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CHAPTER 10 notes.docx

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Natural Science
NATS 1745
Robin Metcalfe

CHAPTER 10: 1. What is the "Island Universe" theory? That we are a part of a disk-shaped star system. 2. Galaxy means milk in greek. It was given this name from its milky appearance. We call it the milky way. It is our view through the disks length. 3. William, Caroline, and John used their giant telescopes to catalog over 5,000 nebulae. They were attempting to identify whether nebulae are distant star systems or nearby glowing gas clouds. Because of the large variety of nebulae, the Herschels couldn’t come up with a conclusion. 4. Herschel’s map of visible stars revealed that our galaxy is the shape of a disk. 5. Through his Leviathan telescope, Lord Rosse discovered that the elliptical nebulae are actually spiral nebulae. 6. What is a variable star? What characterizes the Cepheid variables? Explain briefly why Cepheids pulsate. Astar which fluctuates in brightness at regular time intervals. Cepheid: luminous variable stars which pulsate rapidly (at intervals of days to weeks) and has highly opaque surface; its outward photons push surface outwards, which temporarily expands the star and reduces its opacity. 7. SMC is the Small magellanic Cloud LMC is the large magellanic Clouds (they are galaxies). Henrietta Leavitt assumed SMC Cepheids are equally distant from Earth, and found that the brighter the Cepheid, the longer its period. Discovering Cepheid period was valuable because it gives way its luminosity and therefore its distance, which is the distance to its star systems. Harlow Shapley used her discovery to determine the boundaries of our galaxy. 8. What are the 3 components of our galaxy, according to Shapley's model? Where is the Sun located in this model? Where is the Galaxy's centre? In what 2 ways is this model incorrect? (1) Disk is 300,000 light years and (2) implies all nebulae are systems WITHIN our galaxy and (3) SMC is INSIDE our galaxy. The sun resides in OUTSKIRTS of its disk. The Milky Way centre is in Sagittarius Shapley was wrong about where the spiral resided and the size of the disk (he estimated it was three times bigger) 9. The spirals must be contained within our galaxy. If they weren’t, we should see them all around the sky. 10. Spirals are pushed from our disk by radiation pressure from its high density of stars, creating a “Zone of Avoidance”. So, the spirals avoid the Milky Way. Curtis completed a high- resolution survey of spiral nebulae, revealing dust in their disks. Curtis was correct. 11. If the Milky Way’s disk contains a band of dust, and if spirals are outside our galaxy, they aren’t seen in the “Zone of Avoidance”, because they’re obscured by dust. The dust came from microscopic metallic grains (from dying stars), which absorb starlight. 12. Vesto Slipher measured large redshifts in the absorption lines of spiral, suggesting recessional speeds as high as thousands of km/sec. 13. Spirals are pushed from our disk by radiation pressure from its high density of stars, creating a ZONE OFAVOIDANCE. Therefore, the spirals are WITHIN our galaxy. HE ARGUEDAGAINST ISLAND UNIVERSE. Curtis argued if spirals are moving faster than stars in our Galaxy, the spirals must be OUTSIDE our galaxy. CURTISARGUED FOR ISLAND UNIVERSE 1) Spiral radial velocities are larger than star velocities 2) Spiral spectra are indicative of star systems 3) Faintness of what appear to be novae in spirals suggest they’re distant star systems 4) Spirals show evidence of obscuring matter in their disks; If Milky Way is such a spiral, this explains Zone avoidance 14. Edwin Hubble resolved the Shapley-Curtis debate. He found a Cepheid in M31, with the PL-relation he found a distance to M31 of 1 million light years. This proved the spirals are galaxies beyond the Milky Way. NASA’s tribute to this discovery was the Hubble Space Telescope. 15. The four types of galaxies in Hubble’s Classification s
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