPopula- nitude ;thatis,allstarsattheleftofdashed count allstarsbrighterthan—2absolutemag- tions arenormalizedtoequaltotalbrightness. tion IIsystemwillgiveus 2to3timesmore Abstract. ForselectedfieldsintheAndromedaGalaxy,M31,anditsellipticalcompanion,NGC205,resolved I. Introduction.Thestellarcontentofextra- The count-brightnessratioisdefinedhereas The count-brightnessratioisuseful,because © American Astronomical Society • Provided by theNASA Astrophysics Data System A COMPARISONOFSTELLARPOPULATIONSINTHEANDROMEDA GALAXY ANDITSELLIPTICALCOMPANION THE ASTRONOMICALJOURNAL By W.A*BAUMANDM.SCHWARZSCHILD counted starsforthesametotalluminositythan a PopulationIsystem. will containandexceedthecount-brightness narrow rangeofusefulapplicability.Forthe ratio ininformation.Ontheotherhand,for nearest systems,thecolor-magnitudediagram systems outsideourlocalgroup,starcountssuit- able forthepresentpurposearebeyondreach of availableinstruments.However,withinthis comparing thestellarcontentofmainbody elliptical companion,NGC205.Towardthis narrow rangeofapplicabilityliestheproblem of theAndromedaGalaxy,M31,withthatits problem thepresentworkisdirected. are identifiedinFigure2.Themostinformative companion. of thesearefield3forM31and5the the starcountsandbrightnessmeasurements density andsurfacebrightnessbutsufficiently sufficiently closeintoobtainareasonablestar field. Inordertominimizeanyeffectsdue bility ofseveralbackgroundgalaxieswithinthis absence ofnoticeablecloudsandfromthevisi- freedom fromobscurationwasjudgedthe systematic countingerrors,field5wasselected far outtoavoidappreciableobscuration.The sity wasofthesameorderasthatinfield3.. in aregionofthecompanionwherestarden- As acheck,field4,alsoonthesameplate,was Since thesetwoareaspresumablybothrepresent chosen torepresentamixtureoftheAndromeda that bothcouldbecountedonthesameplate. Field 5wasalsosufficientlyclosetofield3so Galaxy anditscompanion. eda Galaxy,onewouldexpectthemtogiveequal the populationofmainbodyAndrom- ness, anylackofagreement betweenthemshould however, instardensityand insurfacebright- count-brightness ratios.Sincetheydiffergreatly, The count-brightnessratiowilllikelyhavea II. Locationoffields.Sevenfieldsselectedfor Field 3wasselectedinthemainbodyofM31,. A furthercheckwasprovidedbyfields1and2. 247 1955AJ 60 . . 247B © American Astronomical Society •Provided bythe NASAAstrophysics Data System 248 THEASRONMICLJU60,No.1230 stellar populations.PopulationIisrepresentedbythe solar neighborhoodandPopulationIIbyMessier3.The two functionsarenormalizedtoequaltotalbrightness. Note thattheordinateislogarithmic. critically showupsystematicerrorsinthemeas- urements. Anothercheckofexactlythesame identify areaswithinwhichstars werecountedandsurfacebrightnessessampled.Thethree letteredcrossesidentify sky patcheswhichservedasthe “zero”referenceforsurfacebrightnesses. Figure I.Comparisonofluminosityfunctionsfortwo Figure 2.ThegalaxyinAndromeda, M31,anditscompanion,NGC205(upperright).The sevennumberedboxes -2 0+2Myis type isprovidedbyfields6and7»bothofwhich fall intheellipticalcompanion. three plates,alltakenwiththe200-inchHale telescope, throughthe//ß.byRosscoma-correc- tor, onEastmanI03a-DplateswithaGG-11 filter. Thefirstplate(PH-35-Bm),whichcovered covered fields3,4,and5,wastakenbyDr. seeing 2+.Thesecondplate(PH-834-B),which fields iand2,wasexposedsixtyminuteswith 65 minuteswithseeing4-5(occasionallythrough thin haze?),anditwascenteredsothatfields3 Baade especiallyforthiswork.Itwasexposed differential opticaleffectsinthecomparisonof and 5layatapproximatelyequaldistancesfrom the opticalaxisinordertoavoidanypossible erosity intakingthisspecialplatewhichmade these twocriticalfields.ItwasDr.Baade’sgen- the presentworkpossible.Thethirdplate(PH- III. Starcounts.Thecountsweremadeon 1955AJ 60 . . 