**6. Stellar Population Comparisons**

One of the outstanding stellar evolution questions is how stellar populations may depend on their environment, namely the properties of the host galaxy, its mass and luminosity, the fraction of interstellar gas and dust, and especially on the chemical composition or metallicity. As the most luminous and visually brightest stars, massive stars provide the first indicator or measurement of how star formation, evolution, and the terminal state, depend on these factors. Mass loss is known to alter stellar evolution, and in the standard picture of line-driven winds from hot stars, mass loss is also expected to be metallicity dependent and significantly less in lower metallicity systems.

The first evidence for significant differences in the properties of the massive star populations was the well-established absolute magnitude dependence of the visually "brightest blue star" on the galaxy type or luminosity in the surveys for extragalactic distance indicators [14]. The smaller, less massive galaxies had fewer of the most massive stars, thus their evolved counterparts were statistically less likely to become the most luminous observed blue stars. These results sugges<sup>t</sup> that the star formation rate for these most massive stars was less in the smaller galaxies. In contrast, the luminosities of the brightest red stars showed little or no dependence on the host galaxy. The stars of somewhat lower initial mass thus existed in sufficient numbers in the smaller galaxies to produce evolved descendants in the red supergiant region, and the luminosities of their brightest members reflected the upper luminosity boundary.

The metallicity dependence of the overall characteristics of the massive star populations in different galaxies has been less apparent. Comparison of the HR Diagrams of the massive stars in our region of the Milky Way with the LMC revealed very similar populations [25]. The LMC oxygen abundance is lower than Solar but by no more than a factor of two. Consequently, a comparison with the outer regions of the Milky Way, also with reduced metallicity, not surprisingly, showed little variation. Among our Local Group galaxies with comprehensive stellar surveys for luminous stars, the SMC has the lowest metallicity, about 1/10th Solar, and a reduced oxygen abundance by about a factor of five.

Early tell-tale evidence for an observable metallicity affect was the distribution of the spectral types of the M supergiants [48]. A preliminary survey of red supergiants in the LMC and SMC compared to known Galactic RSGs, revealed a dramatic shift in their spectral types in the SMC to much earlier spectral types, compared to the LMC and Milky Way stars. Except for one M2 -type star in the SMC, all of the others were type M0 or earlier. This apparent shift, attributed to weaker TiO bands, was due to the lower SMC metallicity resulting in lower opacities in the atmospheres. The spectra thus arise in warmer layers. This result was confirmed in later surveys that extended to fainter magnitudes [37,49].

Stellar wind theory predicts a measurable dependence of the mass loss rate on metallicity [50], decreasing with declining heavy element abundances, but the measured rates [51] are somewhat higher than expected. Clumping in the stellar winds is another complication which when included in the mass loss models reduces the mass loss rates [52–54]. Measurement of the stellar wind properties and mass loss rates in the luminous, hot OB-type stars in nearby galaxies, especially in the Magellanic Clouds, has progressed with the advent of very large telescopes equipped with high resolution spectrographs. Paul Crowther's article in this issue on the FLAMES survey of the luminous, hot stars in the Clouds discusses their winds and mass loss rates.

### **7. The Most Luminous Stars of Different Types**

Since the early work of the 1970s and 1980s to identify the most luminous and brightest stars in nearby galaxies, numerous surveys and studies of the massive stars, primarily in Local Group galaxies, have greatly expanded the completeness of the population samples. These include, in the LMC and SMC, surveys for the yellow and red supergiants [55,56], and, in M31 and M33, studies of the luminous star population [57–61] and surveys for the yellow and red supergiants [62–66]. Surveys and follow-up

spectroscopy of the luminous blue and red stars in the Local Group irregulars NGC 6822 and IC 1613 are less complete [27,67–69], but they provide an additional sample of the massive stars in two smaller galaxies and also with reduced metallicity.

Table 1 presents a summary of the most luminous stars of different spectral types or temperature ranges in six Local Group Galaxies with their morphological types and integrated visual luminosities. Initial samples of the massive stars in the well-studied nearby spirals, M101, M81 and NGC 2403, outside our Local Group, have also been observed [45–47,70–72] and are included here. The bolometric luminosities of the stars are listed for the three highest in each spectral type group for each galaxy. The adopted distance moduli (Table 2) were used to determine the luminosities. The O-type stars are not included because many are eventually recognized as binary or are in multiple systems. Likewise, those stars in extremely crowded fields, including many OB-type stars, are not listed since they may be blended. Consequently, this table does not include those stars that will be the intrinsically most luminous, most massive members of their home galaxies. A few stars of special interest are identified by name in the table. Some of them are discussed in other articles in this issue. Note that the Galactic stars are not included. The upper HR Diagram for the Milky Way needs to re-examined when the Gaia survey is complete with improved distances and very likely with a larger volume sample.


