MilkyWay - 2/2/10 The Milky Way Galaxy Chapter 16 Overview...

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2/2/10 1 The Milky Way Galaxy Chapter 16 Overview & History History of Galactic Astronomy 1610 -Galileo discovered the Milky Way is comprised of many stars 1755 - Immanuel Kant theorized that the galaxy has a planar structure, some nebulae might actually be entire other galaxies or island universes 1774 -1781 - Messier catalog compiled including Andromeda galaxy as M31 1781-1802 - William and Caroline Herschel conducted first “all-sky survey” and cataloged 5000 nebulae, resolving some into their individual stars Later (1845) William Parsons (Lord Rosse), using a 72-inch telescope, classified the nebulae into featureless ellipticals and whirlpool-like spiral nebulae Much later (1888) Dreyer would add to their list to create the New General Catalog (NGC) and Index Catalog (IC)
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2/2/10 2 1785 - Herschel attempted to determine the shape and size of Galaxy Assumptions: All stars have same intrinsic brightness Star are arranged uniformly throughout the MW He could see to the edge of the system then d max = 10 0.2(m_max-M+5) SUN •Early 1900’s - Kapteyn used stellar parallax to estimate the true size of the Galaxy Kapteyn Universe •10kpc diameter and 2kpc thick with the Sun less than a kpc from the center (rather heliocentric) •Tried to estimate Rayleigh scattering due to ISM gas but determined it to be insignificant (because most obscuration is due to ISM dust absorption which has a smaller λ dependence) •Shapley (1919) noted that globular clusters are distributed asymmetrically in the sky and that if one assumes they are distributed about the center of the galaxy, this implies the Sun in not near the center •Estimated distances to GCs using variable stars and P-M relationship •Concluded size to be 100kpc with Sun 15kpc from center Still wrong…didn’t account for dust absorption which makes things look further away Globular Clusters in our Galaxy Actual size of the Galaxy and the Sun’s location not fully determined until 1950’s
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2/2/10 3 In 1920, the National Academy of Science hosted the Great Debate concerning the nature of the Spiral Nebulae: were they island universes outside of the MW? •Shapley had MW size too big and therefore argued “NO”, they are part of MW •Curtis (and many others at that time) believed the Kapteyn model of a much smaller MW and argued “YES”, they are separate galaxies beyond the extent of the MW. Note the date: 6 Oct 1923 His notes about a variable star In 1922-1924 Edwin Hubble resolved the controversy using the superior 100-inch telescope at Mount Wilson. He observed Cepheid variables in Andromeda and, using the P-m relation, determined its distance at 300kpc -- well outside of the MW (still off by a factor of 2 due to poor Cepheid calibrations) Subsequent claims that the SMC and LMC are about 32-34 kpc away Also in the early 1900’s, Lindblad was doing the first kinematic studies of the MW •Estimated mass in MW from all stars in Kapteyn’s model
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This note was uploaded on 02/13/2012 for the course AST 3019 taught by Professor Sarajedini during the Spring '08 term at University of Florida.

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MilkyWay - 2/2/10 The Milky Way Galaxy Chapter 16 Overview...

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