A stellar engine is a hypothetical device built by a Type II Kardashev civilization that uses a significant fraction of a star's radiation, in one of several ways, to generate exergy. (Exergy is the maximum amount of work that can be extracted from a given energy amount, under given environmental parameters. For example, for a heat quantity Q at temperature T, the exergy is Q(1 – T ' / T ) where T ' is the ambient temperature.)

 

Three classes of stellar engines are defined in reference 1 below. A class A stellar engine uses the impulse of the radiation emitted by a star to produce thrust and is also called a Shkadov thruster. A class B stellar engine uses the energy of the radiation emitted by the star to generate mechanical power. It is a particular type of Dyson sphere. Stellar radiation of temperature T makes the inner surface of star's enclosing shell (thickness h, distance R_p from the star) to have a temperature T_p higher than the temperature T_r of the outer shell surface. The temperature difference R_p – R_r is used to drive thermal engines producing work. A class C stellar engine is a combination of class A and class B stellar engines as part of the shell's inner surface is covered by a mirror (see figure). Thus, a class C stellar engine could provide at the same time available work and a thrust. There is a minimum radius for a class C stellar engine to provide useful power. An optimum class C stellar engine radius R_p allowing maximum power delivery is identified in reference 2 below. In the case of our own solar system, this optimum radius is around 450 million kilometers. A and C stellar engines make the star's temperature increase significantly, which affects the thrust acting on the star.

 

References

1. Badescu, V. and Cathcart, R. B. "Stellar engines for Kardashev's type II civilisations," J. British Interplanetary Soc., 53 (9/10), 297–306, 2000.
2. Badescu, V. "On the radius of the Dyson sphere," Acta Astronautica, 36 (2), 135–138, 1995.