# Milky Way Galaxy. A. Star counts. B. Core and Arms. C. Galaxy Rotation. Milky Way. A1a. Milky Way 4 A1b. Milky Way: Galactic Equator 5

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1 Milky Way Galaxy Milky Way A. Star counts B. Core and Arms C. Galaxy Rotation Dr. Bill Pezzaglia Updated: Nov, Aa. Milky Way 4 Ab. Milky Way: Galactic Equator a. Galileo Galilei (64-64) 6 Ac. 7 Thomas Wright 7 Galileo was one of the very first scientists to do experiments to understand Nature He was the first astronomer to use a telescope (in 6) to study the sky. Sees that the Milky Way is made of stars Milky Way is thin shell of stars, which explains why we see a band across the sky.

2 Ac. 784 Herschel s Telescope 8 year study of 4 regions of sky (maps over 9, stars) Ac. The Galactic Equator Herschel s Star Gauging Counts stars in 68 regions Estimates universe is disk shaped Diameter is times thickness Sun appears to be at the center Ac. Kapteyn Universe measured that stars move (rotate around universe) Magnitude Distance Relation Objects further away look fainter If the star is too far away, it will be fainter than our limiting magnitude and we won t see it. Distance (parsecs) magnitude. (Parsec=.6 light years=, x distance to sun) Assume at parsecs an average star has (absolute) magnitude of M=+. Every factor of in distance it gets magnitude fainter m M Log D/ Space Penetrating Power Turn this idea around. From the limiting magnitude of our telescope, we can estimate how deep we are penetrating into the galaxy. Limiting Magnitude Distance (parsecs). Counting Stars The number of stars N seen in a field of view of as a function of the depth we see into space ( space penetrating power) assuming constant density : We need to see deeper than parsecs to be able to see the thickness of the Milky Way. D m/ m m M N V Log( N) Log Log( D ) If density is constant, expect plot of log of star count vs log of distance to have a slope of

3 Stars vs Magnitude Since magnitude is proportional to times log of distance, expect plot of log count vs magnitude to be a line with slope of.6 : a Number of Stars by Magnitude 4 Log ( N) b Log( D) This assumes density of stars is constant. Assume we live in a BIG spherical ball of stars. Count ALL the stars we can see for entire sky, There are only about bright (first magnitude and brighter) stars There are only about 8 stars visible to naked eye There are much more stars with higher magnitude! b Number of Stars by Magnitude Sample in Milky Way 6 Log(Cumulative count) 6 4 Stellar Counts y =.494x +.94 R² = Limiting Magnitude All visible stars up to m=+8. If a ball of stars expect slope of.6 If a thin disk of stars expect slope of.4 Our data is smack dab in the middle of the two. Log of Count (unknown field size).... Samples in Milky Way y =.44x R = Limiting Magnitude If we sample a region of the sky along the Milky Way, we get a slope close to what we expect for living in a disk of stars. Sample far from Milky Way 7 Galactic Longitude 8 Star Counts Centered on Arcturus from Starry Night Program Log of Star Counts per 9' field. y =.44x -.76 R = View looking down on disk of galaxy -. Limiting Magnitude If we sample a region of the sky perpendicular to the Milky Way, we get a much lower slope (closer to ¼ ) which implies we are seeing out of the disk.

4 Galactic Latitude 9 Galactic Coordinate of Some Stars Side view of galaxy What we should see Observational Data: as a Bar Graph Galactic Star Counts 8 Space penetration power D is furthest distance can see with our telescope As galactic latitude increases, we hit the border of disk of stars, so see less stars. Least number of stars seen at galactic pole Average Number of Stars per Field Galactic Latitude Data: Line Graph Thickness of Galaxy 4 Average Number of Stars per Field Galactic Star Counts y =.e -.4x R = Galactic Latitude The data can be fitted with a nice exponential decaying formula. In fact it s a very good fit (perfect would have an Rsquared value of ). Calculate it from the ratio of counts of stars at pole and at equator (assume D= pc) T D () N N 68 parsecs p 8 Average Number of Stars per Field 8 Galactic Star Counts Galactic Latitude N N p 4

