Galaxies -------- Galaxies are gravitationally bound aggregations of stars, gas, and dust that are the next step up in scale from star clusters. Galaxies display a wide range of sizes and luminosities. The smallest dwarf-irregular systems are only 300 parsecs (about 1000 light-yesrs) in diameter. At the opposite extreme, within the galaxy cluster Abell 1413 is a gargantuan elliptical that spans almost 2 million parsecs. The Milky Way is about 100,000 light years across and is a spiral galaxy. The average galaxy has an absolute visual magnitude of about -23 (over ten times greater than the Milky Way). Just as for stars, radial velocities of galaxies are determined by measuring the Doppler shifts of absorption lines in their spectra. This was first accomplished in 1912. by V. Slipher of Lowell Observatory. All but a few (the closer) galaxies display a redshift. In other words they are receding from us. The recessional velocities are directly proportional to the distance, the well known Hubble law, named after its discovere, which indicates that the universe is expanding. This image shows 6500 galaxies for which 3-dimensional coordinates can be calculated. It is approximately 1,000,000,000 parsecs across, one billion (or 3,000,000,000 light years or 10 to the power 23 kilometres!!!).