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We compare mid-infrared (IR) 3.6 and 4.5 micron Warm Spitzer observations for Cepheids in the Milky Way and the Large and Small Magellanic Clouds. Using models, we explore in detail the effect of the CO rotation-vibration band-head at 4.6 micron on the mid-IR photometry. We confirm the temperature sensitivity of the CO band-head at 4.6 micron and find no evidence for an effect at 3.6 micron. We compare the ([3.6]-[4.5]) period-colour relations in the MW, LMC and SMC. The slopes of the period-colour relations for the three galaxies are in good agreement, but there is a trend in zero-point with metallicity, with the lowest metallicity Cepheids having redder mid-IR colours. Finally, we present a colour-[Fe/H] relation based on published spectroscopic metallicities. This empirical relation, calibrated to the metallicity system of Genovali et al., demonstrates that the ([3.6]-[4.5]) colour provides a reliable metallicity indicator for Cepheids, with a precision comparable to current spectroscopic determinations.

Using Spitzer observations of classical Cepheids we have measured the true average distance modulus of the Small Magellanic Cloud (SMC) to be18.96 +/- 0.01 stat +/- 0.03sys mag (corresponding to 62+/- 0.3kpc), which is 0.48 +/- 0.01 mag more distant than the LMC. This is in agreement with previous results from Cepheid observations, as well as with measurements from other indicators such as RR Lyrae stars and the tip of the red giant branch. Utilizing the properties of the mid-infrared Leavitt Law we measured precise distances to individual Cepheids in the SMC, and have confirmed that the galaxy is tilted and elongated such that its eastern side is up to20 kpc closer than its western side. This is in agreement with the results from red clump stars and dynamical simulations of the Magellanic Clouds and Stream.

Distance measures on a coherent scale around the sky are required to address the outstanding cosmological problemsof the Hubble constant and of departures from the mean cosmic flow. The correlation between galaxy luminositiesand rotation rates can be used to determine the distances to many thousands of galaxies in a wide range ofenvironments potentially out to 200 Mpc. Mid-infrared (3.6m) photometry with the Spitzer Space Telescope isparticularly valuable as a source of luminosities because it provides products of uniform quality across the sky.From a perch above the atmosphere, essentially the total magnitude of targets can be registered in exposures ofa few minutes. Extinction is minimal and the flux is dominated by the light from old stars, which is expected tocorrelate with the mass of the targets.In spite of the superior photometry, the correlation between mid-infrared luminosities and rotation rates extractedfrom neutral hydrogen profiles is slightly degraded from the correlation found with I-band luminosities. A colorcorrection recovers a correlation that provides comparable accuracy to that available at the I band (20 1in an individual distance) while retaining the advantages identified above. Without color correction, the relationbetween linewidth and [3.6] magnitudes is Mb,i,k,a [3.6] 20.34 9.74(logWimx 2.5). This description is foundwith a sample of 213 galaxies in 13 clusters that define the slope and 26 galaxies with Cepheid or tip of the red giant branch distances that define the zero point. A color-corrected parameter MC[3.6] is constructed that has reduced scatter: MC[3.6] 20.34 9.13(logWimx 2.5). Consideration of the seven calibration clusters beyond 50 Mpc, outside the domain of obvious peculiar velocities, provides a preliminary Hubble constant estimate of H0 74 4 km s1 Mpc1.

We have observed the dwarf galaxy IC 1613 at multiple epochs in the midinfrared using Spitzer and the in the near-infrared using the new FourStar near-IR camera on Magellan. We have constructed Cepheid period luminosity relations in the J, H, Ks, [3.6] and [4.5] bands and have used the run of their apparent distance moduli as a function of wavelength to derive the line of sight reddening and distance to IC 1613. Using a nineband fit, we find E(BV ) = 0.050.01 mag and an extinction corrected distance modulus of 0 = 24.29 0.03statistical 0.03systematic mag. By comparing our multiband and [3.6] distance moduli to results from the tip of the red giant branch and red clump distance indicators, we find that metallicity has no measurable effect on Cepheid distances at 3.6 m in the metallicity range 1.0 [Fe/H] 0.2, hence derivations of the Hubble constant at this wavelength require no correction for metallicity.

