Why did supernova 1987A's "String of 'Cosmic Pearls'" appear to be so lumpy 20 years later? Is it still? NASA's A String of 'Cosmic Pearls' surrounds an exploding star is beautiful and the page says:

This image shows the entire region around the supernova. The most prominent feature in the image is a ring with dozens of bright spots. A shock wave of material unleashed by the stellar blast is slamming into regions along the ring's inner regions, heating them up, and causing them to glow. The ring, about a light-year across, was probably shed by the star about 20,000 years before it exploded.

Did the ring break up into &quot;pearls&quot; or lumps due to some kind of instability? Why so lumpy? The image was taken 20 years after the supernova was first observed, does the ring still look like this?

Also see https://en.wikipedia.org/wiki/File:HST_SN_1987A_20th_anniversary.jpg
The &quot;lumpiness&quot; is (mostly) inherent to the ring, and is probably the result of instabilities in the outflowing gas that gave rise to it in the first place. E.g., Akashi et al. (2015):

We suggest that clumpy dense outflowing equatorial rings around
evolved giant stars, such as in supernova 1987A and the Necklace
planetary nebula, are formed by bipolar jets that compress gas towards
the equatorial plane. The jets are launched from an accretion disc
around a stellar companion.... we perform 3D numerical simulations,
and show that bipolar jets expanding into a dense spherical shell can
compress gas towards the equatorial plane and lead to the formation of
an expanding equatorial ring. Rayleigh-Taylor instabilities in the
interaction region break the ring to clumps.

What you're seeing in the image is emission due to heating from the impact of the expanding supernova shock wave. Some of the clumpiness may be exaggerated by the fact that denser clumps of gas can radiatively cool more efficiently than diffuse gas, as explained by Fransson et al. (2015):

If the density of a clump is sufficiently high, radiative cooling will
cause the clump to collapse to an even higher density. The result will
be an optically emitting hotspot (Pun et al. 2002). If instead, the
density of a clump falls below the threshold density for radiative
cooling, the gas behind the transmitted shock will radiate X-rays and
very little optical radiation. This threshold density is a sensitive
function of the velocity, ${{V}_{{\rm clump}}}$, of the shock entering
the clump. Thus, at any given time, the requirement for radiative
cooling acts as a high contrast filter, limiting optical radiation to
only the denser clumps.

The brightness of the ring has faded somewhat since that image was made (more on the east side than on the west), as shown in this composite of HST images (Figure 3 of Larsson et al. 2019):

The figure in the paper itself includes an animation of the evolution.