5 c. Interstellar Reddening 7 Extinction of Light 6 There is a lot of gas and dust in the galaxy This absorbs light ( magnitude per parsecs) Makes stars look fainter Hence we think they are further away than they really are. Causes us to overestimate distances Note NGC6 (left) is more red than NGC76 (right) because it is twice as far away. Short wavelength Blue light is absorbed more than Red First measurements of the Milky Way (9s) was hence x bigger than it really is due to this error! Corrected Space Penetration 8 Globular Cluster of Stars 8 Limiting Magnitude vs Space Penetration Power Magnitude absorption mag/kpc no absorption Distance in Parsecs Aa. Shapley Core (94-7) 9 Postulates globular clusters orbit galactic core More in direction of Sagittarius Estimates core is kpc away from sun (error: its 9 kpc) Ac. Robert Trumpler 9 (Lick Observatory) shows that there is dust in the galaxy which absorbs light. Overestimated distances, because did not know about absorption of light by galactic dust Hence, clusters appear fainter, and more distant than they actually are. Shapley s size of universe is 4% too big!

6 Ab. The Central Bulge Central Bulge is 4kpc in size, with a small pc bright radio source Sagittarius A, also bright in the IR (see below) Ac. Black Hole? In the center of the pc Nucleus is an X-Ray source smaller than AU Recent measurements of orbits of stars around this core imply that there is a.6 million solar mass black hole! %99s_Dinner_is_Fast_Approaching_-_Part_.ogv Aa. Mapping Spiral Arms (96) Ab. Our place in the Galaxy 4 944, Hendrik van de Hulst predicted Neutral Hydrogen gas will emit a cm spin flip spectral line 9 First Observed with radio telescope 96 Used to map spiral arms of our galaxy Ac. Rotations of Galaxies Ad. The Winding Dilemma 6 Spiral Galaxies Rotate Slowly Sun takes 6 million years to go around ( km/sec or AU in 8 days) The rotation speed can be measured by the Doppler effect on the cm radio line Outer stars move slower. Why haven t the spiral arms wound up and disappeared a long time ago? 6

7 Ae. The Winding Dilemma 7 Af. Density Wave Theory 8 Bertil Lindblad 9 Shows stars further from center of galaxy should move slower due to weaker gravity 97 Jan Oort proves this with observations 94 Lindblad Proposes density wave theory to explain spiral arms (resolve the winding paradox) Af. Density Wave Theory 9 Af. Density Wave Theory 4 A compression wave through the galaxy causes stellar birth; the bright short-lived O,B stars show the crest of the wave. Af. Emission Neb in M 4 Af. Emission Nebulae 4 This shows stellar formation In in the spiral arms (where Density waves bunch up matter Red is ionized hydrogen gas Emission nebulae are where stars have recently formed. 7

8 A4a. Rotation Curves 4 Assuming most of mass of galaxy is in the core Velocity of a Star predicted by Newton s Gravity: V /R = GM/R A4b. Rotating Rong? Vera Rubin shows rotation curves of galaxies are nearly constant! Implies a lot of missing (dark) matter surrounds galaxies. Or: V R Pivotal Paper: Rotational Properties of Sc Galaxies with a Large Range of Luminosities and Radii from NGC 46 (R=4kpc) to UGC 88 (R=kpc)," Astrophys. J. 8: 47 (98), V.C. Rubin, W. K. Ford, Jr. and N. Thonnard. A4c. What IS Dark Matter? 4 MACHOs (Massive Compact Halo Objects) were looked for: White Dwarfs Brown Dwarfs Black Holes But its not enough! REFERENCES 46 B Carroll and D. Ostlie, An Introduction to Modern Astrophysics (Addison-Wesley, 996), Chapter WIMPs (Weakly Interacting Massive Particles): Must propose exotic things like a neutrino, but with BIG mass ( to,x that of proton). Even though 96% of the universe is made of it, not a single piece of it is in this room. Or maybe there is something wrong with our theory of gravity? 8

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