Using a mid-infrared calibration of the Cepheid distance scale based on recent observations at 3.6 micrometers with the Spitzer Space Telescope, we have obtained a new, high-accuracy calibration of the Hubble constant. We have established the mid-IR zero point of the Leavitt law (the Cepheid period-luminosity relation) using time-averaged 3.6 micrometers data for 10 high-metallicity, MilkyWay Cepheids having independently measured trigonometric parallaxes. We have adopted the slope of the PL relation using time-averaged 3.6micrometers data for 80 long-period Large Magellanic Cloud (LMC) Cepheids falling in the period range 0.8 < log(P) < 1.8.We find a new reddening-corrected distance to the LMC of 18.477 +/- 0.033 (systematic) mag. We re-examine the systematic uncertainties in H(sub 0), also taking into account new data over the past decade. In combination with the new Spitzer calibration, the systematic uncertainty in H(sub 0) over that obtained by the Hubble Space Telescope Key Project has decreased by over a factor of three. Applying the Spitzer calibration to the Key Project sample, we find a value of H(sub 0) = 74.3 with a systematic uncertainty of +/-2.1 (systematic) kilometers per second Mpc(sup −1), corresponding to a 2.8% systematic uncertainty in the Hubble constant. This result, in combination with WMAP7measurements of the cosmic microwave background anisotropies and assuming a flat universe, yields a value of the equation of state for dark energy, w(sub 0) = −1.09 +/- 0.10. Alternatively, relaxing the constraints on flatness and the numbers of relativistic species, and combining our results with those of WMAP7, Type Ia supernovae and baryon acoustic oscillations yield w(sub 0) = −1.08 +/- 0.10 and a value of N(sub eff) = 4.13 +/- 0.67, mildly consistent with the existence of a fourth neutrino species.

Due to an error at the publisher, an incorrect version of Table 3 appeared in the published article. The updated version of Table 3 is given.

We present an overview of and preliminary results from an ongoing comprehensive program that has a goal of determining the Hubble constant to a systematic accuracy of 2%. As part of this program, we are currently obtaining 3.6 micron data using the Infrared Array Camera (IRAC) on Spitzer, and the program is designed to include JWST in the future. We demonstrate that the mid-infrared period-luminosity relation for Cepheids at 3.6 microns is the most accurate means of measuring Cepheid distances to date. At 3.6 microns, it is possible to minimize the known remaining systematic uncertainties in the Cepheid extragalactic distance scale. We discuss the advantages of 3.6 micron observations in minimizing systematic effects in the Cepheid calibration of the Hubble constant including the absolute zero point, extinction corrections, and the effects of metallicity on the colors and magnitudes of Cepheids. We are undertaking three independent tests of the sensitivity of the mid-IR Cepheid Leavitt Law to metallicity, which when combined will allow a robust constraint on the effect. Finally, we are providing a new mid-IR Tully-Fisher relation for spiral galaxies.

The Carnegie Hubble Program (CHP) is designed to improve the extragalactic distance scale using data from the post-cryogenic era of Spitzer. The ultimate goal is a determination of the Hubble constant to an accuracy of 2%. This paper is the first in a series on the Cepheid population of the Large Magellanic Cloud, and focuses on the period-luminosity relations (Leavitt laws) that will be used, in conjunction with observations of Milky Way Cepheids, to set the slope and zero-point of the Cepheid distance scale in the mid-infrared. To this end, we have obtained uniformly-sampled light curves for 85 LMC Cepheids, having periods between 6 and 140 days. Period- luminosity and period-color relations are presented in the 3.6 micron and 4.5 micron bands. We demonstrate that the 3.6 micron band is a superb distance indicator. The cyclical variation of the [3.6]-[4.5] color has been measured for the first time. We attribute the amplitude and phase of the color curves to the dissociation and recombination of CO molecules in the Cepheid s atmosphere. The CO affects only the 4.5 micron flux making it a potential metallicity indicator.

Hipparcos parallaxes have recently become available for a sample of Galactic Cepheids. We have used these new distances to calibrate the Cepheid period-luminosity (PL) relation at six wavelengths (BVIJHK).

