18. Galaxies - Galaxies Outline Our goals for this section:...

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Unformatted text preview: Galaxies Outline Our goals for this section: Describe the overall shape and structure of the Milky Way galaxy Learn about the galactic rotation curve and what it tells us about dark matter To describe the main types of normal galaxies and their properties To describe the large-scale structures to which galaxies belong Optical panorama http://home.arcor-online.de/axel.mell LMC SMC Galactic centre Dust clouds What it isnt: milk a way a cloud What it is: our host galaxy A galaxy is a group of millions or billions of starsplus associated gas, dust, stellar remnants, planets, etc. all bound together by gravity, isolated in space from other major groups of stars The Sun is just one of about 100 billion stars making up the Milky Way Properly, we capitalize galaxy when referring to the Milky Way the Galaxy = the Milky Way the galaxy = some other galaxy Introducing the Milky Way The Milky Way Known since ancient times - easily visible under a dark sky Milky Way (many European languages - from Greek mythology: milk from Heras breast) Camino de Santiago (Spanish, Catalan) Vintergatan = Winter Street (Swedish) The Birds Path (Estonian/Finnish/Latvian/Lithuanian) Silvery River (Chinese) Heavenly Ganges (Hindi) The Milky Way is shaped like a disk with a bulge in the center The diameter of the disk is about 30 kpc (~600,000,000,000,000,000 miles) It has three main components: the Galactic disk, the Galactic bulge, and the Galactic halo The Structure of the Milky Way Up until the early 20 th century, most astronomers believed the whole universe to be a few kpc in diameter and centred on the Sun remember way back when the best minds of the day thought Earth was the centre? In the early 20 th century, an abundance of astronomical evidence was beginning to emerge to suggest that we might live in one of many galaxies But some evidence suggested that other galaxies, then known as spiral nebulae were merely small objects inside our galaxy In 1920, Shapley publicly debated astronomer Heber Curtis The Shapley-Curtis Debate Shapley: Universe is one big galaxy Globular cluster distances imply huge dimensions for Milky Way Central location of the Sun in Kapteyns model is strange Novae in spirals would be ridiculously bright if spirals are similar to Milky Way The central surface brightness of spirals is much brighter than anything seen in the Milky Way Positive radial velocities of spirals might be due to radiation pressure from Milky Way Spirals have rotational periods of ~10 5 years (measured by van Maanen, Shapleys friend) which is impossible if spirals are as large as Milky Way Curtis Our galaxy is one of many Spirals have large radial velocities - if part of the Galactic system, should also have large proper motions. Not observed....
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18. Galaxies - Galaxies Outline Our goals for this section:...

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