The main difference between Constellation and Galaxy is that the Constellation is a one of the 88 divisions of the celestial sphere, defined by the IAU, many of which derive from traditional asterisms and Galaxy is a astronomical structure.
A constellation is a group of stars that forms an imaginary outline or pattern on the celestial sphere, typically representing an animal, mythological person or creature, a god, or an inanimate object.The origins of the earliest constellations likely go back to prehistory. People used them to relate stories of their beliefs, experiences, creation, or mythology. Different cultures and countries adopted their own constellations, some of which lasted into the early 20th century before today’s constellations were internationally recognized. Adoption of constellations has changed significantly over time. Many have changed in size or shape. Some became popular, only to drop into obscurity. Others were limited to single cultures or nations.
The 48 traditional Western constellations are Greek. They are given in Aratus’ work Phenomena and Ptolemy’s Almagest, though their origin probably predates these works by several centuries. Constellations in the far southern sky were added from the 15th century until the mid-18th century when European explorers began traveling to the Southern Hemisphere. Twelve ancient constellations belong to the zodiac (straddling the ecliptic, which the Sun, Moon, and planets all traverse). The origins of the zodiac remain historically uncertain; its astrological divisions became prominent c. 400 BC in Babylonian or Chaldean astronomy,.
In 1928, the International Astronomical Union (IAU) formally accepted 88 modern constellations, with contiguous boundaries that together cover the entire celestial sphere. Any given point in a celestial coordinate system lies in one of the modern constellations. Some astronomical naming systems include the constellation where a given celestial object is found to convey its approximate location in the sky. The Flamsteed designation of a star, for example, consists of a number and the genitive form of the constellation name.
Other star patterns or groups called asterisms are not constellations per se but are used by observers to navigate the night sky. Asterisms often refer to several stars within a constellation or may share stars with more than one constellation. Examples of asterisms include the Pleiades and Hyades within the constellation Taurus and the False Cross split between the southern constellations Carina and Vela, or Venus’ Mirror in the constellation of Orion.
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 (γαλαξίας), literally “milky”, a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) 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 (2×1011) to a suggested 2 trillion (2×1012) or more, containing more stars than all the grains of sand on planet Earth. Most of the galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3000 to 300,000 light years) and separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 30,000 parsecs (100,000 LY) and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs (2.5 million LY).
The space between galaxies is filled with a tenuous gas (the intergalactic medium) having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups, clusters, and superclusters. The Milky Way is part of the Local Group, which is dominated by it and the Andromeda Galaxy and is part of the Virgo Supercluster. 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, which contains the Virgo supercluster.
An asterism, an arbitrary formation of stars perceived as a figure or pattern, or a division of the sky including it, especially one officially recognised by astronomers.
An image associated with a group of stars.
The configuration of planets at a given time (notably of birth), as used for determining a horoscope.
A wide, seemingly unlimited assortment.
A fleet of satellites of the same purpose such as the set of GPS satellites, or Iridium satcom fleet.
A configuration or grouping.
A network of connections that exists between people who are in polyamorous relationships, for example between one person, their partner, and that person’s partner.
The Milky Way; the apparent band of concentrated stars which appears in the night sky over earth. from 14th c.
Any of the collections of many millions or billions of stars, galactic dust, black holes, etc. existing as independent and coherent systems, of which there are billions in the known universe. from 19th c.
a group of stars forming a recognizable pattern that is traditionally named after its apparent form or identified with a mythological figure.
a group of associated or similar people or things
“no two patients ever show exactly the same constellation of symptoms”
a system of millions or billions of stars, together with gas and dust, held together by gravitational attraction.
the galaxy of which the solar system is a part; the Milky Way.
a large group of impressive people or things
“the four musicians have played with a galaxy of stars”