**Table 1.** The Most Luminous Stars (M*Bol*) of Different Spectral Types.

Some comments with respect to the data in this table are helpful. It is clear that data for some of the spectral type groups are lacking or incomplete such as for the YSGs (FGK) in NGC 6822 and IC 1613. Those stars are undoubtedly present, but have just not been identified in the published surveys, and they may also be of somewhat lower luminosity in those two galaxies. A survey to identify and classify the hotter supergiants in M101 has not been completed, and, although a survey identifying RSGs in NGC 2403 exists [72], confirming spectroscopy and photometry is lacking. In general, the numbers for the hot supergiant group (O9.5–B5) may not include the most luminous members because many of these stars are in crowded regions.

The six Local Group galaxies in Table 1 present a diverse group of galaxy types with a wide range of luminosities. The well known dependence of the most luminous "blue" stars on the parent galaxy is especially notable for the evolved, post-main sequence A-type supergiants in the two lowest luminosity and lowest mass galaxies, NGC 6822 and IC 1613. Together with the SMC, these are also the galaxies with the lowest metallicity, but the heavy element abundance in NGC 6822, however, is intermediate between the SMC and LMC. Reduced metallicity was also expected to reduce the opacity in the stellar atmospheres and increase their absolute visual magnitudes but the data for these galaxies show the opposite effect.

Thus, although metallicity undoubtedly plays a role in stellar evolution, the luminosity and mass of the parent galaxy is the primary determinant, to the first order, for the luminosities of the most luminous stars. This is a size of sample effect. Assuming a similar slope for the mass function, the most massive galaxies will have a larger progenitor population of massive stars, and consequently, at any given time, we will therefore be more likely to observe their most luminous, evolved counterparts.

We also see another effect with this group of galaxies related to the galaxy type. M31 (Sb I–II) and M81 (Sb I -II), both high luminosity spirals, have lower luminosity evolved A-type blue stars compared with the Sc-type spirals and Magellanic irregulars. This second effect very likely reflects a dependence on the lower star formation rate, not mass, in the Sb spirals. Both of these spirals have above solar metallicites, although M81 has a metallicity gradient in its disk similar to the Milky Way and the Sc spirals M33 and M101, and solar-type abundances in the outer parts.

This degeneracy, wherein the luminosity of the most luminous blue stars are dependent on the host galaxy, complicated their use as direct distance indicators. The most luminous red supergiants, however, exhibit a nearly constant upper luminosity, related to the Humphreys–Davidson limit, over a wide range of galaxy types and luminosities. The presence of extensive circumstellar dust and uncertain reddening in the most luminous RSGs though limits their usefulness as distance indicators in the visual, but their luminosities in the infrared need to be further studied. Kudritzki [73] introduced a new distance determination method for B- and A-type supergiants, the Flux-weighted Gravity-Luminosity Relation (FGLR), that depends on quantitative analysis of the Balmer lines measured in low resolution spectra with good S/N ratio. It has been successfully applied to several nearby galaxies [47,74,75], demonstrating that the visually brightest stars, the A-type supergiants, have potential as distance indicators at very large distances.

### **8. The Complex Upper HR Diagram**

The upper luminosity boundary to the HR Diagram, or Humphreys–Davidson Limit, complicates our understanding of massive star evolution. Above some initial mass, ≈40–50 M, the stars do not evolve across the HR Diagram to become RSGs. Their evolution to cooler temperatures is most likely halted by proximity to the modified Eddington Limit and the accompanying high mass loss episodes. The opacity "Modified Eddington Limit", well-known since the mid-1980's [76], see the papers and discussion in [77,78], describes an instability that arises when *L*/*M* approaches the classical *L*/*MEdd* value [79]. Models based on this and other instabilities have been proposed to reproduce the upper limit in the HR Diagram [80–82]. See Humphreys & Davidson [83] for a review.

