From Nothing to Everything 12-13 GlossaryGalaxy: A gravitationally bound system of millions, billions, or even trillions of stars. Spiral galaxy: Flattened system of billions of stars that displays a general spiral pattern. Elliptical galaxy: System of millions, billions, or trillions of stars with a circular or elliptical shape as viewed in the sky. Milky Way Galaxy: The spiral galaxy to which our solar system belongs. Cluster of galaxies: A gravitationally bound system of hundreds or thousands of galaxies. Supercluster of galaxies: A collection of clusters of galaxies grouped together. Superclusters are thought to be the largest non-random structures in the universe. Cosmic web: Nickname given to the pattern of superclusters in the universe consisting of interwoven filaments. Void: A large region containing a low density of galaxies. Hubble’s constant (symbol: H0): Slope of the Hubble Law (Fig. 10-2 and eqs. 10-1 and 10-4). Its value at the current epoch of the universe is about 70 km/s/Mpc. Redshift (symbol: z, no units): A measurement of the increase in wavelength of photons caused by the expansion of the universe; see eqs. 10-2 and 10-3. One plus the redshift (1+z) equals by how many times the universe has spread out since the light we now receive left the object being observed. Galaxies and quasars with high redshifts are so distant that the light we now see from them left billions of years ago, so that we view these objects as they existed during early stages in the universe’s development. Dark matter: Material (probably particles that we have not yet discovered in laboratories on the Earth) that does not emit light but whose gravitational influence is apparent in galaxies and clusters of galaxies.