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Galaxies

Galaxies. II. Topics. Galaxies and Dark Matter The Expanding Universe. Island Universes. 1924 – Edwin Hubble Measured the distances to several galaxies using the Luminosity-Period relationship of Cepheid variables, discovered by Henrietta Leavitt (1912). Hubble at the Mount Wilson

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Galaxies

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  1. Galaxies II

  2. Topics • Galaxies and Dark Matter • The Expanding Universe

  3. Island Universes • 1924 – Edwin Hubble • Measured the distances to several galaxies using the Luminosity-Period relationship of Cepheid variables, discovered by Henrietta Leavitt (1912) Hubble at the Mount Wilson Observatory

  4. Galaxy Types Elliptical Spiral Irregular Peculiar

  5. Galactic Clusters • Clusters • On very large scales galaxies are found to form clusters. • The Milky Way, in particular, inhabits a cluster called the Local Group which contains about 30 galaxies of which the Milky Way, M31 and M33 are the three largest.  

  6. Local Group Contains about 30 galaxies http://en.wikipedia.org/wiki/Local_Group

  7. Galactic Clusters – II • SuperClusters • On even greater scales the galaxy clusters themselves form clusters called superclusters that can be tens to hundreds of millions of light years across. • These superclusters resemble huge ribbons or sheets in space. • Between them lie great voids containing relatively few galaxies.

  8. Virgo Supercluster

  9. Slices of the Universe ~ 1 billion light years You are here

  10. Dark Matter

  11. Rotation Curve of Milky Way • Rotation Curve • This is a plot of the orbital speed of stars, about the galactic center, versus distance from the center. • The speed is determined by the amount of matter enclosed within the star’s orbit.

  12. Rotation Curve of Milky Way – II • How Much Mass? • Newton’s version of Kepler’s 3rd Law is • If m1 >> m2 R P = Period of orbit v = Speed of star

  13. Rotation Curve of Milky Way – III • How Much Mass? • Mass enclosed in radius R • v = 200 km / sec • R = 26,000 light years • m1 ~ 1011 Solar masses R P = Period of orbit v = Speed of star

  14. Rotation Curve Puzzle • Puzzle • Kepler’s Law predicts that the rotation curve should decrease with increasing distance from the center of gravity, like 1/√R, as is true in our solar system. • But this law fails for stars orbiting the galactic center.

  15. Rotation Curve Puzzle – Possible Solutions • Possible Solution: A New Law of Gravity • Newton’s law of gravity fails on very large scales. • Possible Solution: Dark Matter • Newton’s law holds even on large scales. • But a non-luminous form of matter must exist. • If so, the dark matter could be new forms of matter or dark stars, or a mixture of both.

  16. Dark Matter • Sub-Stellar Objects (“Failed Stars”) • Would-be stars that did not draw together enough matter to trigger thermonuclear reactions in their core. • These are objects with masses less than about 0.01 solar masses. They shine so feebly that they cannot be seen from Earth. • Sub-stellar objects in our halo are referred to as Massive Compact Halo Objects (MACHOs).  • Black Holes • One intriguing possibility is that the dark matter may be in the form of black holes. 

  17. Dark Matter – II • Weakly Interacting Massive Particles (WIMPS) • These particles could be everywhere. • If the WIMP hypothesis is correct, these particles are traversing your body right now! • But because they interact so weakly with matter it is unlikely that any will actually interact with the particles in your body! • Experiments are being conducted worldwide to test the WIMP hypothesis.

  18. Dark Stars – Gravitational Lenses

  19. Dark Stars – Gravitational Lenses Dark stars have been found but not enough to explain the missing mass.

  20. The Expanding Universe • 1929 – Red Shift • Drawing on his own observations and those of others, Edwin Hubble discovered that the red shift, z = (lo - le) / le of the light from distant galaxies increases with distance d. le = emitted wavelength lo = observed wavelength

  21. The Expanding Universe – II • 1929 – Hubble’s Law • Hubble assumed that the red shift is caused by the motion of galaxies away from us. In this case z = v / c • His observations are summarized in Hubble’s Law v = H0 d where H0 is called Hubble’s constant.

  22. The Expanding Universe – III • Hubble’s Law v = H0 d • The Hubble Time d = v t d = H0 d t t = 1/H0 • For H0= 70 km/s / Mpc • t ~ 14 billion years. 1 Mpc (Mega-parsec) = 3.26 x 106 light years (ly)

  23. t1 = past le t0 = now d(t1) a < 1 d(t0) t2 = future a = 1 d(t) = a(t) d0 lo d(t2) le= a(t) lo a > 1 z= (lo - le) / le 1 + z = 1/a(t) The Universal Scale Factor a(t) is the scale factor of the Universe t is cosmic time

  24. d0, t0 L = c (t0 – t1) d1, t1 How Far Is Far ? t0 – t1 is the look-back time d0 = d(t0) is the proper distance between the two galaxies now d1 = d(t1) is the proper distance between the two galaxies then

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