247B here. and transparencyareofsecondaryimportance minutes withseeing4-5.Aswillbeevidentfrom taken byBaadepreviously.Itwasexposed75 670-B), whichcoveredfields6and7,hadbeen the discussionwhichfollows,differencesinseeing on theplateor12,300squaresecondsofarc,with selected, andallstarswhichappearedbrighter side. Onthestep-scaleanimagenearbutcom- special microscopeinwhichaportionoftheplate larger. the exceptionoffield1whichwasfourtimes 1955 August and animagestep-scalecouldbeviewedsideby exhibit appreciabledifferencesfromobserverto servers, twoofthemcountingallfieldsandthe stars brighterthanthisimagewereexcludedfrom quarter magnitudesbrighterthanthelimiting count. Anotherimage,approximatelyoneanda than thislimitingimagewereincludedinthe fortably abovethethresholdofplatewas This wasdoneforthesecondplate,whichcov- critical fields.Countsonthesameplateeven lished foronlythesecondplatecontaining since thislimitingmagnitudehasbeenestab- directly compared,sincethelimitingmagnitude field I. field 2,secondhalfof sequence was:firsthalfoffield1, each observerfollowedasymmetricpatternin counts weredoneindependentlybythreeob- the countssoastoeliminatemajorityof counting variouspartsofthefieldsoneachplate; second plate.Tominimizesystematicerrors, thus excluded,however,wasverysmall.The foreground stars.Thenumberofbrighterstars image, wasalsoselectedonthestep-scale,and ever, thatthethreeobserversareinfairlygood very similarplateofanother areainwhicha only abouttenpercent. agreement ontheratiosbetweenfieldscounted observer. ThelastsectionofTableIshows,how- is notpreciselythesameonallthreeplatesand for example,onthefirstplatecounting third countingonlythecriticalfieldson ered thecriticalfields,by comparingitwitha also theirabsolutevaluesuseful,itwasnecessary on thesameplate.Thesedatainfactsuggest Counts madeondifferentplatesshouldnotbe to derivethelimitingmagnitudeofcounts. that theuncertaintiesofmeanratiosare The countsweremadewiththehelpofa All fieldscountedwereonesquarecentimeter The resultsofthecountsarelistedinTableI. To makenotonlytheratiosofcountsbut © American Astronomical Society • Provided by theNASA Astrophysics Data System THE ASTRONOMICALJOURNAL TABLE I.STARCOUNTSEXPRESSEDINNUMBERSOFSTARS counts. scale offaintapparentmagnitudeshadprevi- the tabulatednumbersforitareonequarterofactual ously beenestablishedbyphotoelectricobserva- cepheids. Usingphotographictransfersfroma est starsinglobularclusters,whichsupposedly elliptical companion,NGC205,wereestimated photo visualmagnitude,withanestimatedun- on thesecondplatewasfoundtobe22.2apparent tions (Baum1954).Thus,thelimitofcounts a modulus24.2isobtained. have anabsolutephotovisualmagnitudeof—3.0, 21.2. Ifthesestarsareidentifiedwiththebright- present investigation,thebrighteststarsin axy isneeded.Fromtheplatesusedin magnitude, themodulusofAndromedaGal- certainty of±0.2magnitude.Thethreshold of 1.4magnitudesinthezeropointcepheid faint photoelectricsequenceinSelectedArea68 to haveanapparentphotovisualmagnitudeof the platewasaround23rdmagnitude. mate isbasedultimatelyupon thezeropointof period-luminosity relation(Baade1952;Blaauw Baade’s latestestimatebasedontheclassical modulus issignificantinitself.Sinceoneesti- modulus ofM31.WewishtothankDr.Baade he nowalsogetsapproximately24.2forthe and Morgan1954),Dr.Baadeinformsusthat available. for hiskindnessinmakingthisinformation the cluster-typevariableswhile theotherisbased (Baum, unpublished)andassumingacorrection 2 2^-1 2.93.6—3.3 6- t -7 2.21.8—1.9 3 -r-41.51.21.3 5- ^3i-i1-31-21.2 * Sincefield1isfourtimeslargerthantheotherfields, Field ABCMean This modulusisinexcellentagreementwith To convertthiscountinglimitintoanabsolute The agreementbetweenthetwoestimatesof PER 12,300SQ.SEC.OFARCORICMONTHEPLATE 4 I??192139169 7 85106—96 6 183187—185 2 145214—180 5 285299209264 3 269229177225 1 50*59*—54* Counts betweenM31andNGC205 Counts westofNGC205 Counts eastofM31 Ratios ofcounts Observer 249 1955AJ 60 . . 