Public support and enthusiasm for astronomy have been strong in the final decades of the twentieth century. Nowhere is this better demonstrated than with the Hubble Space Telescope (HCT), a grand endeavor, which is enabling astronomers to make giant strides in understanding our universe, our place in it, and our relation to it. The NASAs first infrared observatory, the Space Infrared Telescope Facility (SIRTF), promises to take the crucial next steps towards understanding the formation of stars and galaxies. Toward their completion, the HST and Beyond Committee identifies major goals, whose accomplishment will justify a commitment well into the next century: (1) the detailed study of the birth and evolution of normal galaxies such as the Milky Way; (2) the detection of Earth-like planets around other stars and the search for evidence of life on them; (3) NASA should develop a space observatory of aperture 4m or larger, optimized for imaging and spectroscopy over the wavelength range 1-5 microns; and (4) NASA should develop the capability for space interferometry.

We report the detection of Cepheid Variable stars in the barred spiral galaxy NGC925, using the Hubble Space Telescope (HST) Wide Field and Planetary Camera 2 (WFPC2). Twelve V (F555W), four I (F814W) and three B (F439W) epochs of cosmic ray split observations were obtained. Eighty Cepheids were discovered, with periods from 6 to +-80 days. Light curves of the Cepheids are presented, and their corresponding period-luminosity diagrams are discussed.

We report the unambiguous detection of individually resolved stars in the elliptical galaxy NGC 3379, a luminous member of the Leo I Group.

We report the unambiguous detection of individually resolved stars in the elliptical galaxy NGC 3379, a luminous member of the Leo I Group.

Based on both empirical data for the nearby galaxies, and on computer simulations, we show that measuring the position of the tip of the first-ascent red-giant branch provides a means of obtaining the distances to nearby galaxies with a precision and accuracy comparable to using Cepheids and/or RR Lyrae variables. We present an analysis of synthetic I vs (V-I) color magnitude diagrams of Population 2 systems to investigate the use of the observed discontinuity in the I-band luminosity function as a primary distance indicator. In the simulations we quantify the effects (1) signal to noise, (2) crowding, (3) population size, and (4) non-giant-branch-star contamination, on the method adopted for detecting the discontinuity, measuring its luminosity, and estimating its uncertainity. We discuss sources of systematic error in the context of observable parameters, such as the signal-to-noise ratio and/or surface brightness. The simulations are then scaled to observed color-magnitude diagrams. It is concluded, that from the ground the tip of the red-giant-branch method can be sucessfully used to determine distances accurate to +/- 10% for galaxies out to 3 Mpc (mu approximately 27.5 mag); and from space a factor of four further in distance (mu approximately 30.6 mag) can be reached using HST. This method can be applied whereever a metal-poor population (-2.0 less than Z less than -0.7) of red-giant stars is detected (whose age is in the range 7-17 Gyr), whether that population resides in the halo of a spiral galaxy, the extended outer disk of a dwarf irregular, or in the outer periphery of an elliptical galaxy.

As part of both the Early Release Observations from the Hubble Space Telescope (HST) and the Key Project on the Extragalactic Distance Scale, we have obtained multiwavelength BVR Wide Field/Planetary Camera-2 (WFPC2) images for the face-on Virgo cluster spiral galaxy M100 = NGC 4321. We report here preliminary results from those observations, in the form of a color-magnitude diagram for approximately 11,500 stars down to V approximately 27 mag and a luminosity function for the brightest blue stars which is found to have a slope of 0.7, in excellent agreement with previous results obtained for significantly nearer galaxies. With the increased resolution now available using WFPC2, the number of galaxies in which we can directly measure Population I stars and thereby quantify the recent evolution, as well as test stellar evolution theory, has dramatically increased by at least a factor of 100. Finally, we find that the stars are present in M100 at the colors and luminosities expected for the brightest Cepheid variables in galaxies.

We have used UBV CCD photometry to determine the interstellar reddening toward 14 OB associations in the Magellanic Clouds. The two reddening-free indices available in the UBV system were used to obtain the reddening estimates. The mean color excesses of the associations range from E(B-V) = 0.01-0.26 mag in the Large Magellanic Cloud (LMC) and from E(B-V) = 0.06-0.25 mag in the Small Magellanic Cloud (SMC). We have modeled the observed scatter in the color excesses of individual stars within the associations and find that statisically significant differential reddening exists in at least some of the associations.

