massaradius

Massaradius is a term used to describe the relationship between an object's mass and its radius. It is not a fixed value but a prediction that depends on the object's composition, internal structure, and the state of matter, as described by its equation of state. The concept is central in astrophysics and planetary science for understanding what an object is made of and how it supports itself against gravity.

In planets, the mass–radius relation reflects composition and layering. Rocky planets tend to follow a relatively shallow rise of radius with mass, while planets with substantial volatile or gaseous envelopes show different scaling. For example, terrestrial bodies roughly follow R ∝ M^β with β around 0.25–0.3, whereas gas giants exhibit larger radii for a given mass due to their extended atmospheres and compression effects. Temperature, age, and atmospheric loss can also affect the observed radius.

In stars, the relation varies with evolutionary stage. On the main sequence, the radius roughly scales with mass with a power between about 0.7 and 0.9, depending on metallicity and internal structure. Evolved stars can have radii many times larger than their mass would suggest, due to expansion in later life stages.

Degenerate and compact objects show distinct mass–radius behavior. White dwarfs exhibit an inverse relation: larger mass corresponds to smaller radius, bounded by the Chandrasekhar limit near 1.44 solar masses. Neutron stars have radii around 10–14 kilometers, with the exact value highly sensitive to the nuclear equation of state. Black holes present a different picture, where the Schwarzschild radius scales linearly with mass (Rs = 2GM/c^2), defining the size of the event horizon.

Measurement and modeling combine observations with interior structure calculations. Techniques include transit photometry, spectroscopic and dynamical mass measurements, asteroseismology, and gravitational lensing, all used to constrain mass–radius relations and infer composition and physical state.