247B zero points(Aikfg^1.4)issupportedhere.This established scales. confirmation carriesnewweight,becauseitnow presently acceptedrelationshipbetweenthese upon thezeropointofclassicalcepheids, rests inbothdirectionsuponphotoelectrically 250 of 22.2correspondstoanabsolutephotovisual counts listedinTableIrepresenttheinterval magnitude of—2.0it0.2.Thismeansthatthe its companion,wenotethatthecountinglimit avoided accordingly. employs anEmitron6685fourteen-stageend-on between M=—2.0and—3.3. measurement offaintobjects(Baum1955).It a photon-countingphotometerdesignedforthe at theprimefocusof200-inchtelescopeusing foreground starsandbackgroundgalaxiesbeing patches werechosentoberepresentativeofthe circular patches28"indiameterwereselected of thesevenfieldsnumberedinFigure2,two shaper, apulseamplifier,pulse-heightdiscrimi- photomultiplier tube,apre-amplifier,pulse M 31andNGC205populationsineachfield, for samplingthesurfacebrightnesses.These distances laterallyinthefocalplane,itcanbe current useareBG-12(1mm)+GG-13(4 P nator, andathree-decadescaler.Colorfiltersin the photometerheadcanbemovedmeasured for blueandGG-11(2mm)yellow.Since seen visuallythroughaneyepiecebutwhichcan set onobjectsorareaswhicharetoofainttobe be locatedwithreferencetobrighterobjects star, thesettingcanthenbemaintainedduring the aidofaphotograph.Byuseguide pendently soastopermitcertainvariationsin a photoncountoflongduration.Theguide-scope metered witha28"focal-planediaphragmcen- brightnesses ofallselectedpatcheswerephoto- itself canalsobemovedmeasureddistancesinde- was determinedinadvancefromaphotograph ence starrelativetothecenterofaselectedpatch by settingthephotometerandtelescopein the offsettingprocedure. v eter headwasplacedatthesamexandydis- tered ontheopticalaxis.Thiswasaccomplished the followingway.Thex,ypositionofarefer- by movingthetelescope, therebyputtingthe star wascenteredinthefocal-plane diaphragm of thefield.Then,attelescope,photom- tances fromtheopticalaxis,andreference Accepting themodulus24.2forbothM31and The observationsweremadephotoelectrically IV. Surface-brightnessphotometry.Withineach For thepresentinvestigation,surface © American Astronomical Society • Provided by theNASA Astrophysics Data System THE ASTRONOMICALJOURNAL guide-scope andtheprecisecirclereadingsof moving theguide-scope.Thecoordinatesof star waslocatedandcenteredonthereticleby selected patchontheopticalaxis.Next,aguide optical axis,andphotoncountswereobtained scope settingweredeterminedforasecondpatch. cedure, theguide-scopecoordinatesandtele- Among thefourteenpatchesinsevennum- dinate orientationbetweenthephotometerbase between theirrespectivesettings.Actually,the alternately onthetwopatchesbyshuttlingboth telescope werethennoted.Bythesamepro- counts, fourononepatchandtheother, patches involvedaseriesofeightalternating steps toaccommodatesmalldifferencesincoor- the guide-scopeandtelescopebackforth Finally, thephotometerheadwasputon bered areas,plusthethreeletteredskyreference the durationofeachcountbeing100seconds. and theseveralplatesfromwhichvariouspatch offsetting procedureentailedsomeadditional be carriedoutinmediocreseeingduringwhich patches (Fig.2),32suchintercomparisonswere positions weredetermined. way toblueandyellowmagnitudesinthePo,V more criticalworkcouldnotbepursued. altogether consumedabout3clearnights,could made. Fortunately,theseobservations,which The