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The Whirlpool Galaxy: M51 or NGC 5194


The Whirlpool Galaxy M51 or NGC 5194


The Whirlpool Galaxy is also known as M51 because it was the 51st object Charles Messier saw through his telescope in 1773. Messier only wanted to observe comets, so he took careful note of fuzzy areas in the sky (clouds or "nebulae") that would interfere with unimpeded comet viewing. His telescope was too small to resolve the Whirlpool galaxy into the immense spiral of billions of stars that it is, so he merely identified it as the 51st cloudy spot in the sky to stay clear of.  Then in 1845 Lord Rosse turned his 72" metal behemoth on it and discovered that the cloud had a spiral structure to it, and so it became known as the first "spiral nebula."

Now it turns out that there is a class of stars, Cepheid Variables, that vary in luminosity over time, and how intrinsically bright they are (their "absolute magnitude" or absolute luminosity) has a direct relationship to their pulsation period. So by observing a Cepheid variable's pulsation period its true brightness (absolute luminosity) can be known, and by comparing that to its apparent brightness (or "apparent luminosity") dimmed by its distance from us, its distance can then be inferred. Thus these stars serve as unique and very important distance indicators to wherever they are found in the universe. In 1924 Edwin Hubble established the distance to Cepheid variables in the Andromeda Galaxy (our nearest neighboring galaxy), and proved that they were not members of the Milky Way Galaxy but in fact belonged to another galaxy outside of our own. This finally laid  to rest the question of whether or not the Milky Way galaxy was the entire universe or was just one of a multitude of galaxies in the universe.

The Whirlpool Galaxy consists of 160 billion solar masses (stars as heavy as our Sun) at a distance of 23 million light years from us in the constellation Canes Venatici (hunting dogs). The smaller galaxy NGC 5195 at the bottom is gravitationally bound to it. A much finer image of the Whirlpool galaxy can be found on page 220 in You and the Universe: S. Beckwith (STScl), HHT, (STScl/AURA), ESA, NASA, with superb additional processing by astrophotographer Robert Gendler.



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Carl Woebcke: The Whirlpool Galaxy M51 or NGC5194, 1991-2016. All rights reserved.