The stars then evolve back to warmer temperatures. Having shed a lot of mass, they are now increasingly subject to atmospheric instabilities before their terminal state. They thus have a different evolutionary path and different mass loss histories than their somewhat lower mass counterparts in the same galaxy or even the same cluster. These lower mass supergiants, from ≈9 or 10 up to 30–40 M or so, evolve across the HR Diagram, becoming red supergiants which alters their interior structure and with enhanced mass loss. Some may also evolve back to a warmer state, perhaps as warm hypergiants, to become LBVs, or B[e] supergiants, where their final fate is most likely as core-collapse supernovae.

Consequently, the upper HR Diagram is populated by a diversity of evolved, luminous and variable stars of different types that challenge our understanding of their physics, evolution and eventual fate. Many of them are distinguished by their emission line spectra, and evidence for stellar winds and mass loss. In addition, some of them exhibit periods of enhanced mass loss, such as the LBVs/S Dor variables and the warm and cool hypergiants with their resolved ejecta. In addition, Wolf–Rayet stars of various types, Oe and Of stars, the B[e] supergiants, and the Fe II emission line stars occupy the same parts of the HR Diagram. They may or may not be related. They could be stars of similar initial mass but in different

stages of their evolution or have experienced different mass loss histories, and some may be binaries. This diversity is one of the challenges to understanding massive stars, their evolution, and eventual fate.

The HR Diagrams below (Figure 4) for M31 and M33 show the distribution in the luminositytemperature plane of three of these mass losing classes of stars, the LBVs, B[e] supergiants and the warm hypergiants.

One of the outstanding questions is the final fate of the most massive stars. It used to be simply assumed that all stars much above initial masses of 10 M or so, would end their brief lives as some kind of supernova, but their final stage as core-collapse SNe is now in question. Smartt [84,85] suggested an upper mass limit of ≈18 M for the red supergiant progenitors of the Type II-P SNe, while Jennings et al. [86] found a lack of massive supernova progenitors in M31 and M33 and suggested an upper mass of 35–45 M. Do the most massive stars collapse directly to black holes instead, long suspected for extreme, very massive stars such as eta Carinae?

**Figure 4.** The schematic HR Diagrams for M31 and M33 showing the positions of the confirmed LBVs and candidate LBVs shown respectively, as filled and open blue circles, the warm hypergiants as green circles and the B[e] supergiants as orange circles. The LBV transits during their high mass loss state are shown as dashed blue lines. The LBV/S Dor instability strip is outlined in blue. The 15 and 20 M tracks are from [87] with rotation are shown to provide a reference for the lower mass B[e]sgs, which are probable rotators. (Higher mass tracks are not shown due to crowding.) The supergiant population is shown in the background in light gray. Reproduced from [61].

In addition, the supernova surveys have identified numerous non-terminal giant eruptions, in which the object greatly increases its total luminosity possibly expelling several solar masses and the star survives. Some of these events are confused with true SNe and thus have been called "supernova impostors". This is a diverse group of objects with a range of luminosities and possible progenitors. A few impostors appear to be normal LBV/S Dor variables in their eruptive or maximum light state. Most are giant eruptions, possibly similar to eta Car [83] from evolved massive stars, while some are red transients (ILRTs [88]) from a lower mass population. The origin of the instability in these giant eruptions is unknown, but proximity to the Eddington Limit is crucial [89]. It is not known what role these high mass loss events may play in the final stages of massive star evolution or their relation to the evolved massive star population and to other stars with instabilities such as the LBVs and the warm hypergiants.

Thus, we observe a complex upper HR Diagram with different evolutionary paths dependent on initial mass, and several types of evolved stars not only experiencing continuous mass loss, but also high mass loss events. The study of luminous stars in the nearer galaxies provides us with an improved census of these evolved stars, their relative numbers, physical properties and behavior, and clues to their evolutionary state and possible relationship to each other on the HR Diagram. In this Special Issue on luminous stars in nearby galaxies, the reviews focus on different examples of evolved massive stars; the luminous O and B-type stars in the Magellanic Clouds, LBVs, B[e] supergiants, the warm hypergiants, and the Wolf–Rayet stars.

Although the articles are about different types of evolved massive stars, many are in the same galaxies. We have therefore adopted the following distance moduli based on the Cepheid scale for these nearby galaxies for consistency and for ease of cross-referencing and comparison.


**Table 2.** Adopted Distance Moduli.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
