We have used the Faint Object Spectrograph on the Hubble Space Telescope to observe the spectra of SN 1987A over the wavelength range 2000 – 8000 Å on dates 1862 and 2210 days after the supernova outburst .
Even these pre-COSTAR observations avoid much of the contamination from the bright stars nearby and provide a very useful set of line strengths and shapes for analysis .
The spectrum is formed in an unusual physical setting : cold gas which is excited and ionized by energetic electrons from the radioactive debris of the supernova explosion .
The spectra of SN 1987A at this phase are surprisingly similar to those of the nova shells of CP Puppis and T Pyxidis decades after outburst .
SN 1987A and the novae are characterized by emission from material with electron temperatures of only a few hundred degrees Kelvin , and show narrow Balmer continuum emission and strong emission lines from O ^ { + } .
The Balmer continuum shape requires the electron temperature in the supernova ejecta to be as low as 500 K on day 1862 and 400 K on day 2210 after outburst .
The [ O II ] \lambda \lambda 3726 , 3728 doublet is surprisingly strong and is plausibly powered by collisional ionization of neutral oxygen to excited states of O ^ { + } .

The line intensity ratio of the [ O I ] \lambda \lambda 6300,6364 doublet obtained from Gaussian fits of the line profiles is 1.8 \pm 0.2 , contrary to the optically thin limit of 3 .
This ratio is not due to an optical depth effect , but rather is an artifact of assuming a Gaussian profile to fit the [ O I ] \lambda \lambda 6300,6364 doublet profile .
Specifying the line ratio R = F ( [ { OI } ] 6300 ) / F ( [ { OI } ] 6364 ) = 3 is consistent with the data and allows a calculation of the decomposed line profile .

All the observed strong lines are found to be blueshifted by a similar amount of 400 { km s ^ { -1 } } .
The line profiles are quite similar for lines arising from different chemical elements .
The profiles are all asymmetric , showing redshifted extended tails with velocities up to 10,000 { km s ^ { -1 } } in some strong lines .
The blueshift of the line peaks is attributed to dust that condensed from the SN 1987A ejecta which is still distributed in dense opaque clumps .
The strongest ultraviolet lines are those of Mg I \lambda 2852 and Mg II \lambda \lambda 2795,2802 .
The Mg I \lambda 2852 line is significantly broader than most lines in the optical , which provides a natural explanation for the size differences in the optical and ultraviolet of the SN 1987A ejecta derived from HST direct images .