You are an expert at summarizing long articles. Proceed to summarize the following text: 
we know that nearly all massive stars form in clusters ( see , e.g. de wit , these proceedings ) . therefore , if we wish to understand massive star formation in general , we must understand the clustered mode of star formation . super star clusters ( sscs ) are the most extreme type of stellar cluster ( and also one of the most extreme star forming environments in the universe ) , with stellar densities exceeding @xmath0 stars pc@xmath1 in their cores . as `` extreme '' objects , one might be tempted to simply think of them as cosmic curiosities . however , these objects are not only fascinating because of their extreme nature , but also because they hold clues to an important mode of star formation during the time of galaxy assembly . specifically , most present - day research strongly supports the idea that sscs are the adolescent precursors to the ancient globular clusters that are ubiquitous around massive galaxies in the local universe today . however , one should also bear in mind that the infant mortality rate of sscs in likely to be quite high ( * ? ? ? * ( possibly as high as 99% , fall & zhang 2001 ) ) , and only the most robust of these objects will have survived to the ripe old age of present - day globular clusters . the low survival rate of sscs suggests that they must have been formed prodigiously in the early universe , making this mode of star formation even more important for understanding the early evolution of of today s massive galaxies . sscs are also important components of galaxies in the local universe . these clusters can have an incredible impact on their surrounding interstellar medium ( ism ) , and in some cases even the intergalactic medium ( igm ) . each of these clusters can host up to thousands of massive stars that have powerful stellar winds , and these stars will all die in a violent way within a few million years of each other . the collective effect of these massive stars ( both in their lives and deaths ! ) can cause tremendous outflows , ionize large volumes of the ism and igm , and enrich vast amounts of interstellar material ( * ? ? ? * ; * ? ? ? * ; * ? ? ? * ( e.g. johnson et al . 2000 , heckman 2001 , martin , kobulnicky , & heckman 2002 ) )    moreover , sscs are also important tools for probing the universe . they are the most luminous type of `` simple stellar population '' ( the constituent stars share the same age and metallicity ) , which makes them very useful `` test particles '' for investigating local galaxy properties . in addition , sscs will be among the brightest point sources visible in galaxies out to large distances , which makes them very attractive targets for observations that must worry about surface brightness issues . research on sscs is rapidly approaching a crossroads , and it is therefore worth briefly reviewing the tremendous advances that have been made in this field over roughly the last decade . perhaps most importantly , we have learned that globular cluster formation continues today . this discovery came to the forefront of cluster research with the _ hubble space telescope _ , and caused a major revision of the prevailing theories of globular cluster formation and galaxy evolution . in particular , the previously accepted theories proposed that globular clusters were among the first bound structures formed in the universe , formed directly out of primordial material ( * ? ? ? * ( e.g. peebles & dicke 1968 ) ) . this idea was particularly compelling because of the similarity between the predicted jean s mass for primordial conditions , and the observed mass of globular clusters . the fact that globular clusters continue to form at the present time provides us with a window into the earlier universe when their formation was prevalent . not only did the _ hubble space telescope _ spark the flurry of research in this area , but it has also been the work - horse ; the vast majority of super star cluster studies have utilized _ hubble _ in some way . because of _ hubble _ there is a sample of globular clusters spanning almost every galaxy type , with a range of ages and metallicities . large statistical samples of sscs have been compiled , and their luminosity function ( and presumably mass function ) is well constrained ( a power - law of @xmath2 ) and roughly universal . the effective radii of many sscs have been directly measured ( typically @xmath3  pc ) , and these radii in turn provide constraints on cluster dynamics and evolution . these are only a few of the many results . the advances in this field due to _ hubble _ have been truly remarkable . however , at the time of this writing , there are no missions in the foreseeable future that will have capabilities comparable to _ hubble _ in the optical wavelength regime . given this prognosis , it is clear that further progress in ssc research will depend critically on utilizing other tools . with new observational capabilities becoming available in the infrared to radio wavelength regimes , it is likely that the next decade of ssc research will be dominated by studying their earliest stages of evolution , which these long wavelengths are particularly well - suited to probing . for this reason , understanding the details of individual massive star formation in the milky way has become all the more important for extragalactic research . there is , of course , a rich history of studying star formation in our own galaxy , as is highlighted in much of this volume . while there are clearly many important issues remaining in regard to massive star formation , a great deal has also been learned . much of this knowledge can be applied to extragalactic objects , which can not be observed with as much detail and precision as