Up to ages of ~ 100 Myr, massive clusters are still swamped in large amounts of gas and dust, with considerable and uneven levels of extinction. At the same time, large grains (ices?) produced by type II supernovae profoundly alter the interstellar medium (ISM), thus resulting in extinction properties very different from those of the diffuse ISM. To obtain physically meaningful parameters of stars, from basic luminosities and effective temperatures to masses and ages, we must understand and measure the local extinction law. This problem affects all the massive young clusters discussed in his volume.

We have used archival Hubble Space Telescope (HST) observations obtained with the Advanced Camera for Surveys (ACS) to study the Hα emission properties of main sequence (MS) stars in the globular cluster 47 Tucanae. An accurate photometric measurement of their Hα equivalent width, allows us to identify objects with large Hα emission. We demonstrate that this method allows us to identify and characterise candidate cataclysmic variables (CVs) with high efficiency, reconciling observations with theoretical predictions on the number of dynamically formed CVs expected in a 47 Tucanae-like globular cluster.

We present a study of the infrared properties of 4922 spectroscopically confirmed massive stars in the Large and Small Magellanic Clouds, focusing on the active OB star population. Besides OB stars, our sample includes yellow and red supergiants, Wolf-Rayet stars, Luminous Blue Variables (LBVs) and supergiant B[e] stars. We detect a distinct Be star sequence, displaced to the red, and find a higher fraction of Oe and Be stars among O and early-B stars in the SMC, respectively, when compared to the LMC, and that the SMC Be stars occur at higher luminosities. We also find photometric variability among the active OB population and evidence for transitions of Be stars to B stars and vice versa. We furthermore confirm the presence of dust around all the supergiant B[e] stars in our sample, finding the shape of their spectral energy distributions (SEDs) to be very similar, in contrast to the variety of SED shapes among the spectrally variable LBVs.

The Supernova Working Group was re-established at the IAU XXV General Assembly in Sydney, 21 July 2003, sponsored by Commissions 28 (Galaxies) and 47 (Cosmology). Here we report on some of its activities since 2005.

In 2004 a deep sequence of HST images of the Bulge was used to identify sixteen transiting extrasolar planet candidates (the SWEEPS candidates; Sahu et al. 2006), of which at least seven are likely to be true planets. Of these, SWEEPS-4 is almost certainly in the disk, and was shown through radial velocity followup to contain a planetary companion; the identification of the remaining fifteen candidates was left undetermined.

We have used a repeat visit in 2006 to attach proper motions to some 180,000 objects, including all sixteen SWEEPS candidates. This has allowed us to build a sample of bulge stars to unprecedented purity. A population of more than 13,000 bulge objects is kinematically isolated, with fewer than thirty disk contaminants. We use the mean bulge and disk populations to test the balance of kinematic associations for the sixteen SWEEPS candidates. Assuming both the detectability and the astrophysical false-positive fraction to be similar for disk and bulge, we find the fraction of stars with planets in the bulge to be consistent with that in the disk.

The SWEEPS (Sagittarius Window Eclipsing Extrasolar Planet Search) program was aimed at detecting planets around stars in the Galactic bulge, not only to determine their physical properties, but also to determine whether the properties of planets found in the solar neighborhood, such as their frequency and the metallicity dependence, also hold for the planets in the Galactic bulge. We used the Hubble Space Telescope to monitor 180,000 F, G, K, and M dwarfs in the Galactic bulge continuously for 7 days in order to look for transiting planets. We discovered 16 candidate transiting extrasolar planets with periods of 0.6 to 4.2 days, including a possible new class of ultra-short period planets (USPPs) with P < 1 day. The facts that (i) the coverage in the monitoring program is continuous, (ii) most of the stars are at a known distance (in the Galctic bulge), (iii) monitoring was carried out in 2 passbands, and (iv) the images have high spatial resolution, were crucial in minimizing and estimating the false positive rates. We estimate that at least 45% of the candidates are genuine planets. Radial velocity observations of the two brightest host stars further support the planetary nature of the transiting companions. These results suggest that the planet frequency in the Galactic bulge is similar to that in the solar neighborhood. They also suggest that higher metallicity favors planet formation even in the Galactic bulge. The USPPs occur only around low-mass stars which may suggest that close-in planets around higher-mass stars are irradiately evaporated, or that planets are able to migrate to and survive in close-in orbits only around such old and low-mass stars.

In 2004 the Sagittarius Window Eclipsing Extrasolar Planet Search (SWEEPS) project undertook a very deep ACS/WFC exposure-set of the Sgr-I low-reddening window in the Galactic Bulge, with repeat observations 2.04 years later. The combination of superb first-epoch sampling, wide field of view and high PSF stability of ACS/WFC on Hubble allows proper motions to be extracted for more than 137,000 objects, over 85,000 to accuracy better than 0.3 mas yr−1. We present these proper motions and outline some of the uses to which they have been put, including the separation of a pure-Bulge sample and the inner Galactic rotation curve.

We present the first results of our study of stellar populations in the Large and Small Magellanic Clouds based on multi-band WFPC2 observations of “random” fields taken as part of the “pure parallel” programme carried out with the HST as a service to the community.

