1	Chapter 14 Our Galaxy
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3	Questions How many stars are in the Milky Way? About 200 billion How many galaxies are there? About 40 billion (approximately)
4	Questions How old is the Milky Way Galaxy? It is 15 billion years old and will remain active for a another 10 billion years. Recall: The Sun is ~ 4.5 billion years old.
5	Are all stars members of the Milky Way? No, most stars in the universe are in other galaxies. What is the structure of the Milky Way? We know in general, that it has the shape of a disk with spiral arms and a central bulge.
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7	Where in the Milky Way is our solar system located? The solar system is located in a spiral arm about 28,000 light years (8kpc) from the center of the Milky Way.
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13	Galaxy Formation
14	The Star-Gas-Star Cycle Generations of stars continually recycle the same galactic matter. Supernova stir and heat the interstellar medium and contribute new heavy elements. Stars return mass to interstellar medium in two ways Stellar winds (solar wind) Death events (planetary nebula and supernova)
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16	"A dying low mass star..." A dying low mass star returns gas to the interstellar medium in a planetary nebula. (HST)
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18	Atomic Hydrogen Gas Most of the intersellar medium is composed of atomic hydrogen gas (70%). Atomic hydrogen emits a spectral line with a wavelength of 21 cm – in the radio! Using radio telescopes, Astronomers can detect this 21 cm radiation coming form all different directions.
19	Molecular Clouds Cool clouds of molecular gas from out of hydrogen and other elements. Molecular clouds are formed from: H₂- Hydrogen gas CO – Carbon Monoxide H₂O – Water NH₃– Ammonia Gravitational forces in molecular clouds collect molecules into dense cores, eventually becoming protostars.
20	"Molecular Cloud in the constellation..." Molecular Cloud in the constellation Orion.
21	Completing the Cycle After a few stars begin to form in a cluster, their UV radiation begins to ionize the surrounding gas of the cloud from which they have formed. The surrounding cloud material begins to ionize and give off its own light. We now have an ionization or emission nebula.
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24	Distribution of gas in the Milky Way
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31	Spiral Arms Observational evidence suggests that they result as a consequence of waves generated by star formation. Theoretical models indicate that disturbances in the disk form spiral density waves that are the source of the galaxy’s spiral arms.
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36	Star Velocities Near The Galactic Center
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38	How do we know that we are located in the spiral arm? In 1917 Harlow Shapley discovered that the globular clusters form a huge spherical system that is not centered on the Earth.
39	Finding Our Place
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41	The Great Debate - April 26, 1920
42	RR Lyrae and Cepheid Variable Stars These are both pulsating variable stars. Their pulsation periods are on the order of a few days. Using the period-luminosity relationship, distances to other galaxies can be estimated
43	RR Lyrae & Cepheids on H-R diagram
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45	Light curves for RR-Lyrae and Cepheid Variables
46	New Distance Ladder
47	Messier Catalogue
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50	"In 1923," In 1923, Edwin Hubble was examining photographic plates of the Andromeda Nebula M31 Hubble located three novae, each marked with an "N.“ (he was looking for Novae) One of these novae, however, turned out to be a Cepheid variable, a star that changes predictably in brightness The "N" was crossed out and the star was relabeled "VAR!“ This Cepheid, and others subsequently discovered in the Andromeda Nebula, enabled Hubble to prove that the Nebula was not a star cluster within our own Milky Way, but a galaxy more than a million light years away.
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52	Edwin Hubble
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56	End of Section