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the first known supernova observed with vlbi was the type ii supernova 1979c , which was only the second supernova detected in the radio , and bartel ( 1985 ) reported a determination of its angular size .
the first vlbi image identified as that of a supernova was one of 41.95 + 575 in m82 by wilkinson & de bruyn ( 1984 ) , although the recent results have cast some doubt on the identification as a supernova ( mcdonald et al .
bartel et al .  (
1987 ) published a higher resolution image of 41.95 + 575 , and then bartel et al .
( 1991 ) published an image of sn  1986j .
since then , radio imaging of approximately ten supernovae has been carried out ( see table  1 below ) , where i use the word `` imaging '' somewhat loosely , meaning supernova for which we have some morphological information , including size determinations , from radio observations .
i will attempt to briefly summarize the state of radio supernova ( rsn ) imaging .
radio imaging is an important tool for the study of supernovae , at least those that are radio bright , ie .
if fact , vlbi radio observations are presently the only means to directly image the expanding shell of any supernova much farther away than sn  1987a . the radio emission is generated as the expanding shell of supernova ejecta interacts with the progenitor star s circumstellar medium ( csm ) which generally consists of its stellar wind .
the expanding shell of ejecta generates two shocks , a forward shock which propagates into the csm and a reverse shock which propagates into the stellar ejecta .
chevalier ( 1990 and references therein ) showed that in the case of density distributions for the csm and the ejecta that are power laws in radius , the solution is self - similar and the supernova shell expands such that its radius , @xmath1 , where @xmath2 is time since the explosion and the parameter @xmath3 is commonly known as the deceleration parameter .
the value of @xmath3 will depend on the density distributions of both the csm and of the ejecta , and @xmath4 indicates free expansion .
= 0.60    = 0.86    the interaction region between the ejecta and the csm is an important laboratory for studying shock acceleration , since supernovae are thought to be responsible for the majority of cosmic rays
. we can also learn about both the supernova and the csm .
furthermore , since the speeds of the ejecta are usually several orders of magnitude faster than those typical of red giant winds , the supernova shock over - runs the wind , and thus the interaction region acts like a time - machine , and has the potential to reveal the last thousands of years of the progenitor s wind history . finally , by equating the angular expansion velocity measured with vlbi with the radial expansion velocity measured spectroscopically , a direct , geometric distance determination can be made ( see bartel & bietenholz , this volume ; bartel 1985 ) .
the early vlbi observations of sn  1979c and sn  1986j allowed measurement of their sizes and determination of their expansion curves . figure  [ bieten_f86exp ] shows the expansion curve to date of sn  1986j ( see bietenholz , bartel , & rupen 2002 ; for sn  1979c , see bartel & bietenholz , this volume ) .
a power law , as suggested by the chevalier mini - shell model , indeed provides a good fit to the data , with @xmath5 ( but see prez - torres et al .
2002 for a different result ) .
a radio image of sn  1986j is shown in figure  [ bieten_fgallery ] .
several decades - old supernovae have been observed in m82 ( see e.g. , mcdonald et al .
2001 ) , and images of two are shown in figure  [ bieten_fgallery ] . for 43.31 + 592 ,
@xmath3 could be determined to be 0.7 .
radio images of sn  1987a have been obtained using the australia telescope ( see manchester et al .
2002 ) and are also shown in figure  [ bieten_fgallery ] .
its expansion can be shown to have been strongly decelerated .
= 0.85    = 0.93    by far the best studied rsn so far is sn  1993j in the nearby spiral galaxy m81 .
two independent vlbi campaigns have observed it extensively ( see bietenholz , bartel , & rupen 2003 ; bartel et al .
2000 ; and marcaide et al .
2002b , 1997 ; and references therein ) .
the quality of the data allowed both vlbi groups to independently determine departures from simple power - law expansion , with bartel et al .
( 2002 ) reporting that @xmath6 was near unity early on , decreased to @xmath70.8 by @xmath8  d , and then increased slightly after that ( see figure  [ bieten_f93exp ] ) .
mioduszewski , dwarkadas , & ball ( 2001 ) performed simulations of an expanding supernova shell , based on a particular model of the ejecta profile , and found changes in @xmath6 remarkably similar to those in fact observed for sn  1993j ( e.g. , bartel et al .
after 10 years , sn  1993j has slowed to less than half its original expansion velocity , which implies that the swept up material is now comparable in mass to the ejecta , and for reasonable mass - loss rates of the progenitor , suggests that the ejecta have a relatively low mass of @xmath9 , and that the progenitor had lost most its envelope mass to a binary companion prior to the supernova explosion .
the images of sn  1993j , shown in figure  [ bieten_f93img ] show a wealth of detail ( see http://www.yorku.ca/bartel/snmovie.html for a movie of the expanding supernova ) . in projection
, the supernova has remained very circular , and phase referencing has shown that it expands isotropically from the explosion center to within 5.5% ( bietenholz , et al .
