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Galaxies
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Luminous Stars in Nearby Galaxies
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Luminous Stars in Nearby Galaxies

A special issue of Galaxies (ISSN 2075-4434).

Deadline for manuscript submissions: closed (30 October 2019) | Viewed by 36547

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editor

Prof. Dr. Roberta M. Humphreys

E-Mail Website
Guest Editor
School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
Interests: massive stars; stellar populations; galaxtic structure
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Perhaps the greatest uncertainty in all of astrophysics and especially in stellar structure and evolution, is distance. This is especially true for the most massive, most luminous stars that may be at very large distances in our own galaxy. Studies of stellar populations in nearby galaxies thus have the advantage that all the stars are at the approximately the same distance, a distance that is relatively well known, especially in comparison with the uncertain distances of individual stars in our own galaxy. Surveys and the subsequent spectroscopy of massive stars in different stages of stellar evolution in the relatively nearby resolved galaxies have revealed a complex distribution in the luminosity–temperature plane, the HR Diagram. The fundamentals of massive star evolution are basically understood, but the roles of mass loss, episodic mass loss, rotation, and binarity are still in question. The final stages of these stars of different masses and their possible relation to each other are not understood. The purpose of this volume is a current review of the different populations of evolved massive stars. The emphasis is on massive stars in the Local Group; the Magellanic Clouds and the nearby spirals M31 and M33.

Prof. Roberta M. Humphreys
Guest Editor

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Keywords

  • massive stars
  • stellar evolution
  • mass loss
  • Magellanic Clouds
  • M31
  • M33

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Published Papers (8 papers)

