A Two--Exponent Mass-Size Power Law for Celestial Objects

The Universe that we know is populated by structures made up of aggregated matter that organizes into a variety of objects; these range from stars to larger objects, such as galaxies or star clusters, composed by stars, gas and dust in gravitational interaction. We show that observations support the existence of a composite (two--exponent) power law relating mass and size for these objects. We briefly discuss these power laws and, in view of the similarity in the values of the exponents, ponder the analogy with power laws in other fields of science such as the Gutenberg--Richter law for earthquakes and the Hutchinson--MacArthur or Damuth laws of ecology. We argue for a potential connection with avalanches, complex systems and punctuated equilibrium, and show that this interpretation of large scale--structure as a self--organized critical system leads to two $predictions$: (a) the large scale structures are fractally distributed and, (b), the fractal dimension is $1.65 \pm 0.25$. Both are borne out by observations.