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Spiral Galaxies. Ron Buta University of Alabama. What are spiral galaxies?. Flattened systems of stars, gas, and dust stars seem to concentrate in spiral “arms” all bound by gravity into a single unit typical size:50,000-100,000 light years typical mass: 10billion-500billion suns
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Spiral Galaxies Ron Buta University of Alabama
What are spiral galaxies? • Flattened systems of stars, gas, and dust • stars seem to concentrate in spiral “arms” • all bound by gravity into a single unit • typical size:50,000-100,000 light years • typical mass: 10billion-500billion suns • often there is also a prominent bulge
Why are spirals interesting? • An intriguing morphology • difficult to explain • common and widespread thoughout the Universe • new stars tend to form in spirals but not in other types of galaxies • part of process of galaxy evolution and interaction • our Galaxy is a spiral
Discovery of Spirals • 1845 William Parsons, Third Earl of Rosse, Ireland • 72-inch reflector • visually detected spiral arms in the Whirlpool Nebula M51 • many other “nebulae” found to be spirals
What causes spiral structure? • Natural phenomenon in flat, rotating stellar disks • possibly triggered by an interaction with another galaxy • or generated by bars • “density waves”
Spiral galaxy research at UA • morphology • sense of winding of spiral arms • gravitational torques due to bars and spiral arms • star formation in galactic rings
The de Vaucouleurs Atlas of GalaxiesGerard de Vaucouleurs 1918-1995
The de Vaucouleurs Atlas: what is it? • Authors: R. J. Buta (U. Alabama) • H. G. Corwin, Jr. (Caltech) • S. C. Odewahn (U. Arizona) • publisher: Cambridge Univ. Press • Purpose: to illustrate the de Vaucouleurs revised Hubble classification system with modern digital images
Sense of winding of spiral arms • Trailing arms: follow direction of rotation • Leading arms: oppose direction of rotation • most spiral arms trail • but one galaxy has leading arms!
Barred spiral galaxies • A bar-like pattern of old stars crosses the center • arms break from the ends of this bar • 70% of spirals have a bar, including the Milky Way
How stars move in presence of a bar • In solar system, planets follow elliptical paths with Sun at one focus • no torque on planet as it orbits • in presence of a bar, star may follow a centered elliptical orbit • there is a torque on star as it orbits • amount of torque is a measure of “bar strength”
Rings of Star Formation • Real bar orbit • “lit up” by new stars • often not circular