A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter literally meaning “milky”, a reference to the Milky Way. Galaxies range in size from dwarfs with just a few billion stars to giants with one hundred trillion stars each orbiting its galaxy’s center of mass.
Galaxies are categorized according to their visual morphology as elliptical, spiral and irregular. Many galaxies are thought to have black holes at their active centers. The Milky Way’s central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun.
As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a co-moving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the Big Bang. Previously, as of July 2015, EGSY8p7 was the most distant known galaxy, estimated to have a light travel distance of 13.2 billion light-years away. Approximately 200 billion or, more recently, at least 2 trillion galaxies exist in the observable universe.
Most of the galaxies are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs). The space between galaxies is filled with a tenuous gas having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into associations known as galaxy groups, clusters, and superclusters. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. The largest structure of galaxies yet recognised is a cluster of superclusters, that has been named Laniakea.
The Galactic Center is the rotational center of the Milky Way. The estimates for its location range from 7.6 to 8.7 kiloparsecs (about 25,000 to 28,000 lightyears) from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius where the Milky Way appears brightest. There is strong evidence consistent with the existence of a supermassive black hole at the Galactic Center of the Milky Way.
Scientists at the Max Planck Institute for Extraterrestrial Physics in Germany using Chilean telescopes have confirmed the existence of a supermassive black hole at the Galactic Center, on the order of 4.3 million solar masses.
A complex astronomical radio source Sagittarius A appears to be located almost exactly at the Galactic Center (approx. 18 hrs, -29 deg), and contains an intense compact radio source, Sagittarius A*, which coincides with a supermassive black hole at the center of the Milky Way. Accretion of gas onto the black hole, probably involving a disk around it, would release energy to power the radio source, itself much larger than the black hole. The latter is too small to see with present instruments.