averagebrightnessofthethreeletteredsky The relationshiptotheP,Vsystem(Johnson system (Johnson1952),whichisanultraviolet- surface brightnessbetweentwopatches,and these differenceswerereducedinaconventional derived accordinglyfromtheobservedmagni- ence andthetruesurfacebrightnessesof free approximationtotheInternationalSystem. patches (Fig.2)wasadoptedasazerorefer- and Morgan1953)ismerelyoneofdefinition: small fractionsofthetotallightreceivedby tude differences. fourteen patchesinM31andNGC205were photometer withinthepatchesselected;and,of M 31andNGC205themselvescontributedonly course, thedifferencesbetweenpatches,i.e., the quantitiesbeingmeasured,amountedtostill 20th magnitude“signals”observedagainsta less. Veryroughly,thesedifferenceswere17thto accuracy affordedbyphoton countingwasquite 15th magnitudebackground. Nevertheless,the For eachcolor,thisintercomparisonoftwo Each intercomparisonyieldedadifferencein Owing tothepresenceofnight-skyglow, (B —V)=0.918(P-V)+0.161. 0 60, No.1230 1955AJ 60 . . 247B which makestheunitoflequivalentto25th are definedby sponding luminositiespersquaresecondofarc square secondsofarc,thesedataarenumerically summarized inTableII.ValuesofVrepresent good ;theprobableerrorsareestimatedtobe somewhat redderthanStebbinsandWhitford’s magnitude. tudes ofthepatchesthemselves.Thecorre- ond ofarc.Sincetheareaeachpatchwas630 apparent photovisualmagnitudespersquaresec- at magnitudeV=25.00. skirts ofNGC205.Boththeseregionsare tematically verydifferentfromthoseintheout- of thepatchesinhaloM31arenotsys- 7 magnitudeslargerthanthemeasuredmagni- color indicesbeingtakenproportionaltotheluminosities. per squaresecondofarc. two withineachnumberedfieldofFigure2,are 1955 August ±0.03 unitoflinTableII. v +0.67) andbluerthantheir valueforthenucleus (1952) valueforthenucleus ofNGC205{C= v Field p A —+0.13 C —-0.05 B ——0.08 Values ofVrepresentapparentphotovisualmagnitudes The correspondingluminositiesUarebasedon=1.00 (Po —V)areweightedmeans,theweightsofindividual Surface brightnessesofthefourteenpatches, It isofinteresttonotethatthecolorindices © American Astronomical Society • Provided by theNASA Astrophysics Data System TABLE II.SURFACEBRIGHTNESSESANDCOLORS Comparison Patches(relativetoownmean) 24-39 24.83 23.78 23.81 23.76 23-99 24-45 23.90 23.03 22.78 23.64 23.33 24.74 23.71 Between M31andNGC205 log /„=10—0.4F, West ofNGC205 East ofM31 +0.75 +0.74 +0.74 +0.75 3.00 +0.75 +0.81 -I-0.90 +0.73 3.10 +0.70 +0.79 +0.81 1.66 +0.74 +0.80 +0.81 1.27 Po —V THE ASTRONOMICALJOURNAL 2-55 4.80 2-77 6.16 3.52 7-75 1.77 1.17 lv 30 15 +0.760 +O.769 +O.784 (Po-F) The secondcolumnofthistablegivestheabso- and ikf=—3.3. TABLE III.BRIGHTNESSES,COUNTS,ANDCOUNT-BRIGHTNESS values werederivedfrom the relativelvalues lute brightnessesLofthesevenfieldsinunits square secondofarcwithintheintervalbetweenM=—2.0 defies thattypeofinvestigation. similar colors;whiletheotherfouritemsin represent twodifferentstellarpopulations,have between CandP—Fcouldhardlyaccount per squaresecondofarc. TheseabsoluteL of astarzeroabsolutephotovisualmagnitude star ofzeroabsolutemagnitudepersquaresecondarc. and thosenearNGC205,whicharefoundto a statisticalreddeningof0.06escô,thenthe comparable norlinearlyrelatable.Ifweassume although, asJohnson(1952)hasemphasized, of M31{C=+0.98).Thesubtledistinction two precedingsectionsarecombinedinTableIII. region, whichwouldbethemostinformative, light onthis;unfortunately,however,thenucleus observations arebeingmade,mayshedsome list above,allnominallybelongingtoPopulation following intrinsiccolorsareestimated: these twocolorsystemsareneitherprecisely for theapparentcolordifferencesinthiscase, tude diagramofM31,forwhichphotoelectric It isratherstrikingthatthepatchesnearM31 II, donothaveacommoncolor.Acolor-magni- v v v v p0 v v A normalizedcountNofunitywouldmeanonestarper An absolutebrightnessLofunityisequivalenttoone v Field Lv100NL F. Resultsanddiscussion.Theresultsofthe v 4 1.271.371.08 3 3-31!