their galactic counterparts . observations of natal sscs suggest that the early stages of massive star cluster evolution may parallel those of individual massive stars observed in the milky way . in particular , natal sscs appear to have spectral energy distributions similar to ultra compact hii regions ; these infant clusters typically have been identified at radio wavelengths as compact thermal ( often optically thick ) free - free sources , and they have copious amounts of thermal infrared emission . like individual massive stars , sscs also appear to spend @xmath4% of the massive star lifetimes in a stage analogus to ultra compact hii regions ( * ? ? ? * ; * ? ? ? * ( kobulnicky & johnson 1999 , vacca , johnson , & conti 2002 ) ) . these similarities suggest that we can apply many of the tools , techniques , and ideas regarding massive star formation in our own galaxy to objects that are much more distant . figure  [ cartoon ] illustrates an analogy between the evolution of an individual massive star and a ssc .    while the analogy between individual massive stars and sscs is a good starting point , the extent to which this analogy holds true will depend on a variety of factors . for example , if star formation within a ssc is not `` instantaneous '' to within @xmath5  myr , then the constituent stars in a cluster will not all be in the ultracompact hii region stage simultaneously , which may result in different global seds . extrapolating from what we know about individual massive stars to sscs could also get us into trouble because of the extreme proto - stellar densities present in natal sscs . it seems likely that we will gain the most physical insight not necessarily from the extreme cases alone , but also from trying to understand how the properties of star formation scale between individual massive star formation and ssc formation . to date , the attempts to constrain the physical properties of natal clusters have been fairly crude , largely due to the limited observations available . as a result , we are forced by occam s razor to the most simple solution , one which requires the least free parameters . our current concept is the following : when an ssc is born , the massive stars ionize the surrounding natal material , creating an extremely dense hii region , similar to a galactic ultracompact hii region , but scaled up in mass and luminosity . this dense hii region is in turn surrounded by a dust cocoon that is presumably quite warm on the inner boundary . the constituent ionizing stars may be surrounded by individual hii regions and dust cocoons in the earliest stages , but this is currently unknown . clusters that are more sparse are likely to resemble ultracompact hii region complexes in the milky way ( such as w49a ) , and contain spatially discrete hii regions and cocoons , but the existence of individual cocoons becomes less tenable in the extreme stellar densities of the cores of massive sscs .    these components of a natal cluster have separate observational signatures . the dense hii regions are characterized by an `` inverted '' radio spectral energy distributions due to optically thick free - free emission ( @xmath6 , where @xmath7 ) . the frequency at which the radio emission becomes optically thick is largely dependent on the density of the hii region . in the earliest stages of cluster evolution , most or all of the stellar luminosity is reprocessed by the surrounding dust cocoon , and this cocoon is observable at thermal infrared to sub - millimeter wavelengths . the nature of the resulting spectral energy distribution depends on whether the dust in the cocoon is smooth or clumpy ( as will be discussed in   [ models ] , but this is not currently constrained by observations . existing observations have provided _ estimates _ for some of the physical properties of natal clusters , including size , electron density , pressure , stellar mass , hii mass , dust mass , and age . for the most nearby clusters , their size can be directly measured using high spatial resolution observations . for more distant clusters , model hii regions can be used to fit the radio spectral energy distribution and infer both the radius and electron density . the embedded sscs that have been analyzed in this way ( * ? ? ? * ( e.g. those in he  2 - 10 , johnson & kobulnicky 2003 ) ) have hii region radii of only a few parsecs , and the electron densities have global values of @xmath8  cm@xmath1 , with peak values in excess of @xmath10  cm@xmath1 . assuming temperatures of @xmath11  k , these densities imply global pressures of @xmath12  k  cm@xmath1 and peak pressures reaching values of @xmath13  k  cm@xmath1 . the masses of the natal clusters , hii regions , and dust cocoons can be inferred from radio and infrared observations . the optically - thin thermal flux ( typically measured with the highest available radio frequency ) can be used to infer a total ionizing luminosity , and the ionizing luminosity can be translated into an embedded stellar mass by assuming a stellar initial mass function . using this technique , clusters with a range of masses have been detected ( spanning modest ob - associations to massive sscs ) . the hii mass can be estimated given the radii and densities for the ionized regions . in at least some cases , the hii mass has anomalously low values ( * ? ? ? * ( @xmath14% of the stellar mass , johnson & kobulnicky 2003 ) ) , which we tentatively interpret as a sign of the youth of the hii regions . the mass of the dust cocoons around embedded sscs has also been estimated in a few cases using infrared observations ; these estimates have suggested dust masses of @xmath15 for massive sscs in sbs0335 - 052 and he  2 - 10 ( * ? ? ? * ; * ? ? ? * ( plante & sauvage 2002 , vacca , johnson , & conti 2002 ) ) . a critical and unresolved issue in the study of massive star clusters is whether sscs are just the statistical tail of a continuous mass distribution that extends all the way down to individual massive stars , or whether they can only be formed in special environments . while there is some evidence that supports each side of this debate , the answer has been clouded by observational and evolutionary issues . in typical optical studies , a cluster s age , extinction , and mass are all contributing factors to its observed luminosity and inferred mass . moreover , as clusters evolve , the lower mass clusters will tend to be disrupted on time - scales as short as a few million years ( * ? ? ? * ( portegies zwart et al . 