We discuss the expected properties of the first stellar generations in the Universe. We find that it is possible to discern truly primordial populations from the next generation of stars by measuring the metallicity of high-z star forming objects through the detection and the study of the emission line spectrum from HII regions associated to primordial massive stars. Moreover, the intensity of HeII emission lines can be used as a diagnostic to reveal the presence of massive stars and determine their importance relative to lower mass stars. Thanks to its very low background, the future James Webb Space Telescope (JWST) will be able to image and study first-light sources at very high redshifts. On the other hand, JWST's relatively small collecting area limits its capability in obtaining spectra of z$\,{\sim}\,$10–15 first-light sources to either the bright end of their luminosity function or to strongly lensed sources. With a suitable investment of observing time JWST will be able to detect individual Population III supernovae, thus identifying the very first stars that formed in the Universe.

Using the definitive reductions of the IUE light curves by [15] and an extensive set of HST images of SN 1987A we have repeated and improved our original analysis [8, 9] to derive a better determination of the distance to the supernova. In this way we have obtained an absolute size of the ring Rabs = (6.23 ± 0.08) × 1017 cm and an angular size R″ = 808 ± 17 mas, which give a distance to the supernova d(SN1987A) = 51.4±1.2 kpc and a distance modulus (m − M)sN1987A = 18.55 ± 0.05. Allowing for a displacement of SN 1987A position relative to the LMC center, the distance to the barycenter of the Large Magellanic Cloud is also estimated to be d(LMC) = 51.7±1.3 kpc, which corresponds to a distance modulus of (m − M)LMC = 18.56 ± 0.05.

We review ten years of radio continuum and X-ray monitoring of the Type IIb SN 1993J in M81. The supernova (SN) has been observed continuously, since only a few days after explosion, by our group with the Very Large Array at a number of radio frequencies, as well as by other groups. As a result, it is among the best-studied radio supernovae. The observed synchrotron radio emission is thought to arise from the interaction of the SN shock with the pre-SN wind-established circumstellar medium around the progenitor star. We describe the properties of the circumstellar interaction, based on the more fully-developed dataset, and compare this to our earlier characterization made in 1994. SN 1993J has also been a target of X-ray satellites, and we briefly discuss the nature of the X-ray emission and, together with the radio emission, describe the implications for the nature of the SN’s progenitor.

Detailed, multi-frequency radio observations of supernovae have shown that the radio emission can be understood as a blastwave interacting with a structured circumstellar medium (CSM) and modeling of the radio light curves allows estimation of the physical conditions in this medium. CSM structures, properties of the presupernova system, and the evolution of the system in the last stages before explosion can then be derived.

Supernova 2001ig in NGC 7424 has been observed with the Australia Telescope Compact Array at ~2 week intervals since its discovery, making this the best-studied Type IIb radio supernova since SN 1993J. We present radio light curves for frequencies from 1.4 to 20 GHz, and preliminary attempts to model the observed behavior. Since peaking in radio luminosity at 8.6 and 4.8 GHz some 1-2 months after the explosion, SN 2001ig has on at least two occasions deviated significantly from a smooth decline, indicative of interaction with a dense circumstellar medium and possibly of periodic progenitor mass-loss.

We present the preliminary results of a detailed theoretical investigation on the pulsation properties and modal stability of red supergiant (RSG) stars at solar chemical composition and for stellar masses ranging from 10 to 20 M⊙. We find that the main parameter governing their pulsation behavior is the effective temperature, and indeed when moving from the blue edge toward lower effective temperatures the models show an increase in the pulsation amplitudes. For temperatures cooler than 3800 K the pulsation amplitudes are still large but the radial displacements become aperiodic.

We present the results of a study of the stellar population around SN1987A in the Large Magellanic Cloud based on an analysis of multi-band HST-WFPC2 images.

Using the new reductions of the IUE light curves by Sonneborn et al. (1997) and an extensive set of HST images of SN 1987A we have repeated and improved Panagia et al. (1991) analysis to obtain a better determination of the distance to the supernova. In this way we have derived an absolute size of the ring Rabs = (6.23 ± 0.08) x 1017 cm and an angular size R″ = 808 ± 17 mas, which give a distance to the supernova d(SN1987A) = 51.4 ± 1.2 kpc and a distance modulus m–M(SN1987A) = 18.55 ± 0.05. Allowing for a displacement of SN 1987A position relative to the LMC center, the distance to the barycenter of the Large Magellanic Cloud is also estimated to be d(LMC) = 52.0±1.3 kpc, which corresponds to a distance modulus of m–M(LMC) = 18.58±0.05.

The radio emission from supernovae (SNe) is nonthermal synchrotron radiation of high brightness temperature, with a “turn-on” delay at longer wavelengths, power-law decline after maximum with index β, and spectral index α asymptotically decreasing with time to a final, optically thin value. Radio supernovae (RSNe) are best described by the Chevalier (1982) “mini-shell” model, with modifications by Weiler et al. (1990). RSNe observations provide a valuable probe of the SN circumstellar environment and constraints on progenitor masses. We present a progress report on a number of recent RSNe, as well as on new behavior from RSNe 1979C and 1980K, and on RSNe as potential distance indicators. In particular, we present updated radio light curves for SN 1993J in M81.

Radio observations have shown that some supernovae are powerful radio emitters which increase rapidly in brightness to radio luminosities which are hundreds to thousands of times greater than even the brightest known supernova remnant, Cas A. They then fade over a period of weeks, months, or years. This radio emission has been found to provide important information about the nature of the progenitor stars, their mass loss rates, and the circumstellar material surrounding them. RSN observations may also offer the possibility of extragalactic distance measurements and the presence of radio emission appears to be indicator of strong x-ray emission and late time optical emission.