-830.55 6 1.451.50Ï-03 5 1-532.141.40 2 1.981.460-74 7 0.700.78i.ii 1 0.700.440.63 Nucleus ofNGC205+0.50 Globular clustersinM31+0.52 Nucleus ofM31+0.81 E-type galaxies+0.80 Patches nearM31+0.61 Patches nearNGC205+0.59 RATIOS FORTHESEVENMEASUREDFIELDS Between M31andNGC205 West ofNGC205 East ofM31 100N 251 1955AJ 60 . . 247B The lastcolumnofTableIIIshowsasignificant which thecountswerebased.Onemayconclude ratio. Table IhavebeennormalizedsothatNrepre- in limitingmagnitudeforthethreeplateson can bemade,becauseoftheunknowndifferences further intercomparisonsbetweenthecheckfields holds truefortheresultsoffields6and7.No to referthesamestellarpopulation.The be expectedsincethesetwofieldswerechosen counts, equalcount-brightnessratios;thiswasto counts butgive,withintheuncertaintiesof sents thenumberofstarspersquaresecond eda Galaxyis24.2.InthethirdcolumnofTable serious systematicerrorsneednotbefearedin the programhavebeensatisfactoryandthat that thethreeobservationalchecksincludedin of thestellarpopulationsfields3and5.Fields since field4waschosentorepresentamixture between thesetwofields;thiswastobeexpected and 5,hasacount-brightnessratiointermediate lower brightnessandcountthanfields3 five checkfields.Field4,inspiteofhavingboth arc. Thelastcolumngivesthecount-brightness while themodulusacceptedhereforAndrom- 2.1. Thelatterfactorarisesfromthecircum- by field5.Itconfirmsanearlierconclusion ratios giveninboldtype,amountingtoafactor difference betweenthetwocount-brightness lows fromadirectcomparisonoffields3and5. the finalresult. fields 3and5,wemayconsidertheresultsfor a starofapparentphotovisualmagnitude25.0 stance thatthelvaluesaremeasuredinunitsof ments foreachfieldanddividingtheaverageby given inTableIIbyaveragingthetwomeasure- between themass-luminosityratioofAn- of 2.5.Thisindicatesanessentialdifferencebe- and thatoftheellipticalcompanionrepresented the AndromedaGalaxyrepresentedbyfield3 tween thestellarpopulationofmainbody 252 dromeda Galaxyandthatofellipticalgalaxies. the count-brightnessratiosobservedherewith III themeancountednumbersofstarsfrom count-brightness ratiocanbecomputedforany those computedfromknownluminosityfunc- cedure. First,integratethe luminosityfunction hypothetical countinglimit bythefollowingpro- tions. Fromanygivenluminosityfunctionthe I and2showbigdifferencesinbrightnesses v (Schwarzschild 1954)baseduponthedifference The firstmainresultoftheinvestigationfol- Before discussingtheresultsforcritical Further resultscanbeobtainedbycomparing © American Astronomical Society • Provided by theNASA Astrophysics Data System THE ASTRONOMICALJOURNAL Table IVasafunctionofthehypotheticallimit- borhood (Bok1937)andfromBandage’s(1954) computed brightnessZtt.inthesameunitas magnitude wasadoptedastheunithere. observed ratios,onemustofcourseexpressthe luminosity functionforMessier3arelistedin the observedbrightness;astarofzeroabsolute computed count-brightnessratioisthen: magnitudes toobtainthetotalbrightness.The stars whichcomposeitandintegrateoverall down tothislimitingmagnitudeobtainthe luminosity functionbytheluminositiesof number ofcountablestars.Second,multiplythe Rhijn’s luminosityfunctionforthesolarneigh- In ordertocomparethiscomputedratiowith shown inFigure3arethecount-brightnessratios mated uncertaintyofthelimitingmagnitude. ratios, whilethehorizontalbarsindicateesti- represented bythetwocurvesinFigure3.Also first pertainingtotheAndromedaGalaxyand measured hereforM31(field3)andNGC205 ing