2002 ) ) , causing further confusion when interpreting cluster luminosity and mass functions . radio observations of natal clusters are immune to reddening and sample only the youngest clusters before they have had time to evolve . as a result , only radio observations are capable of directly sampling the cluster mass function at the time of their birth . the starburst galaxy henize 2 - 10 is one of the nearest galaxies ( 9  mpc ) known to host multiple natal sscs detected in radio and infrared observations , as well as numerous sscs that have already emerged to be visible in optical light ( * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ; * ? ? ? * ( kobulnicky & johnson 1999 , johnson et al . 2000 , vacca , johnson , & conti 2002 , johnson & kobulnicky 2003 , cabanac , vanzi , & sauvage 2005 ) ) . these properties make henize  2 - 10 a very important case study for examining the properties of natal sscs . in our previous radio observations , four natal sscs were detected , all with inferred masses in the range of @xmath16 . although the observations were sensitive enough to detect sscs with somewhat lower masses , less massive natal clusters masses were not apparent . given the linear resolution of the observations used for these previous radio studies of @xmath17  pc , it seemed to be likely that some lower mass clusters were missing due to crowding and confusion .    in order to address the question of whether henize  2 - 10 is only forming sscs with masses of @xmath18 , or whether lower mass clusters are present , but had not yet been detected or disentangled from the more luminous sources , we obtained new high - resolution observations with the very large array using the additional pie town antenna . a comparison between the old and new radio observations is shown in figure  [ he210 ] ( * ? ? ? * ( biswas & johnson in prep ) ) . in the new observations , it is clear that many of the previously known radio sources are composed of multiple components . although these observations are also roughly an order of magnitude more sensitive than the previous observations , it is not clear that any additional natal sscs exist outside of the main star forming region apparent in the previous radio observations . the analysis is still underway at the time of this writing , but it is clear that these new high - resolution and extremely sensitive radio observations will shed light on many issues related to ssc formation . with the _ spitzer space telescope _ providing a wealth of new observations , and _ herschel _ , _ sofia _ , and alma on the horizon , appropriate physical models of natal sscs are required in order to analyze data from these facilities . while the simple conceptual picture discussed in [ properties ] is a good place to start , we know that the universe is complicated , and sometimes nice well - behaved uniform spheres just will not fit the data . one of the main difficulties that must be overcome in order to properly model star forming environments is that the ism is not smooth , and therefore 1-d models can not reproduce physical structures that are consistent with the observed ism . we have produced a suite of models of natal sscs using the 3-d monte carlo radiation transfer code described by ( * ? ? ? * whitney et al . ( 2003 ) ) . these models allow for arbitrary density distributions , and we have invoked a clumpy fractal structure , consistent with observations of the ism . the differences in the spectral energy distribution ( sed ) between smooth and clumpy models can be very significant , as shown in figure  [ sed ] . one of the main differences is that clumpy dust allows more near - infrared light ( both scattered and direct from the central source ) to escape . more reprocessing of the photons also occurs in smooth dust cocoons , which changes the shape of the resulting sed . spectral features , such as the silicate line at @xmath19 m can also vary dramatically depending on the clumpy fraction for model with otherwise identical parameters ( the silicate feature is much more strongly absorbed in models with a larger smooth fraction ) . one important result is that the mid - infrared sed of the same object can vary significantly depending on the viewing angle if the dust cocoon is moderately clumpy . this is very bad news from an observational perspective ; this degeneracy makes it impossible , regardless of the number of data points that sample a given sed , to determine a unique set of physical parameters that will fit the observations . the good news is that the degeneracy introduced by clumping is less significant at far - infrared wavelengths , although these wavelengths also suffer from poor spatial resolution . these results should serve as a strong warning that can not be overstated to investigators with thermal - infrared observations : _ using 1-d models could lead to a significant misinterpretation of the data . _    there is a vast amount of parameter space to explore , even without allowing for 3-d structure . the current study is limited to investigating the effects of a varying the