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Review

27 pages, 5016 KiB  
Review
Luminous Blue Variables
by Kerstin Weis and Dominik J. Bomans
Galaxies 2020, 8(1), 20; https://doi.org/10.3390/galaxies8010020 - 29 Feb 2020
Cited by 44 | Viewed by 6026
Abstract
Luminous Blue Variables are massive evolved stars, here we introduce this outstanding class of objects. Described are the specific characteristics, the evolutionary state and what they are connected to other phases and types of massive stars. Our current knowledge of LBVs is limited [...] Read more.
Luminous Blue Variables are massive evolved stars, here we introduce this outstanding class of objects. Described are the specific characteristics, the evolutionary state and what they are connected to other phases and types of massive stars. Our current knowledge of LBVs is limited by the fact that in comparison to other stellar classes and phases only a few “true” LBVs are known. This results from the lack of a unique, fast and always reliable identification scheme for LBVs. It literally takes time to get a true classification of a LBV. In addition the short duration of the LBV phase makes it even harder to catch and identify a star as LBV. We summarize here what is known so far, give an overview of the LBV population and the list of LBV host galaxies. LBV are clearly an important and still not fully understood phase in the live of (very) massive stars, especially due to the large and time variable mass loss during the LBV phase. We like to emphasize again the problem how to clearly identify LBV and that there are more than just one type of LBVs: The giant eruption LBVs or η Car analogs and the S Dor cycle LBVs. Full article
(This article belongs to the Special Issue Luminous Stars in Nearby Galaxies)
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29 pages, 1376 KiB  
Review
Radiation-Driven Stellar Eruptions
by Kris Davidson
Galaxies 2020, 8(1), 10; https://doi.org/10.3390/galaxies8010010 - 5 Feb 2020
Cited by 12 | Viewed by 3410
Abstract
Very massive stars occasionally expel material in colossal eruptions, driven by continuum radiation pressure rather than blast waves. Some of them rival supernovae in total radiative output, and the mass loss is crucial for subsequent evolution. Some are supernova impostors, including SN precursor [...] Read more.
Very massive stars occasionally expel material in colossal eruptions, driven by continuum radiation pressure rather than blast waves. Some of them rival supernovae in total radiative output, and the mass loss is crucial for subsequent evolution. Some are supernova impostors, including SN precursor outbursts, while others are true SN events shrouded by material that was ejected earlier. Luminous Blue Variable stars (LBV’s) are traditionally cited in relation with giant eruptions, though this connection is not well established. After four decades of research, the fundamental causes of giant eruptions and LBV events remain elusive. This review outlines the basic relevant physics, with a brief summary of essential observational facts. Reasons are described for the spectrum and emergent radiation temperature of an opaque outflow. Proposed mechanisms are noted for instabilities in the star’s photosphere, in its iron opacity peak zones, and in its central region. Various remarks and conjectures are mentioned, some of them relatively unfamiliar in the published literature. Full article
(This article belongs to the Special Issue Luminous Stars in Nearby Galaxies)
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22 pages, 6972 KiB  
Review
Red Supergiants, Yellow Hypergiants, and Post-RSG Evolution
by Michael S. Gordon and Roberta M. Humphreys
Galaxies 2019, 7(4), 92; https://doi.org/10.3390/galaxies7040092 - 3 Dec 2019
Cited by 14 | Viewed by 4899
Abstract
How massive stars end their lives remains an open question in the field of star evolution. While the majority of stars above ≳9 M ⊙ will become red supergiants (RSGs), the terminal state of these massive stars can be heavily influenced by their [...] Read more.
How massive stars end their lives remains an open question in the field of star evolution. While the majority of stars above ≳9 M ⊙ will become red supergiants (RSGs), the terminal state of these massive stars can be heavily influenced by their mass-loss histories. Periods of enhanced circumstellar wind activity can drive stars off the RSG branch of the HR Diagram. This phase, known as post-RSG evolution, may well be tied to high mass-loss events or eruptions as seen in the Luminous Blue Variables (LBVs) and other massive stars. This article highlights some of the recent observational and modeling studies that seek to characterize this unique class of stars, the post-RSGs and link them to other massive objects on the HR Diagram such as LBVs, Yellow Hypergiants and dusty RSGs. Full article
(This article belongs to the Special Issue Luminous Stars in Nearby Galaxies)
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23 pages, 4750 KiB  
Review
Massive Stars in the Tarantula Nebula: A Rosetta Stone for Extragalactic Supergiant HII Regions
by Paul A. Crowther
Galaxies 2019, 7(4), 88; https://doi.org/10.3390/galaxies7040088 - 8 Nov 2019
Cited by 41 | Viewed by 4942
Abstract
A review of the properties of the Tarantula Nebula (30 Doradus) in the Large Magellanic Cloud is presented, primarily from the perspective of its massive star content. The proximity of the Tarantula and its accessibility to X-ray through radio observations permit it to [...] Read more.
A review of the properties of the Tarantula Nebula (30 Doradus) in the Large Magellanic Cloud is presented, primarily from the perspective of its massive star content. The proximity of the Tarantula and its accessibility to X-ray through radio observations permit it to serve as a Rosetta Stone amongst extragalactic supergiant HII regions since one can consider both its integrated characteristics and the individual properties of individual massive stars. Recent surveys of its high mass stellar content, notably the VLT FLAMES Tarantula Survey (VFTS), are reviewed, together with VLT/MUSE observations of the central ionizing region NGC 2070 and HST/STIS spectroscopy of the young dense cluster R136, provide a near complete Hertzsprung-Russell diagram of the region, and cumulative ionizing output. Several high mass binaries are highlighted, some of which have been identified from a recent X-ray survey. Brief comparisons with the stellar content of giant HII regions in the Milky Way (NGC 3372) and Small Magellanic Cloud (NGC 346) are also made, together with Green Pea galaxies and star forming knots in high-z galaxies. Finally, the prospect of studying massive stars in metal poor galaxies is evaluated. Full article