We have used UBV photometry of stars in 14 associations in the Large and Small Magellanic Clouds (LMC/SMS) (Hill, Madore, & Freedman) to derive luminosity and mass functions for the most massive stars. The main-sequence luminosity functions for the associations are quite similar, having an average slope of s = 0.03 +/- 0.06. The mass functions for the associations have slopes that span similar ranges about a common mean for both the LMC and SMC and there is no strong evidence for a significant variation in the slopes from one association to another. Accordingly, metal abundance does not appear to have a strong effect on the initial mass function (IMF), at least for the range in metallicity observed between the Magellanic Clouds. The average slope of the IMF for the Magellanic Cloud associations is Gamma = -2.0 +/- 0.5 for M greater than 9 Solar Masses. The range in the derived slopes is more likely due to the large uncertainties associated with the calculations of the mass functions, rather than to real variations in the IMF. There may be some evidence for a decrease in the slope of the IMF at masses below 9 Solar Masses, but incompleteness and the larger photometric errors associated with the faintest stars make this conclusion tentative.

The Extragalactic Distance Scale (H(sub o)) Key Project for Hubble Space Telescope (HST) aims to employ the Cepheid period-luminosity (P-L) relation to measure galaxy distances out as far as the Virgo Cluster. The vital steps in this program are (1) to obtain precise photometry of stellar images from the Wide Field Camera (WFC) exposures of selected galaxies, and (2) to calibrate this photometry to obtain reliable distances to these galaxies from the Cepheid P-L relation. We have used the DAOPHOT II and ALLFRAME programs to determine 28 instrumental magnitudes -- 22 of F555W (of about V) and six of F785LP (of about I) -- of all stars brighter than V of about 25 in each of two 2.56 arcmin x 2.56 arcmin WFC fields of M81. The reductions use a varying point-spread function to account for the field effects in the WFC optics and yield instrumental magnitudes with single epoch precision ranging from 0.09 to 0.24 mag, at V of about 21.8 to 23.8 -- the magnitude range of the 30 Cepheids that we have now identified in M81. For brighter stars (V of about 22), single epoch magnitudes are precise to 0.09 mag. The photometric calibration onto the Johnson V and Kron-Cousins I systems was determined from independent ground-based CCD observing at the Canada-France-Hawaii Telescope (CFHT) 3.6 m (confirmed by the Kitt Peak National Observatory (KPNO) 4.0 m) and from the Palomar 5.0 m (using the wide-field COSMIC camera) and 1.5 m telescopes. Secondary standards, taken from the COSMIC and CFHT frames, were established in each of the WFC fields in V and I, allowing a direct transformation from ALLFRAME magnitudes to calibrated V and I magnitudes, giving mean V of about 23 magnitudes accurate to of about +/- 0.1 mag. The stellar populations in M81 have been analyzed in terms of the luminosity functions and color magnitude diagrams (CMD) derived from these data, from which we identify numerous supergiants, and a CMD morphology similar to M33.

We report on the discovery of 30 new Cepheids in the nearby galaxy M81 based on observations using the Hubble Space Telescope (HST). The periods of these Cepheids lie in the range of 10-55 days, based on 18 independent epochs using the HST wide-band F555W filter. The HST F555W and F785LP data have been transformed to the Cousins standard V and I magnitude system using a ground-based calibration. Apparent period-luminosity relations at V and I were constructed, from which apparent distance moduli were measured with respect to assumed values of mu(sub 0) = 18.50 mag and E(B - V) = 0.10 mag for the Large Magellanic Cloud. The difference in the apparent V and I moduli yields a measure of the difference in the total mean extinction between the M81 and the LMC Cepheid samples. A low total mean extinction to the M81 sample of E(B - V) = 0.03 +/- 0.05 mag is obtained. The true distance modulus to M81 is determined to be 27.80 +/- 0.20 mag, corresponding to a distance of 3.63 +/- 0.34 Mpc. These data illustrate that with an optimal (power-law) sampling strategy, the HST provides a powerful tool for the discovery of extragalactic Cepheids and their application to the distance scale. M81 is the first calibrating galaxy in the target sample of the HST Key Project on the Extragalactic Distance Scale, the ultimate aim of which is to provide a value of the Hubble constant to 10% accuracy.

UBV charge coupled device (CCD) photometry has been obtained for 14 OB associations in the Magellanic Clouds using the University of Toronto's 0.6 m telescope and the Carnegie Institution of Washington's 1.0 m reflector, both on Las Campanas, Chile. The data are presented and used to construct color-magnitude diagrams for the purposes of investigating the massive-star content of the associations.

Based on newly acquired UBVRI CCD photometry, we present a deep color-magnitude diagram for the Galactic open cluster NGC 129, which contains the 8.00 day period Cephid DL CAS.

