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Astronomy Chapter 9 - textbook.docx

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Western University
Astronomy 1021
Stacey Hallman

Astronomy Chapter 9 The Milky Way Galaxy Cepheid Variable Stars: variable stars with pulsation periods of 1 to 60 days whose period of variation is related to their luminosity Instability Strip: the region of the H-R diagram in which stars are unstable to pulsation. Astar evolving through this strip becomes a variable star Period-Luminosity Relation: the relation between period of pulsation ad intrinsic brightness among Cepheid variable stars Proper Motion: the rate at which a star moves across the sky, measured in seconds of arc per year Calibrate: to make observations of reference objects, checks on instrument performance, calculations of unit conversions, and so on, needed to completely understand measurements of unknown quantities Disk Component: all material confined to the plane of the galaxy SpiralArms: long spiral pattern of bright stars, star clusters, gas, and dust. Spiral arms extend from the center to the edge of the disk of spiral galaxies Spherical Component: the part of the galaxy including all matter in a spherical distribution around the center (the halo and central bulge) Halo: the spherical region of a spiral galaxy, containing a thin scattering of stars, star clusters, and small amounts of gas Central Bulge: the spherical cloud of stars that lies at the center of our galaxy Rotation Curve: a graph of orbital velocity versus radius in the disk of a galaxy Dark Halo: the low-density extension of the halo of our galaxy, believed to be composed of dark matter Dark Matter: nonluminous matter that is detected only by its gravitational influence Spiral Tracer: object used to map the spiral arms – for example, O and B associations, open clusters, clouds of ionized hydrogen, and some types of variable stars Density Wave Theory: theory proposed to account for spiral arms as compressions of the interstellar medium in the disk of the galaxy Self-Sustaining Star Formation: the process by which the birth of stars compress the surrounding gas clouds and triggers the formation of more stars Population I Star: stars rich in atoms heavier than helium, nearly always relatively young stars found in the disk of the galaxy Population II Star: stars poor in atoms heavier than helium, nearly always relatively old stars found in the halo, globular clusters, or the central bulge Metals: in astronomical usage, all atoms heavier than helium Monolithic Collapse Model: an early hypothesis that says that the galaxy formed from the collapse of a single large cloud of turbulent gas over 13 billion years ago Sagittarius A*: the powerful radio source located at the core of the Milky Way Galaxy THE DISCOVERY OFTHE GALAXY The Great Star System • The hazy band of the Milky Way is our wheel-shaped galaxy seen from within, but its size and shape are not obvious • William and Caroline Herschel counted stars over the sky to show that it seemed to be shaped like a disk with the Sun near the center, but they could not see very far into space because of interstellar gas and dust Star Clusters and the Center of the Galaxy • Globular clusters are much too far away to have measurable parallaxes, but they do contain variable stars • In the early 20 century, Harlow Shapley calibrated Cepheid variable stars to find the distance to globular clusters and demonstrated that our galaxy is much larger than the part you can see easily and that the Sun is not at the center Analysis of the Galaxy • Modern observations suggest that our galaxy contains a disk component about 80 000 ly in diameter and that the Sun is two-thirds of the way from the center to the visible edge • The central bulge around the center and an extensive halo containing old stars and little gas and dust makeup the spherical component • The mass of the galaxy can be found from its rotation curve • Kepler’s third law tells astronomers the galaxy contains over 100 billion solar masses, and the rising rotation curve at great distance from the center shows that the halo must contain much more mass o Because that material is not emitting detectable electromagnetic radia
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