(This article belongs to the Special Issue Luminous Stars in Nearby Galaxies)
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36 pages, 1317 KiB  
Review
A Census of B[e] Supergiants
by Michaela Kraus
Galaxies 2019, 7(4), 83; https://doi.org/10.3390/galaxies7040083 - 29 Sep 2019
Cited by 45 | Viewed by 4938
Abstract
Stellar evolution theory is most uncertain for massive stars. For reliable predictions of the evolution of massive stars and their final fate, solid constraints on the physical parameters, and their changes along the evolution and in different environments, are required. Massive stars evolve [...] Read more.
Stellar evolution theory is most uncertain for massive stars. For reliable predictions of the evolution of massive stars and their final fate, solid constraints on the physical parameters, and their changes along the evolution and in different environments, are required. Massive stars evolve through a variety of short transition phases, in which they can experience large mass-loss either in the form of dense winds or via sudden eruptions. The B[e] supergiants comprise one such group of massive transition objects. They are characterized by dense, dusty disks of yet unknown origin. In the Milky Way, identification and classification of B[e] supergiants is usually hampered by their uncertain distances, hence luminosities, and by the confusion of low-luminosity candidates with massive pre-main sequence objects. The extragalactic objects are often mistaken as quiescent or candidate luminous blue variables, with whom B[e] supergiants share a number of spectroscopic characteristics. In this review, proper criteria are provided, based on which B[e] supergiants can be unambiguously classified and separated from other high luminosity post-main sequence stars and pre-main sequence stars. Using these criteria, the B[e] supergiant samples in diverse galaxies are critically inspected, to achieve a reliable census of the current population. Full article
(This article belongs to the Special Issue Luminous Stars in Nearby Galaxies)
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21 pages, 3043 KiB  
Review
The History Goes On: Century Long Study of Romano’s Star
by Olga Maryeva, Roberto F. Viotti, Gloria Koenigsberger, Massimo Calabresi, Corinne Rossi and Roberto Gualandi
Galaxies 2019, 7(3), 79; https://doi.org/10.3390/galaxies7030079 - 18 Sep 2019
Cited by 6 | Viewed by 2780
Abstract
GR 290 (M 33 V0532 = Romano’s Star) is a unique variable star in the M33 galaxy, which simultaneously displays variability typical for luminous blue variable (LBV) stars and physical parameters typical for nitrogen-rich Wolf-Rayet (WR) stars (WN). As of now, GR 290 [...] Read more.
GR 290 (M 33 V0532 = Romano’s Star) is a unique variable star in the M33 galaxy, which simultaneously displays variability typical for luminous blue variable (LBV) stars and physical parameters typical for nitrogen-rich Wolf-Rayet (WR) stars (WN). As of now, GR 290 is the first object which is confidently classified as a post-LBV star. In this paper, we outline the main results achieved from extensive photometric and spectroscopic observations of the star: the structure and chemical composition of its wind and its evolution over time, the systematic increase of the bolometric luminosity during the light maxima, the circumstellar environment. These results show that the current state of Romano’s Star constitutes a fundamental link in the evolutionary path of very massive stars. Full article
(This article belongs to the Special Issue Luminous Stars in Nearby Galaxies)
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14 pages, 1067 KiB  
Review
The Complex Upper HR Diagram
by Roberta M. Humphreys
Galaxies 2019, 7(3), 75; https://doi.org/10.3390/galaxies7030075 - 23 Aug 2019
Cited by 3 | Viewed by 3863
Abstract
Several decades of observations of the most massive and most luminous stars have revealed a complex upper HR Diagram, shaped by mass loss, and inhabited by a variety of evolved stars exhibiting the consequences of their mass loss histories. This introductory review presents [...] Read more.
Several decades of observations of the most massive and most luminous stars have revealed a complex upper HR Diagram, shaped by mass loss, and inhabited by a variety of evolved stars exhibiting the consequences of their mass loss histories. This introductory review presents a brief historical overview of the HR Diagram for massive stars, highlighting some of the primary discoveries and results from their observation in nearby galaxies. The sections in this volume include reviews of our current understanding of different groups of evolved massive stars, all losing mass and in different stages of their evolution: the Luminous Blue Variables (LBVs), B[e] supergiants, the warm hypergiants, Wolf–Rayet stars, and the population of OB stars and supergiants in the Magellanic Clouds. Full article
(This article belongs to the Special Issue Luminous Stars in Nearby Galaxies)
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27 pages, 1242 KiB  
Review
The Wolf–Rayet Content of the Galaxies of the Local Group and Beyond
by Kathryn Neugent and Philip Massey
Galaxies 2019, 7(3), 74; https://doi.org/10.3390/galaxies7030074 - 21 Aug 2019
Cited by 24 | Viewed by 4795
Abstract
Wolf–Rayet stars (WRs) represent the end of a massive star’s life as it is about to turn into a supernova. Obtaining complete samples of such stars across a large range of metallicities poses observational challenges, but presents us with an exacting way to [...] Read more.
Wolf–Rayet stars (WRs) represent the end of a massive star’s life as it is about to turn into a supernova. Obtaining complete samples of such stars across a large range of metallicities poses observational challenges, but presents us with an exacting way to test current stellar evolutionary theories. A technique we have developed and refined involves interference filter imaging combined with image subtraction and crowded-field photometry. This helps us address one of the most controversial topics in current massive star research: the relative importance of binarity in the evolution of massive stars and formation of WRs. Here, we discuss the current state of the field, including how the observed WR populations match with the predictions of both single and binary star evolutionary models. We end with what we believe are the most important next steps in WR research. Full article
(This article belongs to the Special Issue Luminous Stars in Nearby Galaxies)
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