We examine the consequences of a 17 Mpc M100 distance, questioning its consistency with supernova and other distances.

Using the Canada-France-Hawaii Telescope (CFHT) we have obtained three closely spaced epochs of calibrated Blue Violet Red Infrared (BVRI) CCD imaging of two fields in M81, each known to contain a thirty-day Cepheid. Calibrated BVRI photometry of the brightest stars in these fields is presented. The slope of the luminosity function from the brightest 3-4 mag of the main-sequence blue plume is consistent with similar determinations of the apparent luminosity function in other resolved galaxies, thereby removing the one potential deviation from universality noted by Freedman in a photographic study of luminosity functions in nearby resolved galaxies. Under the assumption that the two Cepheids are representative, a reddening-law fit to the multiwavelength BVRI period-luminosity moduli give a true distance modulus of (m-M)sub 0 = 27.79 mag for M81, corresponding to a linear distance of 3.6 Mpc. An error analysis shows that the derived true distance modulus has a random error of +/- 0.28 mag (due to the photometric uncertainties in the BVRI data), with a systematic uncertainty of +/- 0.10 mag (accounting for the combined effects of unknown phasing of the data points, and the unknown positioning of these particular stars within the Cepheid instabiliy strip).

We present a catalog of blue and red stars in M33 based on photographic photometry of over 65,000 objects extracted from plates taken with the 3.6 m Canada-France-Hawaii Telescope (CFHT) and the 2.0 m Rozhen (Bulgarian) Telescope. The completeness limit of the various surveys are estimated here to be V = 19.5 mag for those stars situated in crowded associations, and V = 20.0 mag for stars in the interarm fields. We list magnitudes and positions for 2112 blue stars, defined by (U - V) less than 0.0 mag, (U - B) less than 0.0 mag, and V less than 19.5 mag, and 389 red stars defined by (B - V) greater than 1.8 mag and V less than 19.5 mag. Of these, 1156 are candidate O stars on the basis of (U - V) less than -0.9 mag.

The study presents BVRI CCD photometry of about 5300 stars in the central area of the dwarf elliptical galaxy NGC 185 in the Local Group. The color-magnitude diagram shows three distinct stellar populations: a dominant RGB population, AGB stars located above the tip of the RGB stars, and a small number of young stars having blue to yellow colors. The foreground reddening is estimated to be 0.19 +/- 0.03 mag using the (B - V) - (V - I) diagram for the bright foreground stars with good photometry. Surface photometry of the central area of NGC 185 is presented; it shows that the colors become rapidly bluer inside R of about 10 arcsec. Structural parameters indicate that the mass-to-luminosity ratio ranges from 3 to 5.

A true distance modulus of (m - M) sub 0 = 26.66 +/- 0.10 mag (corresponding to 2.1 +/- 0.1 Mpc) has been determined for the Sculptor Group spiral galaxy NGC 300. New CCD data have been obtained for a sample of known Cepheids in this galaxy from which apparent distance moduli at B, V, R, and I wavelengths are determined. Combining the data available at different wavelenghts, and assuming a true distance modulus to the LMC of 18.5 mag, a true distance modulus is obtained for NGC 300, corrected for the effects of interstellar reddening. The availability of a new distance to NGC 300 brings to five the total number of galaxies with new CCD photometry of Cepheids, useful for calibration of the Hubble constant.

Deep charge-coupled device images of a field in IC 1613 away from the star forming central regions were taken with the '4-shooter' on the Hale 5 m telescope, and processed to find photometrically measure variable stars. Fifteen RR Lyrae stars were found, and periods, light curves, and finding charts for them are presented. The mean magnitude of RR Lyrae stars in this galaxy is deduced to be 24.90 mag. Assuming the absolute g magnitude for RR Lyraes to be Mg = 0.73 mag, and using extinction Ag = 0.07 mag, a distance modulus of 24.10 +/- 0.27 mag is derived. The finding of RR Lyrae stars indicates the unambiguous presence of an old population. The distance modulus derived from them is smaller than that derived from the Cepheids by an amount which is dependent upon the RR Lyrae zero-point calibration adopted, and may be as large as 0.3 mag. The difference has the same sense and magnitude as the discrepancy in the LMC. The probable sources of the problem are discussed.

