12 Globular cluster system Centered on the center of the Milky WayExtends far above and below the planeBy observing globular clusters, we can determine the direction to the center of the Milky Way (and, later, our distance from the center).
18 Use known luminosity to determine distance Flux versus luminosity relationWe can figure out the luminosity of a pulsating star by timing the pulsations. Since, we can measure its flux, we can then find the distance to the star.
20 Luminosities of components With distances, can do star counts, correct for dust absorption and estimate luminositiesDisk: 19109 LBulge: 2109 LHalo: 2109 LTotal: 23109 LAverage star is dimmer than the Sun, total number stars is ~200 billion.
21 Review QuestionsWhat are globular clusters? How are they distributed in the Galaxy?What are Galactic coordinates?Why do some stars pulsate? Why are pulsating stars useful in measuring distances?What is the size and shape of the Milky Way?
22 Orbits of stars in the Milky Way Stellar orbits in disk and haloFinding the mass from the orbitMass of the Milky WayRotation curvesDark matter
23 What keeps the planets in orbit around the Sun? The force of gravity from the SunTo orbit, a planet at a particular distance from the Sun must have a particular orbital speed.
25 Orbits of stars in the Milky Way The orbit of a star is determined by the total mass lying inside the orbitBy measuring the speed of the star’s orbit and its distance from the center, we can figure out the total mass lying inside the orbit of the star
26 Stellar Orbits in the Galaxy Stars in the disk all orbit the Galactic center:in the same directionin the same plane (like planets do)they “bobble” up and downthis is due to gravitational pull from the diskthis gives the disk its thicknessStars in the bulge and halo all orbit the Galactic center:in different directionsat various inclinations to the diskthey have higher velocitiesthey are not slowed by disk as they plunge through itnearby example: Barnard’s Star
28 The distance between the Sun and the Galactic center is closest to: 800 pc8 kpc8,000 kpc8 MpcVariable, between 1 and 10 kpc
29 Mass of the GalaxyWe can use Kepler’s Third Law to estimate the mass of the Milky Way inside the Sun’s orbitSun’s distance from center of Milky Way: ,500 pc = 1.8 x 109 AUPeriod of Sun’s orbit around the center of the Milky Way: 230 million years (2.3 x 108 yr)
30 Simplified form of Kepler’s 3rd law using convenient units Where M in solar massesa in AUP in Earth years
31 Mass of the Milky Way within the Sun’s orbit Where M in solar masses, a in AU, P in Earth yearsMass within Sun’s orbit is 1011 MTotal mass of MW Galaxy is ~ 1012 MTotal number of stars in MW Galaxy 2 x 1011
32 Kepler’s 3rd Law applied to Binary Stars Where:G is gravitational constantG = 6.67·10-11 m3/kg-s2 in SI unitsm1, m2 are masses (kg)P is binary period (sec)A is semi-major axis (m)
33 Kepler’s 3rd Law applied to Galaxy Where M(r) is mass inside r (kg)Change from P to velocity = v
38 Dark Matter Dark – it doesn’t produce light (any kind) Does have mass, produces gravityNature is unknownMight be normal matter in a form that doesn’t emit much light – very small and dim star, little black holesMore likely it is elementary particles other than normal matter
39 What properties of the sun could be used to measure the total mass enclosed within the sun's orbit? mass and orbital speedmass and distance from the centermass and ageorbital speed and distance from the center
40 The Milky Way in various wavelengths Milky Way in infrared, radioThe 21 cm line of Hydrogen
42 Scattering of light Blue light is scattered more Red light is transmitted moreStars seen through dust appear redder than they really areIf we want to try to see through dust, what kind of light should we use?
56 Why can't we see visible radiation from the central region of the galaxy? No visible light is emitted by the central region of the galaxy.Interstellar dust blocks our view.Too many stars are in the way.Gravity curves the light away from the earth and Sun.
61 Spiral arms are patterns According to the density-wave theory, spiral arms are created by density waves that sweep around the GalaxyThe gravitational field of this spiral pattern causes stars and gas to slow down near the armThis compresses the interstellar clouds, triggering the formation of starsThe entire arm pattern rotates around the Milky Way once every 500 million years
65 Which of the following objects are not found primarily in the spiral arms of the Galaxy? white dwarf starsHII regionssupernovasO and B stars
66 Review Questions How can we measure the mass of the Milky Way? Why do stars behind dust clouds appear red?Why is the sky blue?Why are wavelengths of light outside the visible useful in studying the Milky Way?How is the 21 cm line of Hydrogen produced?Describe the spiral arms of the Milky Way and what causes them.
67 The nature of the dark matter in the Milky Way is: Definitely in the form of brown dwarfsProbably cold, dark hydrogen moleculesLikely super-massive black holesDefinitely cold gas, unknown compositionNot known
68 From the rotation curve of a hypothetical galaxy as shown in the figure, one could infer A concentration of dark matter inside 2 kpcA spherical distribution of dark matterDark matter only outside 2 kpcUniformly distributed dark matter in the diskNo dark matter is needed to explain this plot
69 The Galactic Center Center of the Milky Way Stars orbiting the black holeX-ray flares from the black hole
74 Mass of Sgr A* can be measured using stellar orbits Fastest moving star moves at 2% of the speed of light, 5000 km/sMass is about 3 million solar massesEmits radio and X-raysAlmost certainly a black hole
75 X-ray image, central 3 lySgr A* is the bright object in the center of the image.Makes flares in X-rays.Movie.
76 The best estimate of the mass of the black hole at the galactic center was made using: Observations of nearby star orbits in IRRadio observations of accretion diskOrbit of stars in spiral arms [Kepler’s 3rd law]X-ray observations of coronal gasDark matter studies using rotation curves
77 Review QuestionsWhat properties of a star’s orbit around the Galaxy enable one to measure the mass inside its orbit?What is the shape of the rotation curve of the Milky Way and why is was it unexpected?What lies at the center of the Milky Way?