The Galactic center is a hotbed of star-formation activity, containing the most massive-star-formation site and three of the most massive young star clusters in the Galaxy. Given such a rich environment, it contains more stars with initial masses above 100 M⊙ than anywhere else in the Galaxy. This review concerns the young stellar population in the Galactic center as it relates to massive-star formation in the region. The sample includes stars in the three massive stellar clusters, the population of younger stars in the present sites of star formation, the stars surrounding the central black hole, and the bulk of the stars in the field population. The fossil record in the Galactic center suggests that the recently formed massive stars there are present-day examples of similar populations that must have been formed through star-formation episodes stretching back to the time period when the Galaxy was forming.

High-mass stars form in deeply embedded cores with very high visual extinction. Such star-forming regions are typically located at distances > 1 kpc from the Sun. Radio interferometric observations are hence vital for studying such regions at spatial resolutions of < 1000 AU. I will review radio observations of high-mass young stellar objects in our Galaxy, with emphasis on recent results from the Submillimeter Array. There now exists a large sample of sources which represent the earliest stages of high-mass star formation. Radio observations of these sources in dust continuum and molecular line emission have shown that they share many characteristics with low-mass star formation. Stars with masses up to ∼20 M⊙ may form via the disk-accretion mechanism instead of merging of lower-mass stars. Several questions regarding masses and stability of such disks still remain outstanding, such as driving mechanisms of the outflows, and multiplicity of sources. Detailed observations of higher-mass stars, which are at > 2 kpc, will be possible with the next generation of radio interferometers, such as the Atacama Large Millimeter Array, which will help address these questions.

The sequence of massive-star supernova types IIP (plateau light curve), IIL (linear light curve), IIb, IIn (narrow line), Ib, and Ic roughly represents a sequence of increasing mass loss during the stellar evolution. The mass loss affects the velocity distribution of the ejecta composition; in particular, only the IIP's typically end up with H moving at low velocity. Radio and x-ray observations of extragalactic supernovae show varying mass-loss properties that are in line with expectations for the progenitor stars. For young supernova remnants, pulsar wind nebulae and circumstellar interaction provide probes of the inner ejecta and higher velocity ejecta, respectively. Among the young remnants, there is evidence for supernovae over a range of types, including those that exploded with much of the H envelope present (Crab Nebula, 3C 58, 0540–69) and those that exploded after having lost most of their H envelope (Cas A, G292.0+1.8).

Massive stars as a population are the source of various feedback effects that critically impact the evolution of their host galaxies. We examine parameterizations of the high-mass stellar population and self-consistent parameterizations of the resulting feedback effects, including mechanical feedback, radiative feedback, and chemical feedback, as we understand them in the local universe. To date, it appears that the massive-star population follows a simple power-law clustering law that extends down to individual field massive stars, and the robust stellar IMF appears to have a constant upper-mass limit. These properties result in specific patterns in the H II-region luminosity function, and the ionization of the diffuse, warm, ionized medium. The resulting supernovae generate a population of superbubbles whose distributions in size and expansion velocity are also described by simple power laws, and from which a galaxy's porosity parameter is easily derived. A critical star-formation threshold can then be estimated, above which the escape of Lyman-continuum photons, hot gas, and nucleosynthetic products is predicted. A first comparison with a large sample of Hα observations of galaxies is broadly consistent with this prediction, and suggests that ionizing photons are likely to escape from starburst galaxies. The superbubble size distribution also offers a basis for a Simple Inhomogeneous Model for galactic chemical evolution, which is especially applicable to metal-poor systems and instantaneous metallicity distributions. This model offers an alternative interpretation of the Galactic halo metallicity distribution and emphasizes the relative importance of star-formation intensity, in addition to age, in a system's evolution. The fraction of zero-metallicity, Population III stars is easily predicted for any such model. We emphasize that all these phenomena can be modeled in a simple, analytic framework over an extreme range in scale, offering powerful tools for understanding the role of massive stars in the cosmos.

I review our knowledge of metal-free stars in the early universe—based on highly detailed new data on the most metal-poor stars in the Galaxy, and interpreted with new models of nucleosynthesis and chemical evolution. Present data supports the theoretical prediction that metal-free gas did not form stars of M ∼ 1 M⊙, but indicates a characteristic mass M ∼ 10 M⊙, lower than the M ∼ 100 M⊙ suggested by ab initio simulations. This field is expected to grow dramatically in the next decade with future instruments and large surveys of metal-poor Galactic stars.

Observational and theoretical evidence in support of metallicity-dependent winds for Wolf-Rayet stars is considered. Well-known differences in Wolf-Rayet subtype distributions in the Milky Way, LMC and SMC may be attributed to the sensitivity of subtypes to wind density. Implications for Wolf-Rayet stars at low metallicity include a hardening of ionizing flux distributions, an increased WR population due to reduced optical line fluxes, plus support for the role of single WR stars as gamma-ray burst progenitors.

