“A galaxy is composed of gas and dust and stars – billions upon billions of stars. Every star may be a sun to someone.”
— Carl Sagan
Our Solar System is located within the “Milky Way Galaxy” about 2/3 out from the center, the name deriving from its appearance as a band of hazy light when seen from Earth. It looks as it does because the Earth view of the Milky Way is from inside the galaxy structure. The Milky Way is a ‘barred spiral’ shaped galaxy with a diameter between 100,000 to 180,000 light years, and contains an estimated 100 to 400 billion stars and 100 billion planets or more. Getting a good shot of the Milky Way is tough even with all the space telescopes floating around because of location and dust cloud coverage; therefore, the 2015 artist’s concept drawn using data sourced from NASA’s Wide-field Infrared Survey Explorer (“WISE”) was the best image available until the European Space Agency (‘ESA’) released the amazing Gaia’s Sky in Colour on April 25, 2018 showing an all-sky view of our Milky Way based on measurements, brightness and color of nearly 1.7 billion stars observed by the ESA satellite Gaia between July 2014 and May 2016. Now, we can see just how beautiful our home galaxy is!
Brighter regions seen on the image indicate denser concentrations of exceptionally bright stars, while the darker regions correspond to patches of sky where there are fewer bright stars. The bright horizontal structure is the Galactic plane (the flattened disc that contains the majority of the stars). In the middle of the image, the Galactic centre appears vivid and teeming with stars. In the lower right side of the image, the two bright objects are the Large and Small Magellanic Clouds, two dwarf galaxies that orbit the Milky Way.
But, as incredible as it is, our Milky Way is just one of many billions.
It’s estimated there are 200 billion or more galaxies within the Universe, ranging in size from dwarf (containing a few billion stars) to giants with 100 trillion stars. In addition to stars, galaxies contain stellar remnants (endpoints of star evolution), gas, dust and dark matter, all held together by a gravitational pull while orbiting a center mass, and they are classified by their form and structure: elliptical (almost featureless, bright profile and stars orbit randomly), spiral (flat rotating disk shaped with spiral-like arms), barred spiral (a spiral galaxy with a bar-shaped formation in its center, e.g. Milky Way), and irregular (no shape, chaotic in appearance). About 25% of all galaxies are irregular, and while they may seem ordinary when compared to the other galaxies with symmetry and shape, irregulars are invaluable to the study of galaxy evolution and interaction.
The formation of a galaxy involves gravity, gases, star formation, and stellar explosions as well as a lot of time, but exactly how it happens has not yet been determined. There are different theories as to how spiral galaxies such as our Milky Way are formed that can be generally categorized into two basic types: 1) “top-down” which describes a large scale collapse of a large gas cloud during a period of 100 million years, and 2) “bottom-up” referring to small ‘clumps’ of stars (or globular clusters) merging to create a large galaxy.
WATCH the video Galaxies Across Space and Time for a quick exploratory VR trip through the Universe to see galaxies via the Hubble Space Telescope. It’s a bit like taking a ride with Captain Kirk and a NASA tour guide on the bridge of the Starship Enterprise.
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Feature image is the Messier 74 galaxy (aka NGC 62) taken using the NASA/ESA Hubble Space Telescope. Located about 32 million light-years from Earth in the direction of the constellation Pisces, M74 is a stunning example of a spiral galaxy with about 100 billion stars (making it slightly smaller than the Milky Way). M74 can be viewed from Earth nearly face-on, allowing observers to see its perfectly symmetrical spiral arms emanating from the central nucleus along with dotted clusters of young blue stars and glowing pink regions of ionized hydrogen. Credit: NASA, ESA, and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration, Public Domain
Image of Milky Way (2015 artist’s concept) NASA/JPL-Caltech, Public Domain
Images of galaxies in gallery are courtesy of NASA/ESA Hubble Space Telescope, Public Domain
Wikipedia: Galaxies, Milky Way, Galaxy formation and evolution