Radii. Masses. The Hertzsprung Russell Diagram. M A + M B = a AU 3 P y. M A a A =M B a B

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1 Radii 1) For two stars of same spectral type and known distances, make use of Stephan-Boltzmann Law. 2) For binary stars, can use eclipses. 3) Can measure directly through optical interferometry for the brightest stars. Radius of Sun is 6.96 x cm. M A + M B = a AU 3 P y 2 Masses (M A and M B in units of solar masses) M A a A =M B a B Many thousands of spectroscopic binaries, thousands of visual binaries and 50 eclipsing binaries. Best measurements for eclipsing binaries, as we know orbit orientation. Mass of Sun is 2 x g. The Hertzsprung Russell Diagram Same temperature, but much brighter than MS stars Same temperature, but fainter 1

2 Radii of Stars in the HR Diagram 1,000 times the Sun s radius 100 times the Sun s radius As large as the Sun 100 times smaller than the Sun Masses of Stars in the Hertzsprung -Russell Diagram The higher a star s mass, the more luminous (brighter) it is: L ~ M 3.5 High-mass stars have much shorter lives than low-mass stars: t life ~ M -2.9 Sun: ~ 10 billion yr. 10 M sun : ~ 30 million yr. 0.1 M sun : ~ 3 trillion yr. 2

3 Color-Magnitude Diagram from Hipparcos Some stars of the same spectral type may have very different luminosities. Note the stellar luminosity classes. Sun is a G2V star. Star Clusters Open clusters: stars. Young (Population I) stars. 10s-100s of pc away. In Galactic plane. Globular clusters: 10 6 stars. Old (Population II) stars. Kpc - Mpc away. Distributed around Galactic center. Provide useful snapshots of populations. 3

4 HR Diagram of Hyades High-mass stars evolved onto the giant branch Turn-off point Low-mass stars still on the main sequence The Milky Way 1. Structure 2. Motions of Stars 3. Populations 4. The ISM 5. The Local Group Infrared View of the Milky Way Near-infrared image Galactic plane Nuclear bulge Spitzer Space Telescope view of Milky Way 4

5 The Structure of the Milky Way Disk contains stars, open star clusters and lots of dust and gas. Sun is in disk at 8.5 kpc from center of Galaxy (D = 25 kpc). Halo contains only 2% as many stars as the disk, and very little gas and dust. We can t detect halos of other galaxies. Nuclear bulge has radius of 2 kpc and contains little gas and dust. Orbital Motions in the Milky Way Differential Rotation Sun orbits around Galactic center at 220 km/s. 1 orbit takes approx. 240 million years. We have completed roughly 20 orbits. Mass determination from orbital velocity: The more mass there is inside the orbit, the faster the Sun has to orbit around the Galactic center. Combined mass: M = 100 billion M sun M = 25 billion M sun M = 4 billion M sun 5

6 The Mass of the Milky Way If all mass was concentrated in the center, rotation curve would follow a modified version of Kepler s 3rd law. Rotation Curve = orbital velocity as function of radius. The Mass of the Milky Way (2) Total mass in the disk of the Milky Way: Approx. 200 billion solar masses Additional mass in an extended halo: Total: Approx. 1 trillion solar masses Most of the mass is not emitting any radiation: dark matter! 6

7 Possible dark matter sources Neutrinos Massive compact halo objects Brown dwarfs Black holes Gas Planets Other exotic objects How old is the Galaxy? Stellar Populations Population I: Young stars: metal rich; located in spiral arms and disk Population II: Old stars: metal poor; located in the halo (globular clusters) and nuclear bulge Population III: Stars with no metals. These have never been found. How old is the Galaxy? Stellar Populations Our Sun is an intermediate Population 1 star. 7

