The Milky Way Galaxy Shape & Size Structure & Contents Stellar Populations Gas & Dust Motion of Stars & Gas The Galactic Center Formation

Shape & Size “Galaxy” from Greek “Galactos” (meaning “milk”) Shape – Thomas Wright & Immanuel Kant (mid 1700s) Suggest flattened disk of stars Argument: if spherical, would see a lot of stars in every direction – We don’t. See more stars when we look through the disk than when we look out of the disk

Sir William Herschel (late 1700s) – Produced cross-sectional sketch of Milky Way by counting stars in every direction – Got major features roughly correct

Size (early 1900s) – Jacobus Kapteyn Disk ~ 65,000 ly in diameter with Sun at center Assumed “space was transparent” – Harlow Shapely More quantitative measurements based on distances to globular clusters Used Period-Luminosity law for variable stars in these clusters – Overestimated diameter by factor of ~ 3, but got Sun’s relative position right

Under- and overestimates due to presence of dust and IS clouds – Dimming effects – Distances overestimated: dimness not due to distance, but due to gas and dust absorbing some starlight Shapely was ignorant of the different kinds of variable stars, turned out he was measuring mainly RR Lyrae stars (Mira variables)

Structure & Contents Disk – About 100,000 ly in diameter Halo – More stars outside main disk Bulge – Center star population, flattened/elongated Spiral Arms – Complex groupings of stars If MW were the size of Earth, the solar system would be only a few inches across

Plane of MW tilted at 60 ° with respect to ecliptic Sun orbits galactic center at ~ 220 km/sec – Takes ~ million years to complete one orbit

Age – Oldest stars ever observed ~ 13 billion years old (recall white dwarf cluster) – “Calculate” another 10 billion years or so, there will be no more IS dust to make new stars Finite lifetimes Galaxies start to dim (made almost completely of cooling white dwarfs, neutron stars, and black holes)

Stars of the MW – All sizes – Giants, dwarfs, hot, cool, young, old, stable, exploding – Average star is like our Sun Small, dim, and cool Most stars have M = 0.1 – 0.5 M Sun