magnitudeofthecounts.Thesedataare uncertainty inthemeasuredcount-brightness eda Galaxyseemstoagreewiththatofthesolar brightness ratioofthemainbodyAndrom- the uncertaintiesofobservations,count- the secondtoellipticalcompanion.Within neighborhood. Thismaybeinterpretedinthe of theAndromedaGalaxywasdiscussedrecently same termsinwhichthemass-luminosityratio 0 defined ascomprisingall the commontypesof stars inthesolarneighborhood' witharandom -3.1 O.I4— -2.1 O.55I.32 — 2.3O.42O.81 — 2.7O.24O.15 — 2.9O.I9 -2.5 O.32O.39 (field 5).Theverticalbarsindicatetheestimated — I.7O.942.72 -I.9 O.72I.95 (Schwarzschild 1954).LetoldPopulationIbe Mv v.R.M3 TABLE IV.COUNT-BRIGHTNESSRATIOCOMPUTEDASA Count-brightness ratioscomputedfromvan Two resultsmaybereadfromFigure3,the FUNCTION OFTHELIMITINGMAGNITUDEFROM VAN RHIJN’SLUMINOSITYFUNCTIONAND Ltot. 100 N/Ltot. N FROM THATFORMESSIER3 -0.7 3.698.8 -0.5 4.910.5 — I.I2.125.9 — O.92.807.2 -1.3 i.614.70 —0.1 8.816.4 -0.3 6.512.8 -1.5 1.233.65 Mv v.R.M3 60, No.1230 100 N/Ltot. 1955AJ 60 . . 247B 1955 August Figure 3.Thecount-brightnessratioasafunctionofthe velocity dispersionoftheorder25km/sec, velocity dispersion.Accordingly,theluminosity but asexcludingtheveryyoungstarswithlow extreme PopulationIIintheproportionssug- extreme brightend,wherethelatterisdominated function, andhencethecount-brightnessratio, main bodyoftheAndromedaGalaxyiscom- by youngbluegiants.Ifoneassumesthatthe by thatofthesolarneighborhoodexceptat of oldPopulationIshouldbewellrepresented Andromeda Galaxyandthesolarneighborhood gested earlier(Schwarzschild1954),thenthe posed ofamixtureoldPopulationIand Andromeda Galaxyshouldbedominatedbyold brighter partoftheluminosityfunctionin that withintheuncertaintyofobservations ness ratios.Sinceitisknownthatthelargest NGC 205andMessier3haveequalcount-bright- able. Population I;hence,theagreementbetween in count-brightnessratioisdirectlyunderstand- The otherresulttobereadfromFigure3is © American Astronomical Society • Provided by theNASA Astrophysics Data System limiting magnitudeofthecount. THE ASTRONOMICALJOURNAL part ofthetotalbrightnessMessier3iscon- lute magnitude,theequalityincount-brightness ratio suggeststhatalsoinellipticalgalaxiessuch tributed bystarsbrighterthan,say,+2inabso- as NGC205theintrinsicallyfaintstarscontrib- ute littletothetotallight.Thisfurtherimplies must existinellipticalgalaxiestoaccountfor that theenormousnumberoffaintstarswhich contribute appreciablytothetotalbrightness. than +12inabsolutemagnitudesoasnotto their largerelativemasses,mustmostlybefainter eda Galaxyanditsellipticalcompaniondiffer .1955,SkyandTelescope14,264330. Andromeda Galaxyandthesolarneighborhood significantly incount-brightnessratio,indicating conclusions :(a)ThemainbodyoftheAndrom- Johnson, H.L.,andMorgan,W.1953,Ap.J.117,313. Johnson, H.L.1952,Ap.J.116,272. in count-brightnessratioindicatingthesimilarity of theAndromedaGalaxyisdominatedbyold the suppositionthatlightofmainbody agree incount-brightnessratio,substantiating an essentialdifferenceinstellarcontent,(b)The Stebbins, J.,andWhitford,A.E.1952,Ap.J.115,284. Schwarzschild, M.1954,A.J.59,273. brighter absolutemagnitudes. of theirrespectiveluminosityfunctionsforthe NGC 205andtheglobularclusterMessier3agree Population Istars,(c)Thedwarfellipticalgalaxy Sandage, A.R.1954,J.59,162. Blaauw, A.,andMorgan,H.R.1954,B.A.N.12,95. Baade, W.1952,Trans.I.A.U.8,397. Bok, B.J.1937,.DistributionofStarsinSpace(U. Baum, W.A.1954,J.59,422. Chicago Press),p.19. In summarizingwemaystatethefollowing Mount WilsonandPalomarObservatories, and PrincetonUniversityObservatory, REFERENCES Carnegie InstitutionofWashington, California InstituteofTechnology, Pasadena, Calif., Princeton, N.J., 1954 November. 253