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Physics LibreTexts

13: Galaxies

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“There are at least as many galaxies in our observable Universe as there are stars in our galaxy. ”

Martin Rees
Lord Rees of Ludlow
Astronomer
(1942 -)

Learning Objectives

Upon completion of this module, the student will be able to:Upon completion of this module, the student will be able to:

  • Define what a galaxy is and the broad characteristics of galaxies, including galaxy types.
  • Explain the history of observing galaxies.
  • Recognize Hubble’s Law and its implications in astronomy.
  • Differentiate among galaxy-related objects, including Quasars, Blazars, galactic grouping, WIMPs, gravitational lensing

This module presents an overview of the islands of stars found throughout the Universe: the galaxies. Like many objects we have discovered, galaxies vary in characteristics such as size, shape, and age.

  • 13.1: What do you think?
  • 13.2: Galaxies
    This page explains that galaxies are vast systems made up of stars, remnants, interstellar medium, and dark matter, named after the Greek word for milk. There are approximately 170 billion galaxies in the Universe, many likely hosting planets around stars. It highlights the presence of supermassive black holes, such as Sagittarius A* in the Milky Way, at galaxy centers, which might influence active galactic nuclei (AGN) known for their high luminosity.
  • 13.3: The History of Studying Galaxies
    This page outlines the historical study of galaxies, tracing back to Abd al-Rahman al-Sufi's observations in 964 AD, and includes significant contributions from Thomas Wright, who theorized the Milky Way's structure, Immanuel Kant, who introduced the term "Island Universe," and advancements by Charles Messier, William Herschel, and Lord Rosse in cataloging and classifying deep sky objects.
  • 13.4: Hubble’s Major Contributions
    This page discusses Edwin Hubble's pivotal role in galactic astronomy after he joined Mt. Wilson Observatory in 1919. He identified certain nebulae as distant galaxies, reshaping the understanding of the universe. Hubble classified galaxies into elliptical, spiral, and irregular forms through the Hubble Sequence. Additionally, he formulated Hubble's Law, linking a galaxy's distance to its redshift and receding velocity, although establishing Hubble's constant continues to pose challenges.
  • 13.5: Hubble’s Law
    This page discusses the controversy surrounding Hubble's Law, which connects distance and velocity in an expanding Universe. Georges Lemaître proposed similar ideas prior to Hubble but did not receive equal recognition, possibly due to bias from Harlow Shapley, Lemaître’s advisor, who was opposed to Hubble's research. As a result, Lemaître's contributions were overshadowed by Hubble's more prominent acknowledgment in the scientific community.
  • 13.6: Classification of Galaxies
    This page discusses Edwin Hubble's pioneering classification of galaxies into three main types: Elliptical, Spiral, and Irregular. It highlights his development of the Hubble Galactic Tuning Fork, which illustrated concepts of galactic evolution. Additionally, it notes that modern astronomers now recognize four major groups of galaxies, including the previously mentioned types and a fourth category of Active Galaxies.
  • 13.7: Elliptical Galaxies
    This page discusses elliptical galaxies, which are categorized by their shape from round (E0) to elongated (E5), contain little dust and gas, and are among the oldest galaxy types. Despite their diverse forms, they make up only about 10% to 15% of all galaxies in the Universe.
  • 13.8: Spiral Galaxies
    This page discusses spiral galaxies, noting their flattened disk shape and central bulge with spiral arms. They are categorized into three main types, including lenticular spirals that lack distinct spiral arms and appear older, such as NGC 2787.
  • 13.9: Irregular Galaxies
    This page discusses irregular galaxies, which make up around 25% of all galaxies and lack a defined shape, setting them apart from elliptical and spiral types. They are generally smaller, contain some dust but less than spirals, and are often found as satellites to larger galaxies. It is suggested that irregular galaxies were more prevalent in the early Universe.
  • 13.10: Active Galaxies
    This page discusses active galaxies, which make up approximately 10% of all galaxies and are distinguished by their high energy output. It highlights different types, including Seyfert galaxies, radio galaxies, Starburst galaxies, Quasars, and Blazars. Notable examples mentioned are Centaurus A, Hoag’s Object, and M82, each representing unique characteristics within the category of active galaxies.
  • 13.11: Quasar-stellar Objects
    This page discusses quasars, extremely luminous and powerful objects in the universe powered by super-massive black holes. Typically found at the edge of the observable universe, they provide insights into cosmic history through their high red shifts. A significant quasar discovered in 2010 is located 13.3 billion light-years away. Quasars are believed to have originated from early galactic collisions that formed their central black holes.
  • 13.12: Blazars- Quasar-like Object
    This page discusses Blazars, which are compact, Quasar-like objects with energy jets moving at near-light speed, causing them to appear exceptionally bright. Both Blazars and Quasars are types of Active Galactic Nuclei (AGN), and some objects previously mistaken for variable stars or radio sources have been reclassified as such. Additionally, faint traces of the host galaxy can often be detected in Blazars.
  • 13.13: Galaxies, Distance, and Age
    This page discusses the distinct characteristics of galaxies that influence their shapes, brightness, and distances. It highlights the Tully-Fisher Relationship for correlating rotation speed with brightness and the use of supernovae of white dwarfs as standard brightness measures. Hubble’s Law is also mentioned as a means to infer the age of galaxies and the universe by relating distance and velocity, allowing for time calculations using t=d/V.
  • 13.14: The Ages of Galaxies and What that Reveals
    This page discusses the formation of galaxies from hydrogen and helium clumps after the Universe's inception. Initial expansion slowed, leading to Protogalactic clouds that birthed stars and galaxies. The dominant theory posits that galaxies, including the Milky Way, started small and grew by gathering intergalactic gas and merging. A 2010 study suggests some galaxy segments may stem from a single gas and dust blob collapse.
  • 13.15: Galactic Groups
    This page discusses how galaxies cluster in groups, exemplified by the Milky Way and Andromeda in The Local Group, which includes over 54 galaxies. These smaller groups are part of larger structures called galaxy superclusters, with the Local Group within the Virgo Supercluster that contains at least 100 galaxy groups and clusters.
  • 13.16: Dark Matter
    This page discusses Dark Matter, an invisible component of the Universe identified by its gravitational effects, constituting the majority of its mass. Direct observation is impossible since it neither emits nor absorbs light. Its presence is inferred through techniques like weighing galaxies and analyzing Mass-to-Light ratios. Understanding Dark Matter is essential for predicting the Universe's future, impacting scenarios such as continuous expansion or a potential Big Crunch.
  • 13.17: WIMPs and String Theory
    This page discusses WIMPs as theoretical Dark Matter candidates with minimal interaction, primarily clustering in galaxies. It also covers String Theory, which aims to unify the four fundamental forces of nature by depicting them as vibrations of one-dimensional strings, a concept that emerged in the late 1960s.
  • 13.18: Gravitational Lensing
    This page discusses gravitational lensing, a phenomenon where massive objects bend light due to their gravitational fields, as explained by Einstein's General Theory of Relativity. Initially proposed by Fritz Zwicky in 1937 and confirmed in 1979, it includes various scales such as strong and microlensing. The page also highlights Edwin Hubble's contributions to astronomy, especially in identifying galaxies beyond the Milky Way, noting he did not receive a Nobel Prize in Physics before his death.


This page titled 13: Galaxies is shared under a CC BY 4.0 license and was authored, remixed, and/or curated by Lumen Learning via source content that was edited to the style and standards of the LibreTexts platform.

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