Milky Way Galaxy 1 Dr. Bill Pezzaglia Updated: Nov 25, 2012

Milky Way 2 Star counts B. Core and Arms C. Galaxy Rotation

A1a. Milky Way 4

A1b. Milky Way: Galactic Equator 5

2a. Galileo Galilei (1564-1642) 6 Galileo was one of the very first scientists to do experiments to understand Nature He was the first astronomer to use a telescope (in 1610) to study the sky. Sees that the Milky Way is made of stars

A1c. 1750 Thomas Wright 7 Milky Way is thin shell of stars, which explains why we see a band across the sky.

A1c. 1784 Herschel’s Telescope 20 year study of 2400 regions of sky (maps over 90,000 stars)

A1c. The Galactic Equator 9 1784 Herschel’s Star Gauging Counts stars in 683 regions Estimates universe is disk shaped Diameter is 5 times thickness Sun appears to be at the center

A1c. Kapteyn Universe 9 1897 measured that stars move (rotate around universe) (Parsec=3.26 light years=200,000 x distance to sun)

Magnitude Distance Relation 10 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. Assume at 10 parsecs an average star has (absolute) magnitude of M=+2.5 Every factor of 10 in distance it gets 5 magnitude fainter Distance (parsecs) magnitude 10 2.5 32 5 100 7.5 316 1000 12.5 3162 15

Space Penetrating Power 11 Space Penetrating Power Limiting Magnitude Distance (parsecs) 2.5 10 5 32 7.5 100 316 12.5 1000 15 3162 Turn this idea around. From the limiting magnitude of our telescope, we can estimate how “deep” we are penetrating into the galaxy. We need to see deeper than 500 parsecs to be able to see the thickness of the Milky Way.

Counting Stars 12 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 : If density is constant, expect plot of log of star count vs log of distance to have a slope of 3

Stars vs Magnitude 13 Since magnitude is proportional to 5 times log of distance, expect plot of log count vs magnitude to be a line with slope of 0.6 : 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,

3a Number of Stars by Magnitude 14 There are only about 15 bright (first magnitude and brighter) stars There are only about 8000 stars visible to naked eye There are much more stars with higher magnitude!

3b Number of Stars by Magnitude 15 All visible stars up to m=+8.5 If a ball of stars expect slope of 0.6 If a thin disk of stars expect slope of 0.4 Our data is smack dab in the middle of the two.

Sample in Milky Way 16 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 17 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”.

Galactic Longitude 18 View looking down on disk of galaxy

Galactic Latitude 19 Side view of galaxy

Galactic Coordinate of Some Stars 20

What we should see 21 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

Observational Data: as a Bar Graph 22

Data: Line Graph 23 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 1).

Thickness of Galaxy 24 Calculate it from the ratio of counts of stars at pole and at equator (assume D=1000 pc) N0 Np

2c. Interstellar Reddening 17 Note NGC3603 (left) is more red than NGC3576 (right) because it is twice as far away. Short wavelength Blue light is absorbed more than Red

Extinction of Light 16 There is a lot of gas and dust in the galaxy This absorbs light (1 magnitude per 1000 parsecs) Makes stars look fainter Hence we think they are further away than they really are. Causes us to overestimate distances First measurements of the Milky Way (1920s) was hence 10x bigger than it really is due to this error!

Corrected Space Penetration 18 Corrected Space Penetration

Globular Cluster of Stars 28

A2a. Shapley Core (1914-17) 29 Postulates globular clusters orbit galactic core More in direction of Sagittarius Estimates core is 15kpc away from sun (error: its 9 kpc) Overestimated distances, because did not know about absorption of light by galactic dust

A1c. Robert Trumpler 30 1930 (Lick Observatory) shows that there is dust in the galaxy which absorbs light. Hence, clusters appear fainter, and more distant than they actually are. Shapley’s size of universe is 40% too big!

A2b. The Central Bulge 31 Central Bulge is 4kpc in size, with a small 5 pc bright radio source “Sagittarius A”, also bright in the IR (see below)

A2c. Black Hole? 32 In the center of the 5 pc Nucleus is an X-Ray source smaller than 100 AU Recent measurements of orbits of stars around this core imply that there is a 2.6 million solar mass black hole! http://en.wikipedia.org/wiki/File:A_Black_Hole%E2%80%99s_Dinner_is_Fast_Approaching_-_Part_2.ogv

A3a. Mapping Spiral Arms (1960) 33 1944, Hendrik van de Hulst predicted Neutral Hydrogen gas will emit a 21 cm “spin flip” spectral line 1951 First Observed with radio telescope 1960 Used to map spiral arms of our galaxy

A3b. Our place in the Galaxy 34

A3c. Rotations of Galaxies 35 Spiral Galaxies Rotate Slowly Sun takes 226 million years to go around (220 km/sec or 1 AU in 8 days) The rotation speed can be measured by the Doppler effect on the 21 cm radio line

A3d. The “Winding Dilemma” 36 Outer stars move slower. Why haven’t the spiral arms wound up and disappeared a long time ago?

A3e. The “Winding Dilemma” 37

A3f. Density Wave Theory 38 Bertil Lindblad 1925 Shows stars further from center of galaxy should move slower due to weaker gravity 1927 Jan Oort proves this with observations 1940 Lindblad Proposes “density wave theory” to explain spiral arms (resolve the winding paradox)

A3f. Density Wave Theory 39 A compression wave through the galaxy causes stellar birth; the bright short-lived O,B stars show the crest of the wave.

A3f. Density Wave Theory 40

A3f. Emission Neb in M51 41 This shows stellar formation In in the spiral arms (where Density waves bunch up matter

A3f. Emission Nebulae 42 Red is ionized hydrogen gas Emission nebulae are where stars have recently formed.

Addenda: Rotation of Galaxies 43 The way galaxies should rotate B. Galaxies rotating too fast C. Theory of “dark matter”

A4a. Rotation Curves Assuming most of mass of galaxy is in the core 44 Assuming most of mass of galaxy is in the core Velocity of a Star predicted by Newton’s Gravity: V2/R = GM/R2 Or: V  1/R

A4b. Rotating Rong? 45 1980 Vera Rubin shows rotation curves of galaxies are nearly constant! Implies a lot of “missing” (dark) matter surrounds galaxies. Pivotal Paper: Rotational Properties of 21 Sc Galaxies with a Large Range of Luminosities and Radii from NGC 4605 (R=4kpc) to UGC 2885 (R=122kpc)," Astrophys. J. 238: 471 (1980), V.C. Rubin, W. K. Ford, Jr. and N. Thonnard.

A4c. What IS Dark Matter? 46 MACHOs (Massive Compact Halo Objects) were looked for: White Dwarfs Brown Dwarfs Black Holes But its not enough! WIMPs (Weakly Interacting Massive Particles): Must propose exotic things like a neutrino, but with BIG mass (10 to 10,000x 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?

REFERENCES 47 B Carroll and D. Ostlie, “An Introduction to Modern Astrophysics” (Addison-Wesley, 1996), Chapter 22 http://en.wikipedia.org/wiki/Supermassive_black_hole