8 Metal Abundances in the Universe All elements heavier than He are very rare. Logarithmic Scale Linear Scale Metals in Stars Absorption lines almost exclusively from Hydrogen: Population II Many absorption lines also from heavier elements (metals): Population I At the time of formation, the gases forming the Milky Way consisted exclusively of hydrogen and helium. heavier elements ( metals ) were later only produced in stars. => Young stars contain more metals than older stars. The History of the Milky Way Quasi-spherical gas cloud fragments into smaller pieces, forming the first, metal-poor stars (pop. II). Rotating cloud collapses into a disk-like structure. Later populations of stars (pop. I) are restricted to the disk of the Galaxy. Oldest GCs are 13 billion years old. 8

9 Exploring the structure of the Milky Way with O/B Associations Perseus arm Orion-Cygnus arm Sagittarius arm O/B Associations trace out 3 spiral arms near the Sun. Distances to O/B Associations determined using Cepheid variables. Star Formation in Spiral Arms (1) Shock waves from supernovae, ionization fronts initiated by O and B stars, and the shock fronts forming spiral arms trigger star formation. Spiral arms are stationary shock waves, initiating star formation. Density wave theory. Star Formation in Spiral Arms (2) Spiral arms are basically stationary shock waves. Stars and gas clouds orbit around the galactic center and cross spiral arms. Shocks initiate star formation. Star formation selfsustaining through O/B ionization fronts and supernova shock waves. 9

10 The Galactic Center (1) Our view (in visible light) towards the Galactic center (GC) is heavily obscured by gas and dust: Extinction by 30 magnitudes Only 1 out of optical photons makes its way from the GC towards Earth! galactic center Wide-angle optical view of the GC region Measuring the Mass of the Black Hole in the Center of the Milky Way By following the orbits of individual stars near the center of the Milky Way, the mass of the central black hole could be determined to be ~ 4 million solar masses. 10

11 The Interstellar Medium (ISM) The space between the stars is not completely empty, but filled with very dilute gas and dust, producing some of the most beautiful objects in the sky. We are interested in the ISM because a) dense interstellar clouds are the birth places of stars b) dark clouds alter and absorb the light from stars behind them Structure of the ISM The ISM is 99% interstellar gas and comprises 10-15% of the visible mass of MW. It occurs in two main types of clouds: HI clouds (molecular clouds) Cold (T ~ 100 K) clouds of neutral hydrogen (HI); moderate density (n ~ 10 a few hundred atoms/cm3); size: ~ 100 pc Hot intercloud medium: Hot (T ~ a few 1000 K), ionized hydrogen (HII); low density (n ~ 0.1 atom/cm3); gas can remain ionized because of very low density. HI clouds (molecular clouds) 11

12 Our Galaxy Cluster: The Local Group Our local group is a poor cluster: Milky Way > 30 galaxies Andromeda Galaxy 1 Mpc diameter Of bright galaxies, 14 elliptical 3 spiral 4 Small irregular Magellanic Cloud Most of galaxies are Large dwarf Magellanic Cloud ellipticals. Largest members are: Our Galaxy Cluster: The Local Group Milky Way Milky Way Andromeda Galaxy Andromeda (M31) Triangulum (M33) Andromeda is the largest but we think MW may be the most Small massive. Magellanic Cloud Large Magellanic Cloud Largest members are: Our Galaxy Cluster: The Local Group Milky Way Milky Way Andromeda Galaxy Andromeda (M31) Triangulum (M33) Small Magellanic Cloud Large Magellanic Cloud 12

13 Our Galaxy Cluster: The Local Group Largest members are: Milky Way Andromeda Galaxy Milky Way Andromeda (M31) Triangulum (M33) Small Magellanic Cloud Large Magellanic Cloud Magellanic Clouds: Local group dwarfs Mergers of Galaxies Milky Way and Andromeda are moving towards each other at 500,000 km/hour and are expected to merge in about 3 billion years. 13

14 The Universe About nomenclature Numbers with M in front of them are Messier objects, cataloged by Charles Messier between 1758 to These were about 100 diffuse structures often mistaken for comets. About nomenclature NGC means New General Catalog of nebulae and star clusters, compiled by John Dreyer in Contains 8000 objects. 14