We review the recent measurements on the cosmic infrared background (CIB) and their implications for the physics of the first-stars era, including Population III stars. The recently obtained CIB results range from the direct measurements of CIB fluctuations from distant sources using deep Spitzer data to strong upper limits on the near-IR CIB from blazar spectra. This allows us to compare the Population III models with the CIB data to gain direct insight into the era of the first stars, the formation and evolution of Population III stars, and the microphysics of the feedback processes in the first halos of collapsing material. We also discuss the cosmological confusion resulting from these CIB sources and the prospects for resolving them individually with NASA's upcoming space instruments, such as the James Webb Space Telescope (JWST).

Multiple observations reinforce the binarity of Eta Carinae including the 5.54-year periodicity in x-rays, spectroscopic excitation of the Weigelt blobs and the behavior of the stellar line profiles. The Hubble Space Telescope (HST) STIS observations from 1998.0 to 2004.3 provide considerable new evidence of the binary system. We focus on the lines of He I, HI, FeII and [N II] and provide initial visualizations of the binary system. Recent observations with VLTI/AMBER are consistent with a binary model.

Large numbers of young stars are formed in merging galaxies, such as the Antennae galaxies. Most of these stars are formed in compact star clusters (i.e., super star clusters), which have been the focus of a large number of studies. However, an increasing number of projects are beginning to focus on the individual stars as well. In this contribution, we examine a few results relevant to the triggering of star and star cluster formation; ask what fraction of stars form in the field rather than in clusters; and begin to explore the demographics of both the massive stars and star clusters in the Antennae.

The current generation of millimeter interferometers have revealed a population of compact (r ≳ 0.1 pc), massive (M ∼ 100 M⊙) gas cores that are the likely progenitors of massive stars. I review models for the evolution of these objects from the observed massive-core phase through collapse and into massive-star formation, with particular attention to the least well-understood aspects of the problem: fragmentation during collapse, interactions of newborn stars with the gas outside their parent core, and the effects of radiation-pressure feedback. Through a combination of observation, analytic argument, and numerical simulation, I develop a model for massive-star formation by gravitational collapse in which massive cores collapse to produce single stars or (more commonly) small-multiple systems, and these stars do not gain significant mass from outside their parent core by accretion of either gas or other stars. Collapse is only very slightly inhibited by feedback from the massive star, thanks to beaming of the radiation by a combination of protostellar outflows and radiation-hydrodynamic instabilities. Based on these findings, I argue that many of the observed properties of young star clusters can be understood as direct translations of the properties of their gas-phase progenitors. Finally, I discuss unsolved problems in the theory of massive-star formation, and directions for future work on them.

A list of 50 optically observable O stars that are likely on or very near the ZAMS is presented. They have been selected on the basis of five distinct criteria, although some of them exhibit more than one. Three of the criteria are spectroscopic (He II λ4686 absorption stronger than in normal luminosity class V spectra, abnormally broad or strong Balmer lines, weak UV wind profiles for their spectral types), one is environmental (association with dense, dusty nebular knots), and one is photometric (derived absolute magnitudes fainter than class V). Very few of these stars have been physically analyzed, and they have not been considered in the current framework of early massive stellar evolution. In particular, they may indicate that the earliest, embedded phases are not as large a fraction of the main-sequence lifetimes as is currently believed. Detailed analyses of these objects will likely prove essential to a complete understanding of the early evolution of massive stars.

The Chandra X-ray Observatory is providing fascinating new views of massive-star–forming regions, revealing all stages in the life cycles of massive stars and their effects on their surroundings. I present a Chandra tour of some of the most famous of these regions: M17, NGC 3576, W3, Tr14 in Carina, and 30 Doradus. Chandra highlights the physical processes that characterize the lives of these clusters, from the ionizing sources of ultracompact H II regions (W3) to superbubbles so large that they shape our views of galaxies (30 Dor). X-ray observations usually reveal hundreds of pre-main sequence (lower-mass) stars accompanying the OB stars that power these great H II region complexes, although in one case (W3 North) this population is mysteriously absent. The most massive stars themselves are often anomalously hard x-ray emitters; this may be a new indicator of close binarity. These complexes are sometimes suffused by soft diffuse x-rays (M17, NGC 3576), signatures of multi-million-degree plasmas created by fast O-star winds. In older regions we see the x-ray remains of the deaths of massive stars that stayed close to their birthplaces (Tr14, 30 Dor), exploding as cavity supernovae within the superbubbles that these clusters created.

The stellar initial mass function (IMF) in star clusters is reviewed. Uncertainties in the observations are emphasized. We suggest there is a distinct possibility that cluster IMFs vary systematically with density or pressure. Dense clusters could have additional formation processes for massive stars that are not present in low-density regions, making the slope of the upper-mass IMF somewhat shallower in clusters. Observations of shallow IMFs in some super star clusters and in elliptical galaxies are reviewed. We also review mass segregation and the likelihood that peculiar IMFs, as in the Arches cluster, result from segregation and stripping, rather than an intrinsically different IMF. The theory of the IMF is reviewed in some detail. Several problems introduced by the lack of a magnetic field in SPH simulations are discussed. The universality of the IMF in simulations suggests that something more fundamental than the physical details of a particular model is at work. Hierarchical fragmentation by any of a variety of processes may be the dominant cause of the power-law slope. Physical differences from region to region may make a slight difference in the slope and also appear in the low-mass turnover point.