SN 1885 was a probable subluminous Type Ia supernova which occurred in the bulge of the Andromeda galaxy , M31 , at a projected location 16 ^ { \prime \prime } from the nucleus .
Here we present and analyze Hubble Space Telescope images of the SN 1885 remnant seen in absorption against the M31 bulge via the resonance lines of Ca i , Ca ii , Fe i , and Fe ii .
Viewed in Ca ii H & K line absorption , the remnant appears as a nearly black circular spot with an outermost angular radius of 0 \hbox { $ . ^ { \prime \prime } $ } 40 \pm 0 \hbox { $ . ^ { \prime \prime } $ } 025 , implying a maximum linear radius of 1.52 \pm 0.15 pc at M31 ’ s estimated distance of 785 \pm 30 kpc and hence a 120 yr average expansion velocity of 12 { , } 400 \pm 1400 { km } { s } ^ { -1 } .
The strongest Ca ii absorption is organized in a broken ring structure with a radius of 0 \hbox { $ . ^ { \prime \prime } $ } 2 ( = 6000 km s ^ { -1 } ) with several apparent absorption ‘ clumps ’ of an angular size around that of the image pixel scale of 0 \hbox { $ . ^ { \prime \prime } $ } 05 ( = 1500 km s ^ { -1 } ) .
Ca i and Fe i absorption structures appear similar except for a small Fe i absorption peak displaced 0 \hbox { $ . ^ { \prime \prime } $ } 1 off-center of the Ca ii structure by a projected velocity of about 3000 km s ^ { -1 } .

Analyses of these images using off-center , delayed-detonation models suggest a low ^ { 56 } Ni production similar to the subluminous SN Ia explosion of SN 1986G .
The strongly lopsided images of of Ca i and Fe i can be understood as resulting from an aspherical chemical distribution , with the best agreement found using an off-center model viewed from an inclination of \sim 60 ^ { \circ } .
The detection of small scale Ca II clumps is the first direct evidence for some instabilities and the existence of a deflagration phase in SNe Ia or , alternatively , mixing induced by radioactive decay of ^ { 56 } Ni over time scales of seconds or days .
However , the degree of mixing allowed by the observed images is much smaller than current 3D calculations for Rayleigh-Taylor dominated deflagration fronts .
Moreover , the images require a central region of no or little Ca but iron group elements indicative for burning under sufficiently high densities for electron capture taking place , i.e. , burning prior to a significant pre-expansion of the WD .
Using time-dependent ionization calculations , we show that the presence today of neutral ions in this 120 yr old remnant can be understood as ejecta self-shielding from the UV radiation in the M31 bulge .