2001 , 2003 ) .
the shell structure is clearly visible in all the adequately resolved images .
there are , however , distinct departures from circular symmetry even at the earliest resolved epochs , where there is a pronounced minimum to the west and a maximum to the east - southeast ( fig .
[ bieten_f93img ] ) . even after 10 years
, no sign of a pulsar nebula has yet been seen at the center .
however , it seems probable that the ejecta in the interior of the shell are dense enough that the radio optical depth due to free - free absorption will remain large for several decades , and hence obscure any emerging pulsar nebula .
l l c c c c l c name & host & distance&type & expansion & @xmath10 & morphology & ref.@xmath11 + & galaxy & & & speed & + & & ( mpc ) & & ( @xmath12 km  s@xmath0 ) & + sn 1979c & m100 & 16 & ii & 12 & 0.95 & circular & 3 + sn 1980k & ngc6946 & 6 & ii & @xmath13 & & & 1 + sn 1986j & ngc891 & 10 & ii & 6 & 0.7 & distorted shell & 5 + 41.95 + 575@xmath14 & m82 & 3 & & @xmath15 & & asymmetric & 7 + 43.31 + 592 & m82 & 3 & & 9.7 & 0.7 & circular shell & 7 + 44.01 + 596@xmath14 & m82@xmath16 & 3 & & & & circular shell & 7 + sn 1987a & lmc & 0.05 & iipec & @xmath17 & @xmath18 & circular shell & 6 + sn 1993j & m81 & 4 & ii & 8.5 & 0.81 & circular shell & 2 + sn 1994i & m51 & 8 & ic & @xmath19 & & & 4 + sn 2001gd & ngc5033 & 13 & ii & @xmath20 & & & 8 +    table  1 gives an overview of the characteristics of the rsne for which we have some morphological information . of the well - resolved supernovae , all but 41.95 + 575 , whose identification as a supernova is uncertain , show a discernible , fairly circular shell morphology .
the observed shell structures are clear evidence that the bulk of the radio emission originates from the interaction of the expanding shells with their surroundings , rather than from nebulae around the young pulsars , as had also been suggested . despite the relatively circular shells ,
significant intensity modulation with position angle seems the norm .
none of the imaged rsne show a clear case of bisymmetric structure , as might be expected from axisymmetric stellar winds .
there is evidence , however , most compelling from sn  1986j , for some more asymmetric structure , possibly in the form of protrusions or jets .
the origin of the asymmetries and protrusions is not yet well understood .
the observed mostly circular morphologies , do , however , suggest mostly spherical shells , which allow direct distance determinations to be made ( e.g. , bartel & bietenholz , this volume ; bartel 1985 ) .
power - law expansion curves , with @xmath21 , provide a first order fit to all supernovae whose sizes have been determined over a reasonable time baseline except for sn  1987a , which is probably in a much lower density environment than the other radio - bright supernovae .
however , at least in the case of sn  1993j , distinct departures from strict power - law expansion can be seen , implying significant structure in the ejecta and/or the csm . in both
sn  1993j and sn  1986j , the source structure has been shown to change non self - similarly as the supernova expands .
there is an intriguing change in the radio spectrum of sn  1986j , with a new spectral component that peaks at @xmath2220  ghz emerging after @xmath2210  yr , possibly related to a new compact component in the images , that may perhaps be related to a pulsar nebula ( see bietenholz et al .
2002 ) .
there is still considerable scope for the more detailed understanding of supernova shell dynamics , and for determining the structure of the csm and of the ejecta through hydrodynamic simulations , particularly in the case of sn  1993j where exceptionally good data are on hand .
currently , there are only a couple of rsne per decade that are close enough and bright enough to image .
future improvements in sensitivity , e.g. , the mark  iv vlbi recording system , the evla , and the ska , will allow us to image somewhat more rsne , but will only enable a modest increase in the resolution available to global vlbi ( by going to higher observing frequencies ) .
the practical distance limit is on the order of 30  mpc , where global vlbi at 22  ghz gives a linear resolution of @xmath238000  au .
there have been approximately 8 rsne per decade with a 22-ghz flux density @xmath24  mjy since 1980 , so even with the ska , the number of rsne available for imaging will be limited .
it will therefore be important to image those rsne that are available to us , since they will provide us with direct distance estimates , test our understanding of the supernova process , which may be crucial to understanding the distant , un - resolved supernovae upon which cosmological results are based , and allow us to probe the circumstellar environments and thus the wind - history of evolved stars , and allow us maybe to see the birth of a pulsar nebula .
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as the shock and expanding shell of a supernova plow out through the circumstellar material at thousands of km s@xmath0 , radio emission is generated . 
 vlbi observations of this radio emission are presently the only means to directly image the expanding shell of any supernova much farther away than sn  1987a . 
 the last decade has seen great progress in vlbi imaging of radio supernovae . in particular 
 , sn  1993j in the galaxy m81 provided a rare opportunity to closely study an expanding supernova , and has been intensively observed . 
 i summarize some of the results on sn  1993j and other radio supernovae , and compare the different observed radio supernovae . 
 i briefly discuss the future prospects of radio supernovae imaging .    
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