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The Hidden Lives of Galaxies - Characteristics of Galaxies

I. The Lives of Galaxies

A. The Characteristics of Galaxies

Like all galaxies, the Milky Way is held together by gravity. Gravity also holds the stars, planetary bodies, gas, and dust in orbit around the center of the galaxy. Just as the planets orbit around the Sun, the Sun orbits around the center of the Milky Way.

Galaxies come in a variety of shapes. In the 1920s Edwin Hubble was the first to study the morphology of galaxies. Using the 100-inch Hooker reflector telescope at Mount Wilson Observatory in California between 1922-1926, Hubble photographed numerous galaxies. He categorized these shapes or basic schemes as spiral, barred spiral, elliptical, irregular, and peculiar. This system was known as the Hubble morphological sequence of galaxy types. First, he noted that some galaxies, like the M31- Andromeda Galaxy, appeared as disks and had arms of stars and dust which appeared in a spiral pattern. Like M31, these galaxies appeared nearly uniform in brightness. In addition, Hubble observed that in some of these types of galaxies the arms were more tightly wound around the galaxy. He called these spiral galaxies. Our Galaxy, the Milky Way, is an example of a spiral galaxy. Secondly, he noted that some spirals had a bright bar of gas through the center, and called these barred spirals. Thirdly, Hubble discovered galaxies that were slightly elliptical in shape, while others were nearly circular, such as M32. He called these elliptical galaxies. The fourth type of galaxy observed was neither spiral nor elliptical, but was irregular in shape. These galaxies were called irregular. An example of this is the Magellanic Clouds. Finally, there were some galaxies that fit none of these descriptions. These were called peculiar galaxies, one example of which is Centaurus A.

Astronomers now have decided that the morphology classification should consist of only two types of galaxies: the spirals and the ellipticals. Barred spirals are a subclass of spirals. Irregulars may be either spiral or barred spiral. Peculiars are not fundamentally a different type. They are simply galaxies in the act of colliding; the collision distorts their shape and makes them appear "peculiar."


A service of the High Energy Astrophysics Science Archive Research Center (HEASARC), Dr. Alan Smale (Director), within the Astrophysics Science Division (ASD) at NASA/GSFC

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