A galaxy is a system of stars, stellar remnants, interstellar gas, dust, dark matter, bound together by gravity. The word is derived from the Greek galaxias, literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a hundred million stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies. Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many 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.

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A Mysterious Wave-Like Structure in Our Galaxy Found to Be Slowly Slithering

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New app always points to the supermassive black hole at the center of our galaxy

Cosmic Dust Could Spread Life from World to World Across the Galaxy

James Webb Space Telescope tracks a galaxy's history back to just after the Big Bang

Hubble Spots a Galaxy Shrouded by Stars

The JWST Discovers a Galaxy That Shouldn’t Exist

Watch Amazon's new trailer for cosmic comedy 'The Second Best Hospital in the Galaxy' (video)

Hubble Views a Dim but Distinct Galaxy

Unraveling the Mysteries of Star Creation in the Whirlpool Galaxy

Study: Stars travel more slowly at Milky Way’s edge. The findings suggest our galaxy’s core may contain less dark matter than previously estimated.

Stars travel more slowly at Milky Way's edge: Galaxy's core may contain less dark matter than previously estimated

'Trainwreck' galaxy reflects the aftermath of a violent galactic collision (image)

Hubble Views a Galaxy Settling into Old Age

Webb Telescope Unravels Cosmic Puzzle: Galaxy Mergers Illuminate Early Universe Mystery

This Galaxy Hosted One of the Most Powerful Supernovae Ever Seen

Earth is being bombarded by mysterious bursts of intergalactic radio waves. And it has scientists puzzled. Fast radio bursts originating from beyond the galaxy were first detected almost 20 years ago.

Puzzling Baby Stars at the Galaxy’s Core: Astronomers Unravel Age Mystery

Astronomers have snapped a new photo of the black hole in galaxy M87

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