page_title: Galaxy
section_title: Early galaxy formation
hierarchical_section_title: Galaxy / Formation and evolution / Formation / Early galaxy formation
caption_attribution_description: English: The three panels show different components of near-infrared background light detected by the Hubble Space Telescope in deep-sky surveys. The one on the left is a mosaic of images taken over a 10-year period. When all the stars and galaxies are masked, the background signals can be isolated, as seen in the second and third panels. The middle panel reveals "intra-halo light" from rogue stars torn from their host galaxies, and the panel on the right captures the signature of the first galaxies formed in the universe.
context_page_description: A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias, literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million 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, or irregular. Many galaxies are thought to have supermassive black holes at their 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 comoving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the Big Bang.
  Research released in 2016 revised the number of galaxies in the observable universe from a previous estimate of 200 billion to a suggested two trillion or more and, overall, as many as an estimated 1×10²⁴ stars.
context_section_description: The detailed process by which early galaxies formed is an open question in astrophysics. Theories can be divided into two categories: top-down and bottom-up. In top-down correlations (such as the Eggen–Lynden-Bell–Sandage [ELS] model), protogalaxies form in a large-scale simultaneous collapse lasting about one hundred million years. In bottom-up theories (such as the Searle-Zinn [SZ] model), small structures such as globular clusters form first, and then a number of such bodies accrete to form a larger galaxy.
  Once protogalaxies began to form and contract, the first halo stars (called Population III stars) appeared within them. These were composed almost entirely of hydrogen and helium, and may have been massive. If so, these huge stars would have quickly consumed their supply of fuel and became supernovae, releasing heavy elements into the interstellar medium. This first generation of stars re-ionized the surrounding neutral hydrogen, creating expanding bubbles of space through which light could readily travel.
  In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z = 6.60. Such stars are likely to have existed in the very early universe (i.e., at high redshift), and may have started the production of chemical elements heavier than hydrogen that are needed for the later formation of planets and life as we know it.
caption: Different components of near-infrared background light detected by the Hubble Space Telescope in deep-sky surveys[107]
exif: {"Image ImageDescription": "The three panels show different components of near-infrared background light detected by the Hubble Space Telescope in deep-sky surveys. The one on the left is a mosaic of images taken over a 10-year period. When all the stars and galaxies are masked, the background signals can be isolated, as seen in the second and third panels. The middle panel reveals \"intra-halo light\" from rogue stars torn from their host galaxies, and the panel on the right captures the signature of the first galaxies formed in the universe. Link: NASA Press release", "Image Software": "Adobe Photoshop CS4 Macintosh"}