Paul Parsons in New Scientist:
When supernovae explode, they leave behind either a black hole or a dense remnant called a neutron star. However, recent calculations suggest a third possibility: a quark star, which forms when the pressure falls just short of creating a black hole.
Astronomers believe these form after the neutron star stage, when the pressure inside a supernova rises so high the neutrons disintegrate into their constituents – quarks. These form an even denser star than neutrons.
Observing a quark star could shed light on what happened just after the big bang, because at this time, the universe was filled with a dense sea of quark matter superheated to a trillion °C. While some groups have claimed to have found candidate quark stars, no discovery has yet been confirmed.
Now Kwong-Sang Cheng of the University of Hong Kong, China, and colleagues have presented evidence that a quark star formed in a bright supernova called SN 1987A (pictured), which is among the nearest supernovae to have been observed.
More here. [Thanks to Angus Faulkner.]