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 isn’t: – milk – a way – a cloud • What it is: our host galaxy • A galaxy is a group of millions or billions of stars—plus 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 Hera’s breast) • “Camino de Santiago” (Spanish, Catalan) • “Vintergatan” = Winter Street (Swedish) • “The Bird’s 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 Kapteyn’s 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, Shapley’s 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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