Editing Second law of thermodynamics (section)

== Gravitational systems ==
Commonly, systems for which gravity is not important have a positive [[heat capacity]], meaning that their temperature rises with their internal energy. Therefore, when energy flows from a high-temperature object to a low-temperature object, the source temperature decreases while the sink temperature is increased; hence temperature differences tend to diminish over time.

This is not always the case for systems in which the gravitational force is important: systems that are bound by their own gravity, such as stars, can have negative heat capacities. As they contract, both their total energy and their entropy decrease<ref>{{cite web |last1=Baez |first1=John |title=Can Gravity Decrease Entropy? |url=http://math.ucr.edu/home/baez/entropy.html |website=UC Riverside Department of Mathematics |publisher=University of California Riverside |access-date=7 June 2020 |date=7 August 2000 |quote=... gravitationally bound ball of gas has a negative specific heat!}}</ref> but [[Kelvin-Helmholtz mechanism|their internal temperature may increase]]. This can be significant for [[protostars]] and even gas giant planets such as [[Jupiter]]. When the entropy of the [[black-body radiation]] emitted by the bodies is included, however, the total entropy of the system can be shown to increase even as the entropy of the planet or star decreases.<ref>{{cite web |last1=Baez |first1=John |title=Can Gravity Decrease Entropy? |url=http://math.ucr.edu/home/baez/entropy2.html |website=UC Riverside Department of Mathematics |publisher=University of California Riverside |access-date=7 June 2020 |date=7 August 2000}}</ref>