A Xeelee flower is a small flower like device used to make construction material. It works when the control unit is moved into direct sunlight.
The Xeelee Flower, written 1986, is a short story and the first published work by Stephen Baxter in the Xeelee sequence. In it, the device known as the Xeelee flower is introduced. In 4922 A.D, during the Squeem occupation, a star know as Goober's Star is reaching the end of its lifespan. A planet in orbit around the star is inhabited by Xeelee, who are evacuating in preparation for the supernova. (In this first work, the Xeelee are far less advanced than in the rest of the sequence, a property which is mostly retconned in the later series.)
The Squeem employ Jones, a human to travel to the planet after the Xeelee exodus and before the explosion, to hunt for Xeelee technology carelessly left behind in the evacuation. The sole device found is the flower, described as the following:
"Six angular petals, which looked as if they were made of Xeelee sheeting, were fixed to a small cylindrical base; the whole thing was about the size of my open hand ... Half the base clicked off in my hand."
It was found that in light, the mass of the flower would slowly increased, and the mass gained corresponded to the perfectly efficient conversion of the energy of the light to mass. The device was considered highly significant by the Squeem, who ordered the immediate return to the mothership before the detonation of Goober's Star.
The Squeem miscalculated, however, and they were forced to abandon Jones on the surface when the star exploded early, who then made it to orbit and used the flower as a shield. The energy of the nova was converted to Xeelee construction material, which was easily capable of resisting the blast. The flower grew enormous in size, and Jones deduced that the detachable base regulated whether the energy-absorbing abilities of the flower were active. He then planned to allow the growth of the flower to continue unchecked, which would disrupt the structure of the galaxy within mere centuries. He kept the base a secret, and promised to reveal how to stop the growth if humankind were given the secret of the Squeem hyperdrive.
The plan was successful, and the humans easily threw off the primative Squeem occupation of the solar system when their only technological advantage was removed. Humans were again able to expand into the universe, recontacting old extrasolar colonies and expanding faster than light with the new hyperdrive technology.