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Charles Hakes Fort Lewis College 1 Charles Hakes Fort Lewis College 2 Chapter 15-16 The Milky Way Dark Matter Extending the Distance Scale Charles Hakes Fort Lewis College 3 Mapping the Milky Way Charles Hakes Fort Lewis College 4 Spiral Galaxies • A view of spiral galaxies from faceon and edge-on. Charles Hakes Fort Lewis College 5 Figure 14.1 Galactic Plane Charles Hakes Fort Lewis College 6 Mapping the Milky Way • Radio observations can determine much of the structure and rotation rates. Charles Hakes Fort Lewis College 7 Mapping the Milky Way • Radio observations can determine much of the structure and rotation rates. • Orderly rotation in the plane. • Random orbits in the halo. Charles Hakes Fort Lewis College 8 Figure 14.12 Stellar Orbits in Our Galaxy Charles Hakes Fort Lewis College 9 Figure 14.10 Observations of the Galactic Disk Charles Hakes Fort Lewis College 10 Mass of the Milky Way • Recall Newton’s modification to Kepler’s third law: total mass (solar) = Charles Hakes Fort Lewis College orbit size (A.U.)3 orbit period (years)2 11 Figure 14.18 Galaxy Rotation Curve Charles Hakes Fort Lewis College 12 Mass of the Milky Way • There is apparently more mass than can be seen. • Unseen mass out to ~50 kpc. • Recall radius of observable Milky Way is ~15 kpc. • Dark Matter • Can detect gravitational effects • Cannot detect any other way. Charles Hakes Fort Lewis College 13 Dark Matter • Is not atomic or molecular clouds - we would detect those using spectroscopy. • Could be brown dwarfs or white dwarfs - very difficult to see. • MACHOs - MAssive Compact Halo Objects • Could be exotic subatomic particles • WIMPs - Weakly Interacting Massive Particles Charles Hakes Fort Lewis College 14 Charles Hakes Fort Lewis College 15 Figure 14.19 Gravitational Lensing What observations suggest the mass of the Galaxy goes much farther out than its visible disc? A) the orbits of the open clusters in the disc B) x-ray images of other galaxies' discs from Chandra C) the rotation curve beyond 15kpc D) 21 cm maps of the spiral arms E) infrared observations of distant brown dwarfs Charles Hakes Fort Lewis College 16 What observations suggest the mass of the Galaxy goes much farther out than its visible disc? A) the orbits of the open clusters in the disc B) x-ray images of other galaxies' discs from Chandra C) the rotation curve beyond 15kpc D) 21 cm maps of the spiral arms E) infrared observations of distant brown dwarfs Charles Hakes Fort Lewis College 17 Galaxy Masses Charles Hakes Fort Lewis College 18 Figure 16.4 Galaxy Rotation Curves Charles Hakes Fort Lewis College 19 Galaxy Masses • Galaxy masses determined from Newton’s modification to Kepler’s third law. • Within the visible spiral, radial velocities (and masses) can be measured directly. • Outside the visible spiral, observe multiple galaxy systems. • Only radial velocity determined with Doppler shift. • Reliable statistical information from lots of observation. Charles Hakes Fort Lewis College 20 Figure 16.5 Galaxy Masses • from Newton’s modification of Kepler’s law Charles Hakes Fort Lewis College 21 Galaxy Masses • Galaxies apparently have invisible halos similar to the Milky Way. • All contain 3-10 times the visible mass. • Mass discrepancy is even greater for clusters of galaxies. Charles Hakes Fort Lewis College 22 Figure 16.6 Dark Galaxy? Charles Hakes Fort Lewis College 23 Figure 16.7ab Galaxy Cluster X-Ray Emission • Intergalactic space is filled with superheated gas in this cluster. Charles Hakes Fort Lewis College 24 Figure 16.7c Galaxy Cluster X-Ray Emission Charles Hakes Fort Lewis College 25 Figure 16.8 Head–Tail Radio Galaxy - Could this be a “wake” through intergalactic clouds? Charles Hakes Fort Lewis College 26 Extending the Distance Scale • • • • Variable Stars Tully-Fisher Relationship Supernovae Cosmological Redshift Charles Hakes Fort Lewis College 27 Figure 14.7 Variable Stars on Distance Ladder • Greater distances can be determined than typically available through spectroscopic parallax, because these variables are so bright. Charles Hakes Fort Lewis College 28 Figure 15.12 Local Group Charles Hakes Fort Lewis College 29 Tully-Fisher Relationship Charles Hakes Fort Lewis College 30 Figure 15.9 Galactic “Tuning Fork” • Galaxies are classified according to their shape (Hubble classification) • Elliptical • Spiral • Irregular Charles Hakes Fort Lewis College 31 Figure 15.10 Galaxy Rotation • Rotation rates can be determined using Doppler shift measurements • Blue shift indicates moving towards you • Red shift indicates moving away from you Charles Hakes Fort Lewis College 32 Tully-Fisher Relationship • Rotation speed can be used to determine a galaxy’s total mass. • A close correlation between rotation speed