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Galaxies

Galaxies. I. Topics. The Milky Way Galaxies and Dark Matter Active Galaxies. The Milky Way. History – I. 1785 – William Herschel (German musician)

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Galaxies

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  1. Galaxies I

  2. Topics • The Milky Way • Galaxies and Dark Matter • Active Galaxies

  3. The Milky Way

  4. History – I • 1785 – William Herschel (German musician) • Discovered that the stellar system to which the Sun belongs was shaped like a disk. He did this by counting the number of stars in different directions.  • 1845 – Earl of Rosse • Made first drawing of the Whirlpool Galaxy (M51).

  5. The Whirlpool Galaxy – M51 Drawing by Earl of Rosse * * http://www.seds.org/messier/more/m051_rosse.html

  6. History – II • 1912 – Henrietta Leavitt • Studied Cepheid variables in the Small Magellanic Cloud. • Cepheids dim and brighten in a regular manner. • The time between corresponding points on the light curve is called the period of the variable.

  7. The Large Magellanic Cloud (LMC)

  8. History – III Luminosity-Period Relation of Cepheid Variables Henrietta Leavitt discovered that the period of a Cepheid is related to its absolute luminosity.

  9. History – IV • 1915 – Harlow Shapley • Measured the positions of dozens of globular clusters using RR Lyrae stars. • These stars, like Cepheids, are variables whose absolute luminosities are related to their periods of light variation. • Most of these clusters were found to be located in the direction of the constellation Sagittarius.

  10. History – IV • 1917 • Shapley conjectured that the globular clusters orbit the galactic center and that the latter was in the direction of Sagittarius. • This would explain why the globular clusters appeared to be mostly in that direction of the sky.

  11. History – V • 1920 – The Shapley-Curtis Debate • Harlow Shapley debated Heber D. Curtis. • Shapley argued that the nebulae (which later proved to be galaxies) lie within the Milky Way. • Curtis, however, argued that they resided far from our galaxy. • The issue was not resolved.

  12. History – VI – Island Universes • 1923 – Edwin Hubble • Photographed the Andromeda Galaxy (M31) and showed (in 1924) that M31 is at about 2 million light years from us. • This was conclusive proof that the galaxy lay well beyond the Milky Way. • The universe was much vaster than previously thought.

  13. The Andromeda Galaxy http://www.seds.org

  14. Structure • Disk  • Near the Sun the average density of stars is about 1star per 330 cubic light years. • Bulge • In the galactic nucleus there are about 10 millionstars per cubic light year. • Halo • The globular clusters form a halo about the galaxy.

  15. The Milky Way in Visible Light

  16. Characteristics • Diameter • About 90,000 light years. • The sun is about 26,000 light years from the center and takes about 240 million years to make a complete orbit, at about 220 km/s. • Age • From the oldest stars we know that the galaxy is about 15 billion years old. • Luminous Mass • Between 100 to 500 billion solar masses.

  17. Stellar Populations • Population I   • Stars about 1 million to a few billion years old. • Found in the disk and spiral arms, like the Sun. • Relatively rich in heavier elements (about 3% by mass) because they are of the second or third generation. • Population II • Stars around 10 billion years old that populate the galactic bulge and halo. These stars contain only trace amounts of heavier elements. 

  18. Star Clusters • Open Clusters • Loose groupings of stars that contain a few hundred stars in a volume with a radius between 5 and 20 light years. • The stars are mainly population I stars. • A beautiful example is the Pleiades.

  19. The Pleiades

  20. Star Clusters – II • Globular Clusters • These are spherical groupings of stars that contain thousands to millions of stars. • These clusters have radii in the range 40 to 160 light years. • A good example is the globular cluster, M13 in the constellation Hercules. • The stars are always (old) population II stars.

  21. The Great Globular Cluster in Hercules Eddie Guscott, http://antwrp.gsfc.nasa.gov/apod/ap040511.html

  22. Nebulae • Emission Nebula • Gas is ionized by ultraviolet light from hot stars. Electrons recombine with the ions to form atoms, which then drop down to lower energy levels by emitting photons. • Dark Nebula • Dust clouds that absorbs light from other nebulae and stars. • Reflection Nebula • Clouds of gas and dust that reflect rather than emit light.

  23. Emission Nebula: Orion

  24. Dark Nebula: The Lagoon

  25. Dark Nebula: The Horsehead

  26. Reflection Nebula: The Pleiades

  27. The Galactic Center • The Great Galactic Shroud • Unfortunately, we cannot see the center of our galaxy in visible light because the light from the center is absorbed by the intervening dust. • But, fortunately, radio and infrared waves can penetrate the dust. • We have therefore been able to study the galactic center in detail, even though it is shrouded by dust and is 26,000 ly away.

  28. The Galactic Center

  29. Radio Observations of our Galaxy • 21-cm line of Hydrogen • The most-used radio emission line for studying the Universe is the 21-cm line of hydrogen. • The 21-cm radio photons are created when a spin-flip transition occurs in the spin of the electron.

  30. The Galactic Center – II

  31. The Galactic Center – III • The Galactic Nucleus • Diameter about 10 light years • Emits about 80 million Suns worth of infrared energy • Emits radio waves and variable amounts of x-rays. • Sgr A* • At the very center is a bright region called Sgr A*. It is now possible to measure the rotation speeds of stars very close to Sgr A*. • One star gets within 17 light hours of Sgr A*.

  32. Stellar Orbits about Sgr A*

  33. Sgr A* Mass of Sgr A* can be inferred from an application of Newton’s laws: M = V2 R / G The mass turns out to be about 2.6 million Solar masses http://ircamera.as.arizona.edu/NatSci102/lectures/milkyway.htm

  34. Matter Between the Stars • The Interstellar Medium • The space between stars is filled with about 1 atom of hydrogen per cubic centimeter on average. • H I Regions • Regions of higher density that contain predominantly neutral hydrogen, sometimes in molecular form. • H II Regions • Regions of ionized hydrogen, that is, of electrons and protons.

  35. The Triffid Nebula – H II Region

  36. Summary • The Milky Way • A disk of about 100 billion stars with spiral arms and a central bulge. The younger (Population I) lie in the disk and spiral arms; the old (Population 2) stars populate the bulge and halo. • The galaxy contains open clusters, globular clusters and nebulae of various kinds (emission, dark and reflection). • The Galactic Center • Measurements of stellar orbits around Sgr A* indicate it has a mass of 2.6 million Solar masses. The evidence indicates this could be a black hole.

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