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Chapter 14

Chapter 14 Our Galaxy.docx

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York University
Natural Science
NATS 1740
Randy Hoffman

Chapter 14 Our Galaxy 14.1 The milky way revealed What does our galaxy look like? - It is a Spiral galaxy - It has Spiral arms - Fairly flat disk - Bright central bulge - Disk surrounded by a halo - Our galaxy consists of a flat disk with spiral arms surrounding a central bulge, with a roughly spherical halo surrounding everything  Globular cluster: spherical collection of stars that orbit a galactic core  Interstellar medium: clouds of interstellar gas and dust  2 small galaxies in the milky way are named large Magellanic cloud and small Magellanic cloud - The milky way is a large galaxy and several small galaxies orbit it How do stars orbit in our galaxy? Orbits of disks and stars - Disk stars orbit the galaxies centre in orderly circles that all go in the same direction, bobbing slightly up and down as they orbit Orbits of halo and bulge stars - Orbits of halo and bulge are much less organized than disk - Halo stars swoop high above and below the disk on randomly oriented orbits Using stellar orbits to measure the mass of the galaxy - We can calculate the mass of the galaxy within the suns orbit using the suns orbital properties and Newton’s version of Kepler’s third law - The orbits of the milky way stars reveal that most of the galaxy’s mass consists of invisible dark matter in the halo  The orbital velocity law 2 M r r x v G 14.2 Galactic recycling How is gas recycled in our galaxy?  Star-gas-star cycle: atomic hydrogen clouds  molecular clouds  star formation  nuclear fusion in stars  returning gas  hot bubbles  back to start Gas from dying stars - Supernovae and high speed stellar wings can produce hot bubbles of gas in the interstellar medium - Giant bubbles of hot gas can erupt out of the disk, spreading their contents over a large region of the galaxy  Supernovae remenant: aftermath of the supernova  Cosmic rays: rays caused by supernova that can cause genetic mutations in living organisms Cooling and cloud formation  Atomic hydrogen gas: gas that is cool enough that hydrogen atoms remain neutral rather than being ionized (78% H, 28% He, 2% Other elements) - Atomic hydrogen emits a spectral line with a wavelength of 21 centimeters called the 21 centimeter line  Interstellar dust grain: tiny solid flecks of carbon and silicon that form in the winds of giant red stars - Gas heated by supernovae first cools into atomic hydrogen then cools fither into molecular clouds C
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