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The Milky Way. Types of Stars. Population I – Second generation stars containing elements heavier than Helium ( the Sun is population 1 ) Population II – first generation ( primordial ) stars consisting of only Hydrogen and Helium
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Types of Stars Population I – Second generation stars containing elements heavier than Helium ( the Sun is population 1 ) Population II – first generation ( primordial ) stars consisting of only Hydrogen and Helium Variable Stars – Horizontal branch stars whose luminosity ( and hence their distance ) can be determined by measuring the period of their cycle of brightening/dimming
Types of Clusters Globular – tightly bound elliptically-shaped systems of 10,000 to 1,000,000 old population II stars, typically found orbiting outside the disk of a spiral galaxy Open – loosely bound irregularly-shaped systems of population I stars typically found in the galactic disk
Types of Gas • H I regions • very cold hydrogen gas giving off light at 21 cm due to transitions involving flipping electron spin • Typically found in the disk of a spiral galaxy far removed from stars
Types of Gas • H II regions • Heated ionized Hydrogen gas surrounding young stars or protostars • Emits light characteristic of atomic transitions. Protons capture electrons in an excited state , a photon is emittted when the electron drops into the ground state • Typically found in the spiral arms of a spiral galaxy
Elliptical Galaxies • Elliptical galaxies are not flattened into a disk • The three dimensional configuration is very efficient in producing stars • S0 …. • Ellipticals contain very little dust • Almost all the stars are old population II stars • Stellar orbits are approximately random ( no rotation curves )
Bulge: • Many RR Lyrae stars • A little gas & dust • Halo: • Old metal-poor stars • Globular clusters • RR Lyrae Stars • Dark Matter ! • Thick disk of Stars (~1000 pc thick) • Open Clusters & loose Associations of stars • Mix of young & old stars • Cepheid Stars in young clusters • Thin disk of Gas & Dust (~100 pc thick) • Mostly cold atomic Hydrogen gas • Dusty Giant Molecular Hydrogen Clouds
Interpretation of Rotation Curves • The velocity of rotating stars is a probe of the distribution of mass within the galaxy • The wiggles indicate the presence of spiral arms, where the matter density is higher than average • Rotation curves for spiral galaxies are typically flat out to distances beyond where any stars can be seen. This indicate the presence of DARK MATTER • The data is consistent with spherical Halos of dark matter extending far beyond the visible part of the galaxy
