A galaxy is a vast system of stars, #gas, #dust, #stellar remnants, and dark matter held together by #gravity, with the Milky Way being our own galaxy. Galaxies come in #various shapes, including spirals, ellipticals, and #irregular forms, and can be organized into larger structures like groups, clusters, and superclusters, which together form the large-scale structure of the #universe. The word "galaxy" comes from the #Greek #word galaxias, meaning "milky," which was a reference to the #band of light we see in the night sky, the Milky Way.
#Composition: A galaxy consists of stars, planets, #interstellar gas, dust, and dark matter.
Gravity: #Gravity is the force that holds all these #components together.
Scale: Galaxies can range from containing a few #thousand stars to #billions, with the largest spanning more than a million light-years across.
Supermassive Black Holes: Most large galaxies have a supermassive #black hole at their #center.
#Astronomers classify galaxies primarily based on their appearance, a #system proposed by #Edwin #Hubble:
Spiral Galaxies: #Characterized by a central bulge and #spiral arms, such as the Milky Way.
#Elliptical Galaxies: Smooth, oval-shaped galaxies.
Irregular Galaxies: Galaxies with no defined, orderly shape.
The Milky Way: Our #home galaxy, the Milky Way, contains our #Solar System and is a large barred spiral galaxy.
The #Universe's Structure
Galaxies are not isolated but are organized into larger structures:
Groups: Collections of around 100 or fewer galaxies.
#Clusters: Thousands of galaxies held together by gravity.
#Superclusters: Even larger structures, sometimes called the #cosmic #web, which include groups, clusters, and #voids.