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LNOTES19Galaxies TheUniverse - A view from our planet 4...

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A view from our planet, 4 billion years hence: Our Galaxy, colliding with Andromeda
The Realm of the Nebulae: Galaxies •Is there some order to the stars?A constant in the chaos?
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• A collection of billions of stars is called a galaxy. • Our sun is in a galaxy we call the Milky Way. • The Milky Way is about 100,000 light years across • Galaxies come in different sizes and shapes. • Galaxies are also found in clusters.GALAXIES
vGlobular clusters are concentrated around the center of the galaxy. vHarlow Shapley realized this from observations. vHe used Henrietta Leavitt's discovery of Cepheid variable's period-luminosity law.Globular Clusters
Milky Way Edge On vA near infrared map of the entire sky shows the Milky Way projected left to right with a prominent central bulge. vThis could resemble an edge on view of our Milky Way galaxy.
Star Clusters vOpen clusters are found within the galactic disk. vGlobular clusters are found surrounding the outside of the galaxy. vDetermining the sun's rotation around the galaxy is useful in determining the mass of the galaxy. vKepler's third law, which is the harmonic law, is used to determine mass.
vStars in the galactic plane have nearly circular orbits. vStars in the halo are randomly oriented and have highly elongated orbits.Stellar Orbits in the Galaxy
vGalaxies are not rigid objects. vThe galaxy rotates at different rates outward from the center. vThis is called differential rotation.Galaxy Rotation
vThe galaxy has two types of stars which are called Population I and Population II. vPopulation I are metal rich and young. vPopulation II are metal poor and older. vAstronomically speaking, any element heavier than helium is considered a metal.Population I and II stars
Figure 15.14AvThe most abundant element in the universe is hydrogen. vPopulation II stars are probably made from the original hydrogen. vBy studying the spectra of such stars to determine age, clues to the formation of the galaxy are revealed. vThis is called nucleocosmochronology Abundances and Cosmochronology
vThe galaxy possibly began as a rotating gas cloud, forming much the way we think our solar system did. vGlobular clusters forming from the primordial cloud, move outward forming the halo.But there is evidence that some have been captured. vAs the cloud flattens, increased stellar formation defines the equatorial region and young, ever forming stars.Ideas about Origin of the Milky Way
vThe spiral arm structure of the Milky Way contains young stars. vThese arms are 'dusty' and stars are constantly forming, maintaining the spiral structure. vBy locating hot blue stars, the arms can be traced, since these types of bright stars are found in spiral arms of other galaxies.Spiral Arms
vIn this image of M51, young star clusters are located along the spiral arms (pink regions). vThe dark inner edges of the arms are dust regions.A Galaxy Much Like Our Own: M51
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