the milky way galaxy shape & size structure & contents stellar populations gas & dust...
TRANSCRIPT
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)