To simplify to the extreme, we may say that the physics of these objects is primarily governed by their masses, which are involved on two levels:
• the effect of gravity, which tends to compress the object, liberating gravitational energy,
• nuclear processes, which begin as and when the temperature rises in the object's core.
Mass is therefore an excellent parameter for classifying different astrophysical objects, the unit of comparison being one solar mass (denoted by M0). We may then define three mass-categories, by decreasing mass:
• If M > 0.08M0 (^80 MJ, where MJ is the mass of Jupiter), the mass is sufficient for gravitational contraction to allow the object's core to reach the temperature at which hydrogen fusion reactions begin. The object is then described as a 'star', and its radius is proportional to its mass.
• If 0.013 M0 < M < 0.08M0 (13 Mj < M < 80Mj), the temperature at the core of the object does not start hydrogen fusion reactions, but does initiate deuterium fusion reactions. The object is called a 'brown dwarf', and its radius is inversely proportional to the cube root of its mass.
• If M < 0.013M0 (M < 13MJ), the temperature at the core does not allow any nuclear fusion reactions to occur. The object is called a 'planet'. Within this category, a distinction is generally made between giant planets and terrestrial planets, where the mass of the latter is not sufficient for them to accrete any gas. The boundary between giant planets and terrestrial planets lies at about 10 Earth masses.
• Unlike hydrogen fusion, deuterium fusion plays no role in determining the nature of the object. The limit of 13 Mj between a planet and a brown dwarf is conventional (and also based on consensus).
• A planet is also (and always?) a body orbiting a star. There are brown dwarfs that are not bound to a central star.
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