Structure of the Milky Way. Structure of the Milky Way. The Milky Way

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1 Key Concepts: Lecture 29: Our first steps into the Galaxy Exploration of the Galaxy: first attempts to measure its structure (Herschel, Shapley). Structure of the Milky Way Initially, star counting was used Assume all stars are about the same luminosity See how many stars are in each direction to map the Galaxy First done by the (German-born) British astronomer William Herschel ( ) and his son. The Structure of the Galaxy Our first encounter with Dark Matter The Milky Way Faint band of light circling sky First resolved into stars by Galileo using his telescope Herschel also discovered infrared light Structure of the Milky Way What Herschel found We are in a flattened disk of stars It extends about 6000 light years We appeared to be at or near the center!! SUN

2 Shapley Maps the Galaxy with Globular Clusters Harlow Shapley ( ) RR Lyrae Stars Horizontal branch stars Brightness varies because they pulsate All have the same average luminosity Found in globular clusters Shapley Maps the Galaxy Distribution of globular clusters is not symmetric about our position in the Galaxy 30% of G.C.s are found in only 2% of the sky toward Sagittarius Shapley Maps the Galaxy Shapley s Results Shapley mapped the distribution of globular star clusters Used RR Lyrae Stars to estimate distances RR Lyrae stars all have the same Luminosity Measure Flux (i.e. apparent brightness) Used Inverse Square Law to get Distance, d flux = luminosity /(4π d 2 ) 47 Tucana M5 Sun is not in center of Galaxy! Center of G.C.s is 26,000 LY = 8000 parsecs from Earth Center in direction of Sagittarius (see Links section of class webpage for more on Shapley s debate with Curtis about the size of the Galaxy, our location in it, and the nature of the nebulae.)

3 What Herschel Got Wrong The star counts do indeed end at 6000 light years ~=2000parsecs They are cutoff by thick clouds of dust Dust tends to: Absorb and scatter blue light Has little effect on light with wavelengths longer than the size of the dust grains The actual extent of the Milky Way is much larger than can be seen in visible light The Structure of the Milky Way A thin highly flattened disk 100,000 LY in diameter 1000 LY thick - About the relative thickness of a CD The Sun is in this disk Contains younger stars & gas (called Population I stars: Pop I) Nuclear Bulge - Central region 10,000 LY across Contains old stars (Pop II) A spherical Halo component Contains globular clusters Contains old stars (Pop II) Different Views of the Milky Way The Structure of the Milky Way Hydrogen Gas in the Radio Star Formation Regions traced by hot dust at 12 microns Old Stars in the Near-IR From COBE

4 Weighing the Milky Way Most stars & gas in the MW disk are on roughly circular orbits This allows us to measure the mass of the Galaxy using Newton s version of Kepler s Third Law e.g. for Sun s orbit: M Galaxy + M sun = a 3 /P 2 a = 26,000 LY = 1.6 x 10 9 A.U. P = 2.25 x 10 8 years M Galaxy + M sun M Galaxy = 2x10 11 M sun This estimate tells us how much mass is inside the Sun s orbit. Can we write this in terms of the speed? Newton s version of Kepler s 3rd law M Galaxy + M sun = a 3 /P 2 P = distance / speed P = 2π a / v a / v M Galaxy + M sun a v 2 We see that v is approximately constant in galaxies (does not depend on a). Therefore the Mass inside radius a increases linearly with a. Question? If we were to measure the velocities of more distant parts of the Milky Way and we find that they are moving faster than the Sun, so that their orbital periods are all about the same as the Sun, what does this tell us about the total mass of the Galaxy? Hint in the solar system the more distant planets orbit much more slowly. M Galaxy + M sun = a 3 /P 2 Mass of the Milky Way Most of the luminous matter in the Galaxy drops off at distances > 30,000 LY Mass of Galaxy should be ~ M Sun However there are a few globular clusters and some gas clouds that lie beyond this boundary These objects move faster than the Sun! Must be more gravity than supplied by luminous matter Total Mass of Galaxy ~ M Sun DARK MATTER - Only about 10% of mass is visible

5 The Search for MACHOs Gravitational Lensing See textbook What is the Dark Matter? Some kind of faint star? White dwarfs Brown dwarfs Examples of MAssive Compact Halo Objects MACHOs Exotic subatomic particles? Have mass Don t interact with matter Formed in early universe Weakly Interacting Massive Particles WIMPs