An attempt was made to estimate the reddening for the galaxy IC 342 on the basis of CCD BV photometry obtained for 117 stars brighter than V = 22.0 mag in a field centered near a spiral arm. At V = 19.4 mag, the plume of the main sequence and slightly evolved blue supergiant stars were detected. The plume is centered at a color of (B-V) = 0.54 mag. A comparison with similar color-magnitude-diagram data for the brightest stars in IC 1613 suggests that these stars in IC 342 are reddened by E(B-V) = 0.79 +/- 0.05 mag (A sub B = 3.32 +/- 0.21 mag).

Developments concerning the period-luminosity (PL) relation for classical Cepheids are discussed with particular emphasis on the fact that Cepheids continue to provide the most homogeneous and reliable set of distances to many nearby galaxies. Observational programs relating to extragalactic Cepheids are also reviewed, and new distances and reddenings to individual galaxies are determined from these data using multiwavelength PL relations, based for the first time on self-consistent calibrating data sets. Three recent promising secondary extragalactic distance indicators (Tully-Fisher, planetary nebula luminosity functions, and surface-brightness fluctuations) tie their zero points to the Cepheid distance scale and give globally consistent values for the Hubble constant of about 80 km/sec/Mpc. Tying the Faber-Jackson relation and the supernova distance scales to the Leo and Virgo clusters yields similar results. Such a value for the Hubble constant is marginally inconsistent with new estimates of the ages of globular clusters, and it may prove to be a serious problem for standard cold-dark-matter models of the universe.

A true distance modulus to the nearby spiral galaxy M33 has been determined based on CCD photometry obtained at BVRI wavelengths. M33 is presently one of five nearby galaxies used in the calibration of the IR Tully-Fisher relation, and thereby in the determination of the Hubble constant. Using period-luminosity relations at several wavelengths offers the advantage that the distance moduli derived can be corrected for the effects of interstellar extinction. These data indicate that there is internal reddening affecting the Cepheid photometry in M33 which must be accounted for if a true distance modulus is to be obtained for this galaxy. Adopting a true distance modulus to the LMC of 18.5 mag, the new CCD data yield a true distance to M33 of 24.64 + or - 0.09 mag, corresponding to a linear distance of 840 kpc. A mean value of the total color excess (foreground and internal) for the Cepheids in M33 is estimated to be E(B - V) = 0.10 + or - 0.09 mag, assuming a value for the total mean LMC Cepheid color excess of 0.10 mag.

The results are reported of the first observational test of the sensitivity of the zero point of the Cepheid period-luminosity relation to metal abundance. BVRI time-averaged observations of Cepheids in three radially separated and chemically different fields in M31 are used to find mean reddenings and true distance moduli that differ by only 0.25 mag. Consistent distances are found for all three fields. The large observed differences in the apparent moduli can be totally attributed to different mean extinctions among the three fields.

BVRI CCD photometry is presented for stars brighter than V = 21 mag in four fields in the nearby spiral galaxy M33. V vs (B - V) and I vs (V - I) color-magnitude diagrams clearly show both a young stellar population (as indicated by the blue main sequence and red supergiant plumes) as well as an intermediate-age population of asymptotic giant branch stars. Deep photometry in the outer field (where crowding is less severe) reveals a population consistent in color and magnitude with the tip of the first red giant branch. The M33 distance modulus, 24.6 + or - 0.3 mag, derived from this Population II component is consistent with a recent redetermination of the distance modulus found from Population I Cepheid variables. Finally, some evidence is presented for a radial gradient in the average internal reddening for the fields in M33 reported here.

New photographic, CCD, and near-infrared photometry of stars in the distant globular cluster AM-1 are presented. Based on the photographic and CCD photometry down to the horizontal branch, a distance modulus (m-M)v = 20.4 mag is derived, with E(B-V) = 0.0 mag, corresponding to a distance of 120 kpc from the sun. Using the (V-K) colors of two giants in AM-1, a metallicity of Fe/H = -2.0 + or - 0.2 is deduced, in good agreement with earlier determinations. Using complete photometry and star counts, a core radius of 10 arcsec (6 pc) and a tidal radius of 100 arcsec (58 pc) are derived. Despite the low intrinsic luminosity (Mv = -5.0 mag) and the large galactocentric distance of this globular cluster, the apparent magnitude distribution of the giant branch stars of AM-1 is not found to be unusual.