Nonrotating stars that end their lives with masses 140 M⊙ ≤ M* ≤ 260 M⊙ should explode as pair-production supernovae (PPSNe). Here I review the physical properties of these objects, as well as the prospects for them to be observationally constrained.

In very massive stars, much of the pressure support comes from the radiation field, meaning that they are loosely bound, and that (d lgp/d lg Ρ)adiabatic near the center is close to the minimum value necessary for stability. Near the end of C/O burning, the central temperature increases to the point that photons begin to be converted into electron–positron pairs, softening the equation of state below this critical value. The result is a runaway collapse, followed by explosive burning that completely obliterates the loosely bound star. While these explosions can be up to 100 times more energetic than core collapse and Type Ia supernovae, their peak luminosities are only slightly greater. However, due both to copious Ni56 production and hydrogen recombination, they are brighter much longer, and remain observable for ≈1 year.

Since metal enrichment is a local process, PPSNe should occur in pockets of metal-free gas over a broad range of redshifts, greatly enhancing their detectability, and distributing their nucleosynthetic products about the Milky Way. This means that measurements of the abundances of metal-free stars should be thought of as directly constraining these objects.

In recent years a new generation of model atmosphere codes, which include the effects of metal line blanketing of millions of spectral lines in NLTE, has been used to re-determine the properties of massive stars through quantitative spectral analysis methods applied to optical, IR and UV spectra. This has resulted in a significant change of the effective temperature scale of early-type stars and a revision of mass-loss rates. Observed mass-loss rates and effective temperatures depend strongly on metallicity, both in agreement with theoretical predictions. The new model atmospheres, in conjunction with the new generation of 10-m-class telescopes equipped with efficient multi-object spectrographs, have made it possible to study blue supergiants in galaxies far beyond the Local Group in spectroscopic detail to determine accurate chemical composition, extinction and distances. A new distance determination method, the flux-weighted gravity–luminosity relationship, is discussed as a very promising complement to existing stellar distance indicators.

Observationally, there are still fundamental uncertainties in the determination of stellar mass-loss rates, which are caused by evidence that the winds are inhomogeneous and clumped. This may lead to major revisions of the observed rates of mass loss.

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.