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Stellification is a theoretical process by which a brown dwarf star or Jovian-class planet is turned into a star, or by which the luminosity of dim stars is greatly magnified.

Luminosity magnification
The fusion reaction is strongly dependent upon temperature. For proton-proton reactions such as found in Earth's sun, the reaction rate scales with the fourth power of temperature (T4). For other reactions such as the CNO cycle, the proportionality can be as high as T20. Thus, increasing the temperature of the star even a small amount (for example by using reflective solar sails), would create a large increase in power output, resulting in a much higher equilibrium temperature, and therefore luminosity, of the star.

Black hole seeding
Brown dwarf stars and gas-giant planets do not achieve sustained fusion, as they contain insufficent mass to gravitationally compress the reactants to the degree required to initiate a reaction. If the density of the star or planet could be increased, fusion could be initiated. One such method is to "seed" the body with a black hole. Although the black hole would initially start swallowing the body, the huge output of radiation caused by this would resist the flow of further material. The rate of infall is bound by the Eddington limit, which shows that the luminosity of the resultant star (in Watts) would be equal to approximately six times its mass (in kilograms).

It has been suggested that this could be achieved by placing an asteroid in orbit around the black hole, and using a mass driver to direct a stream of matter into it. This could be used to move the black hole either via simple conservation of momentum, or by harnassing the power generated as a result. Zubrin (1999) suggests that a luminosity 1/10,000th that of our own sun would be required to create Earth-like temperatures on planets in close orbit to a brown dwarf, thus requiring a black hole with a mass of 6.1 × 1021kg (about 8% the mass of Earth's moon).

Thermonuclear ignition
It is well established that Jovian-class planets consist mostly of hydrogen and helium. It is theorised that concentrations of hydrogen and helium isotopes at certain depths of a gas-giant planet may be sufficient to support a fusion chain reaction, if sufficient energy can be delivered to ignite the reaction. Scientists have proposed that a nuclear warhead, heavily shielded and able to withstand pressures of up to 1000 MPa may be able to reach a depth of 1000 km in the atmosphere of Jupiter, potentially deep enough to reach high concentrations of isotopes and ignite a fusion reaction.

Terraforming of Mars
The stellification of Jupiter or Saturn has been proposed as a method of terraforming Mars via the Bosch reaction. The ignition of a gas giant results in the planet shedding its outer layers - about 10% of its mass - into space. This ejecta would consist mostly of hot ionized hydrogen. If the ignition were to take place when Mars and Jupiter were at their closest approach of approximately 3.7AU, the atmosphere of Mars would receive about 7.15 × 1015kg of hydrogen. The same process performed with Saturn would add 0.45 × 1015kg. The hot H+ ions would then react with the CO2 in the Martian atmosphere, depositing graphite on the surface and adding water vapour to the atmosphere. Assuming a 10% efficiency of the Bosch reaction, the Martian surface would be covered with 4.47kg/m2 of graphite (or 0.93kg/m2 in the case of Saturn). The reaction would also raise the temperature of the atmosphere by around 78 K (Jupiter) or 16 K (Saturn).

The effect on Mars would be three-fold. First, the direct heating of the atmosphere would bring the mean surface temperature above 0°C. It is speculated that a temperature rise of 4 − 25 K is required to sublimate the carbon dioxide in the planet's ice-caps, thus triggering a runaway greenhouse effect. Second, the addition of water vapour to the atmosphere would initiate a Martian hydrocycle. This is particularly important as Mars has very little native hydrogen reserves. The water vapour would also contribute to the greenhouse effect. Finally, the deposition of large quantities of dark, light-absorbing graphite on the planet's surface would reduce Mars' albedo, resulting in a further temperature increases. It would also be by far the cheapest terraforming proposal for the red planet, with a mission cost comparable to the $3.27 billion Cassini-Huygens probe.

Examples in fiction

 * In Arthur C. Clarke's 2010: Odyssey Two, an alien construct transforms the hydrogen of Jupiter's atmosphere directly into heavier elements, leading to a subsequent ignition of the planet.
 * In The Saga of Seven Suns by Kevin J. Anderson, humanity uses alien technology to ignite a gas giant, inadvertently wiping out a race of gaseous aliens who dwelt within and thus initiating an interstellar war.
 * In Star Maker by Olaf Stapledon, the eponymous "star maker" is depicted as a rational artist, akin to a god.