Astronomers characterize the morphology (shape) of a globular cluster by means of standard radii: the core radius (rc), the half-light radius (rh), and the tidal or Jacobi radius (rt). The radius can be expressed as a physical distance or as a subtended angle in the sky. Considering a radius around the core, the surface luminosity of the cluster steadily decreases with distance, and the core radius is the distance at which the apparent surface luminosity has dropped by half. A comparable quantity is the half-light radius, or the distance from the core containing half the total luminosity of the cluster; the half-light radius is typically larger than the core radius.
Astronomers characterize the morphology (shape) of a globular cluster by means of standard radii the core radius (rc) the half-light radius (rh) and the tidal or Jacobi radius (rt) The radius can be expressed as a physical distance or as a subtended angle in the sky Considering a radius around the core the surface luminosity of the cluster steadily decreases with distance and the core radius is the distance at which the apparent surface luminosity has dropped by half A comparable quantity is the half-light radius or the distance from the core containing half the total luminosity of the cluster; the half-light radius is typically larger than the core radius