The Milky Way GalaxyShape & 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 ~ 225-240 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 MSun