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Astronomy 101: Galaxies

To understand some of the biggest objects in the universe, astronomers have come up with a classification system for galaxies based on their shape and the number of stars they hold.
M74
Spiral galaxy M74
NASA/ESA/Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration
A galaxy is a huge gravitationally bound grouping of stars. The number of stars a galaxy holds varies greatly, from a few billion to more than a trillion. Astronomers split galaxies into a few different types. Our Milky Way is an example of a spiral galaxy, and it hosts about 200 billion stars.

Giant galaxies, like NGC 1132, are elliptical galaxies and have more than 1 trillion stars. The smallest galaxies are called dwarf galaxies, which hold only a few billion stars.

Noted 20th-century astronomer Edwin Hubble came up with a classification system for most galaxies. This is called the “tuning fork diagram,” and it shows how spiral galaxy structures differ from elliptical. There’s even a branch of spirals that have a central bar — that’s the bottom row, those with abbreviations beginning with uppercase “SB.” This diagram does NOT show a sequence through time, so please don’t read this as an evolutionary diagram.

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