and total luminosity has been observed. • Comparing (true) luminosity to (observed) apparent brightness allows us to determine distance • Distance scale can be extended to ~200 Mpc. Charles Hakes Fort Lewis College 33 Figure 15.11 Extragalactic Distance Ladder Charles Hakes Fort Lewis College 34 Supernovae • Type II Supernovae • Are a result of a very massive star’s core collapse • Can vary in brightness, since the cores can vary in size. • Therefore, they are not a good distance indicator. Charles Hakes Fort Lewis College 35 Supernovae • Type I Supernovae • White dwarf, carbon detonation • Are a result of a white dwarf exceeding its Chandrasekhar limit (1.4 Msolar). • They are all about the same size. • They are very good distance indicators (Standard Candles). Charles Hakes Fort Lewis College 36 Standard Candles • Standard Candles are easily recognizable astronomical objects whose luminosities are confidently known. • Term usually only refers to very luminous objects • Type I supernovae • Other objects might include • Rotating spiral galaxies • Cepheid variables • Main sequence stars Charles Hakes Fort Lewis College 37 Figure 15.11 Extragalactic Distance Ladder Charles Hakes Fort Lewis College 38 Review Questions Charles Hakes Fort Lewis College 39 Which of these does not exist? A) a .06 solar mass brown dwarf B) a 1.3 solar mass white dwarf C) a six solar mass black hole D) a million solar mass black hole E) a 3.3 solar mass neutron star Charles Hakes Fort Lewis College 40 Which of these does not exist? A) a .06 solar mass brown dwarf B) a 1.3 solar mass white dwarf C) a six solar mass black hole D) a million solar mass black hole E) a 3.3 solar mass neutron star Charles Hakes Fort Lewis College 41 A star has an apparent magnitude of +1.0 and an absolute magnitude of +1.0. If the distance between Earth and the star increases, the apparent magnitude would _____, and the absolute magnitude would _____. A) increase; decrease B) decrease; increase C) increase; not change D) decrease; not change E) not change; increase Charles Hakes Fort Lewis College 42 A star has an apparent magnitude of +1.0 and an absolute magnitude of +1.0. If the distance between Earth and the star increases, the apparent magnitude would _____, and the absolute magnitude would _____. A) increase; decrease B) decrease; increase C) increase; not change D) decrease; not change E) not change; increase Charles Hakes Fort Lewis College 43 A star has apparent magnitude of +8.0 before it goes nova and increases its luminosity by 10,000 times. Its apparent magnitude after it goes nova is. A) +8.0 B) +18.0 C) -8.0 D) -2.0 E) +3.0 Charles Hakes Fort Lewis College 44 A star has apparent magnitude of +8.0 before it goes nova and increases its luminosity by 10,000 times. Its apparent magnitude after it goes nova is. A) +8.0 B) +18.0 C) -8.0 D) -2.0 E) +3.0 Charles Hakes Fort Lewis College 45 Using spectroscopic parallax, you find a star’s distance to be 76 parsecs. You now find out that the star isn’t a main sequence star, but is a red giant. Your distance estimate is A) too large B) too small C) fine - no significant change in estimate is needed. Charles Hakes Fort Lewis College 46 Using spectroscopic parallax, you find a star’s distance to be 76 parsecs. You now find out that the star isn’t a main sequence star, but is a red giant. Your distance estimate is A) too large B) too small C) fine - no significant change in estimate is needed. Charles Hakes Fort Lewis College 47 Which is correct? 1 - The new moon rises at noon. 2 - The first quarter moon rises at noon. 3 - The full moon rises at noon. 4 - The third quarter moon rises at noon. Charles Hakes Fort Lewis College 48 Which is correct? 1 - The new moon rises at noon. 2 - The first quarter moon rises at noon. 3 - The full moon rises at noon. 4 - The third quarter moon rises at noon. Charles Hakes Fort Lewis College 49 In Paris, France (50 degrees north latitude), what is the longest day of the year? 1: 2: 3: 4: March 21 June 21 September 21 December 21 Charles Hakes Fort Lewis College 50 In Paris, France (50 degrees north latitude), what is the longest day of the year? 1: 2: 3: 4: March 21 June 21 September 21 December 21 Charles Hakes Fort Lewis College 51 Where along the horizon does the Sun rise on June 21 in Paris, France? 1: 2: 3: 4: Due east North of east South of east Can’t tell with information given Charles Hakes Fort Lewis College 52 Where along the horizon does the Sun rise on June 21 in Paris, France? 1: 2: 3: 4: Due east North of east South of east Can’t tell with information given Charles Hakes Fort Lewis College 53 Three Minute Paper • Write 1-3 sentences. • What was the most important thing you learned today? • What questions do you still have about today’s topics? Charles Hakes Fort Lewis College 54