Dragon's Egg (1980) is a neutron star that has wandered into the solar system; superheavy, only 20 miles in diameter, it rotates five times a second, has a superpowerful magnetic field, a surface gravity of 67 billion g--and is inhabited by sesame-seed-sized, intelligent alien cheela, who live so rapidly that a single human hour is equivalent to a cheela generation! In this sequel, as in Egg, the most fascinating and worthwhile elements are Forward's ingenious ideas on cheela science and engineering: the tiny aliens quickly learn to manipulate superdense objects, black holes, super-strong magnetic fields, gravity, and space-time (in the span of a few hours, they send scouts to visit nearby galaxies). Thus, cheela science soon progresses far beyond that of the human observers. But then a devastating ""starquake"" wipes out most life on Dragon's Egg; the survivors revert to barbarism. To help the cheela, the human observers donate one of the asteroids that allows them to survive the powerful gravity tides generated by Dragon's Egg. And, a few hours later, the cheela are once again progressing in leaps and bounds, pausing only in gratitude to invent a time-warp to save their human rescuers. The recipe as before, then: dazzling, beautifully worked-out scientific extrapolations, along with fairly rudimentary characters, drama, and plot. An adventure that's sure to please fans of ""hard"" sf.