Astrophage

Astrophage (Latin: star eater) is a spacefaring alien microbe that feeds directly on the surface heat of stars. Astrophage have the unique ability to absorb energy, convert it to mass, then back to usable energy, which is how they feed and survive on stars. In the absence of predators, Astrophage feeding can significantly diminish a star's radiant output, to the point where life on its orbiting planets is threatened or even wiped out.

Description
Astrophage is a hyperthermophilic single-celled organism similar in size and complexity to terrestrial eukaryotic cells. An Astrophage cell is spherical and 10 microns in diameter. Despite their alien origin and extremely hot environment, they are very similar to Earth cells - they are carbon-based, contain mostly water, and even have organelles similar to those of Earth eukaryotes. Live Astrophage are always completely black and opaque, but dead Astrophage are translucent enough for the interior to be seen.

Super cross-sectionality
The unique abilities of Astrophage depend on a property called super cross-sectionality, which is poorly understood and seen nowhere else in nature. This property renders it impossible for anything to quantum-tunnel through the Astrophage, effectively making opaque to all forms of radiation. Astrophage is even opaque to wavelengths of light larger than itself and to neutrinos, both of which should be impossible. This allows Astrophage to store neutrinos inside itself as an energy reserve.

Only live Astrophage have this property. It dissipates immediately upon cell death.

Mass-energy conversion
Astrophage feed and maintain their temperature by converting thermal energy into mass. Astrophage cell membrane contains hydrogen ions - free protons - at a temperature of 96.415 degrees Celsius, the Astrophage's "critical temperature." At this temperature, the kinetic energy of two colliding protons can be converted into a pair of neutrinos. If the temperature outside the Astrophage cell is greater than 96.415 degrees Celsius, the protons can collide and produce neutrinos while still having leftover kinetic energy, so that they can continue colliding with other protons and generate more neutrinos. This means that any heat energy above the critical temperature is quickly added to the Astrophage's neutrino store without the Astrophage actually getting any hotter, even on the surface of a star. An Astrophage cell can store up to 1.5 megajoules of energy - about 17 nanograms of neutrinos, significantly more than the Astrophage's actual biomass - in this way.

Any two such neutrinos that collide annihilate and produce photons of the "Petrova wavelength," named after its discoverer - infrared light with a wavelength of 25.984 microns. Astrophage can release this energy to power its metabolism, heat itself if it drops below the critical temperature, and propel itself. Astrophage can release Petrova-wavelength light to propel itself at immense velocities and accelerations, reaching up to 92% of the speed of light.

Metabolism
Astrophage can feed on any heat source hotter than their critical temperature, but the photospheres of stars are their ideal feeding grounds. This environment, generally at a temperature of several thousand Celsius, is the richest possible energy source for Astrophage. While this energy constitutes Astrophage's diet, it is still an organic creature that needs carbon compounds to metabolize, grow, and reproduce, much like a plant does. Astrophage acquires its carbon from atmospheric carbon dioxide. Since carbon dioxide is not present on stars, Astrophage must travel to planets to obtain it.

Reproduction and life cycle
Astrophage reproduces via mitosis. To breed, it travels to a planet with an atmosphere rich in carbon dioxide, where it aerobrakes into the atmosphere. The thermal energy generated by the relativistic Astrophage's collision with the air is converted into neutrinos, giving the Astrophage a store of energy and preventing it from burning up. The Astrophage comes to a stop when the air reaches a pressure of 0.02 atmospheres, at which point it consumes carbon dioxide to make the biomass necessary to make a copy of itself. Once the Astrophage has completed this process and divided, both mother and daughter cell fly back to the star to feed.

The migratory route of an Astrophage population is visible in the Petrova wavelength, appearing as a line that extends from the north pole of the feeding star to engulf the entire surface of the breeding planet. This is called a "Petrova line," and is seen in every system with an Astrophage population.

Astrophage can spore, and in this state can travel as far as eight light-years. How or why they do this is unknown.

Ecology
Astrophage is indigenous to the Tau Ceti system, where it feeds off the star itself and breeds on the planet Tau Ceti e, or Adrian. Adrian is a super-Earth planet, 3.93 times the mass of Earth, with a thick atmosphere dominated by carbon dioxide and methane. Adrian’s atmosphere is home to a rich biosphere of methanogenic microbes, some of which is adapted to live and travel in space. In fact, it is likely that all life in the solar systems near Tau Ceti, including life on Earth and Erid, is ultimately descended from remote ancestors of Adrian life that spread out into interstellar space. Astrophage is a natural part of this biosphere - its Petrova line provides a habitat for some other microbes, and it is food for a predator called Taumoeba.

In Tau Ceti, Astrophage is almost completely benign. Its numbers are kept in check by Taumoeba, ensuring that it never grows numerous enough to significantly lower Tau Ceti’s energy output.

However, Astrophage can travel to other solar systems. Once there, it begins feeding on the star and breeding in the atmosphere of the planet richest in carbon dioxide. With no predators in these other solar systems, Astrophage breeds out of control at an exponential rate. The Astrophage population only levels out once the star’s output is reduced by about 10%, enough to wipe out all life on orbiting planets. This is the fate that threatens both Earth and Erid.