inner and outer radius of the dust cocoon , total initial dust mass of the cocoon ( which directly reflects the star formation efficiency ) , and the fraction of the dust that is clumpy . within this limited parameter space , we have simulated an evolutionary sequence for sscs as they go from deeply embedded to only having a thin dust shell . the results from this evolutionary sequence can be directly compared to observed colors of ultra compact hii regions . figure  [ wc ] shows a comparison between the colors of natal sscs from our pseudo - evolutionary sequence produced by our 3-d models and the observed _ iras _ colors of ultra compact hii regions as well as `` field '' objects ( * ? ? ? * ; * ? ? ? * ( johnson et al . in prep , kurtz , churchwell , & wood 1994 ) ) . overall , there is excellent agreement between our model colors and the observed colors of ultra compact hii regions . in general , as a natal ssc moves through our pseudo - evolutionary sequence ( its inner radius becomes larger and therefore the dust becomes cooler ) , its _ iras _ colors become more red . the most deeply embedded objects lie in the lower left region of the spread of model points , which reflects the extremely hot dust in the inner portions of the dust cocoons . if these objects with such hot dust exist , the lifetime of this state is likely to be extremely short . therefore , embedded star forming regions with such hot dust should be quite rare , possibly accounting for the lack of ultra compact hii regions detected with colors as blue as the models can produce . the model colors also extend further red - ward than observed for ultra compact hii regions , but this is not surprising ; as an embedded star forming region evolves and begins to emerge from its cocoon , its thermal - infrared luminosity will fall precipitously . as a result , we expect that the existing samples of ultra compact hii regions become incomplete at these later stages ( or perhaps objects that are this evolved are no longer considered to be ultra compact hii regions ! ) . there are no lack of questions regarding the birth of sscs , and given the new observatories that are becoming available , it is an exceptionally good time to be asking these questions . a few major questions keep recurring that i want to highlight here :    \(1 ) how does star formation vary between individual massive stars and sscs ( and is this reflected in the stellar initial mass function ) ? \(2 ) what are the environmental requirements necessary to form a ssc , and how do they differ from individual stars ? there are also a variety of properties that it would be nice to know more about . for example , it would be instructive to _ directly _ measure densities , pressures , and temperatures using various line diagnostics at ir to radio wavelengths . another quantity that might be particularly useful for theorists ( that is almost completely unconstrained ) is the star formation efficiency in these objects . we also have no idea how much ionizing radiation could be escaping from the natal cocoons ( and it could be quite a lot if the dust is clumpy ) . in fact , we know almost nothing about the natal dust cocoons ( including the temperatures , grain compositions , and geometry ) . this list is certainly not exhaustive . an understanding of these quantities , and how they relate to the properties of the resulting cluster , will eventually provide a great deal of insight into the requirements for massive star cluster formation . over the next decade ( or so ) , a number of powerful new facilities are going to become available at infrared to radio wavelengths . these observational capabilities are likely to bring about a renaissance in the study of the formation of massive clusters . these facilities will be incredibly well - suited to observing natal sscs ( figure  [ obs ] ) , and will allow us to study natal sscs in amazing detail . surely the new observations will bring about many new discoveries as well as many more new questions . the work has benefited from many fruitful discussions with my collaborators , including i. biswas , p. conti , m. goss , l. hunt , r. indebetouw , h. kobulnicky , w. vacca , b. whitney , and k. wood . i also gratefully acknowledge support for this work provided by the nsf through and astronomy and astrophysics postdoctoral fellowship , and by nasa through the hubble fellowship grant # 01173.02-a through the space telescope science institute , which is operated by the associated universities for research in astronomy , inc . , under nasa contract nas5 - 26555 .
super star clusters are one of the most extreme star forming environments in the universe , and the most massive and dense of these may be proto globular clusters . like individual massive stars , the earliest stages of super star cluster evolution are deeply obscured , and therefore our knowledge about their birth environments is currently very incomplete . 
 however , the study of natal super star clusters has become somewhat of a cottage industry in recent years , and the sample of such objects has been growing rapidly with high - quality long - wavelength data now available from a number of observatories . 
 the natal super star clusters identified in thermal - infrared and radio observations represent the youngest stage of massive star cluster evolution yet observed . 
 their properties appear to be similar to those of ultracompact hii regions in the milky way , but scaled up in total mass and luminosity . 
 i will overview what we think we know about these objects based on existing observations , discuss their relationship to ultracompact hii regions , present new models of their spectral energy distributions based on 3-d simulations , and outline some of the most significant gaps in our current understanding .