Chapter 14 The Milky Way Galaxy
Spiral Galaxy M81 - similar to our Milky Way Galaxy
Our Parent Galaxy A galaxy is a giant collection of stellar and interstellar matter held together by gravity Billions are known Our galaxy is the Milky Way Galaxy or the Galaxy (capital G) The disk contains most of Galaxy s luminous stars and interstellar matter (and sun)
The Milky Way Line of sight perpendicular to disk - see few stars Line of sight within disk - see many stars merging into continuous blur Band of light is the Milky Way
Figure 14.1 Galactic Plane
Spiral Galaxies Our Galaxy is a spiral galaxy consisting of: Galactic disk with spiral arms Galactic bulge at center Galactic halo (roughly spherical)
Figure 14.2a - Andromeda galaxy similar to our own - 800 kpc away
Figure 14.2b,c - Spiral Galaxies a) M101, b) NGC 4565
Measuring the Milky Way Herschel in late 1700 s counted stars Found Galaxy is disk shaped Judged size wrong - didn t know about obscuring gas and dust
Figure 14.3 Herschel s Galaxy Model
Variable Stars Systematically studied near start of 20th century Not eclipsing binaries or novae Pulsating variables of two types: RR Lyrae variables Cepheid variables Recognizable by shape of light curve Are post-main sequence stars temporarily unstable
Figure 14.4 - Variable Stars a) RR Lyrae, b) & c) Cepheid
RR Lyrae variables Periods from 0.5 to 1 day Lower mass horizontal branch (H-R diagram)
Cepheid variables Periods from 1 to 100 days High mass stars
Figure 14.5 Variable Stars on the H-R Diagram
Period-Luminosity relationship All RR Lyrae stars roughly 100X luminosity of sun (averaged over a cycle) Close correlation between luminosity and pulsation period for Cepheid variables Discovered by Henrietta Leavitt in 1908
Discovery 14-1 Early Computers
Figure 14.6 Period-Luminosity Plot
Variable as yardstick Apparent brightness proportional to luminosity/distance 2 Stellar or spectroscopic parallax distance of nearby variables luminosity For distant variables period luminosity distance
Distance measurements Radar to about 1 AU Stellar parallax to about 200 pc Spectroscopic parallax to about 10 kpc Variable stars to about 25 Mpc
Figure 14.7 Variable Stars on Distance Ladder
Globular cluster distribution Early 1900 s Harlow Shapley used RR Lyrae s to map globular clusters - he found: Globulars many kpc s from sun Globulars spherically distributed Center is 8 kpc from sun Globular clusters in Galactic Halo
Figure 14.8 Globular Cluster Distribution
Evolving ideas of our place in the universe Earth at center Sun at center of solar system Sun at center of Galaxy Sun not at center of Galaxy Galaxy bigger than previously thought
Figure 14.9 Stellar Populations in Our Galaxy
Mapping our Galaxy Optical observations useful for halo For disk, visible wavelengths obscured by dust and gas Measure disk with 21 cm radio wavelength Galactic diameter 30 kpc Disk thickness 300 pc Centers of disk and halo roughly coincide
Galactic bulge Use infrared wavelengths to image 6 kpc in plane of disk by 4 kpc perpendicular to plane of disk Football shaped - elongated along disk High gas density at center Vigorous star formation
Figure 14.10 Infrared View of the Milky Way Galaxy
Stellar Populations Halo and bulge redder, disk bluer Disk has all the bright blue stars, open star clusters and star-forming regions Cooler redder stars distributed in disk, bulge and halo
Halo No gas or dust Stars are old - 10 billion years No star formation Stars less abundant in heavy elements Referred to as Population II stars
Disk Active star formation O and B supergiants give it bluish color Younger stars contain heavier elements Young disk stars called Population I
Orbital motion Galactic disk rotates differentially At radius of 8 kpc (sun) orbital speed is 220 km/s and period is 225 million years Globular clusters, stars in halo and bulge orbit in random orientation about center
Figure 14.11 Orbital Motion in the Galactic Disk
Figure 14.12 Stellar Orbits in Our Galaxy
Milky Way Formation Formed from several smaller systems Irregular shape initially Stars formed throughout Gas and dust fell to galactic plane, forming spinning disk Halo stars left behind Star formation ceased in halo New stars form in the disk
Figure 14.13 Milky Way Galaxy Formation
Table 14.1 Overall Properties of the Galactic Disk, Halo, and Bulge
Galactic Spiral Arms Radio studies show spiral arms in Galaxy Typical of other spiral galaxies Young O and B stars and recently formed open clusters found in spiral arms Arms are regions of star formation
Figure 14.14 Gas in the Galactic Disk
Figure 14.15 Milky Way Spiral Structure
Problem Differential rotation can t fully explain spiral arms Spiral arms would wrap up over many rotations
Figure 14.16 Differential Galactic Rotation
Spiral arm explanation Spiral density waves moving through disk Region of compression rotates more slowly than stars and gas Stars formed in spiral arms Material through arm consists of High density dust and gas Dust lane Emission nebulae and young O, B stars Older stars
Figure 14.17 Density-Wave Theory
Density wave Think of work crew slowly moving along freeway Traffic jam (increased density of cars) forms around work crew Cars enter at back and leave at front Jam (spiral arm) moves slower than cars (gas, dust and stars)
Discovery 14-2 Density Waves
Alternative explanation Formation of stars drives waves (rather than other way around) Self-propagating star formation Can t fully explain galaxy-wide spiral arms
Figure 14.18 Self-Propagating Star Formation
Mass of Milky Way Galaxy Using Kepler s third law and sun s orbit, get 1 X 10 11 M Assumes all mass at center (not true) Ignores mass outside of sun s orbit
Figure 14.19a Weighing the Galaxy
Rotation curve Plot rotation speed vs distance from center Within 15 kpc radius (visible edge of galaxy) 2 X 10 11 M Not Keplerian beyond 15 kpc Invisible matter beyond 15 kpc
Figure 14.19b Weighing the Galaxy
Dark matter 6 X 10 11 M lies within 50 kpc 2 X 10 11 M lies within 15 kpc (visible edge of galaxy) Dark halo out to 50 kpc So 2X as much invisible dark matter as visible Not visible at any wavelength - only detected gravitationally
What is dark matter? Possibilities: Brown dwarfs and white dwarfs Exotic subatomic particles WIMPs - Weakly Interacting Massive Particles MACHOs - Massive Compact Halo Objects
Search for dark matter Gravitational lensing of light around a faint massive object such as brown dwarf or white dwarf Rare - use automated telescopes and computer processing to find
Figure 14.20 Gravitational Lensing
Galactic center Bulge and especially nucleus should be densely populated with stars Interstellar medium blocks the view
Figure 14.21 Galactic Center
Infrared and radio observations Can penetrate though interstellar matter At center, 50,000 stars per cubic pc Million times more than at sun s location Clouds rich in dust Ring of molecular gas Bright radio source Sagittarius A Sgr A* at center (supermassive black hole)
Figure 14.22 Galactic Center Close-Up
Sgr A* VLBI measurements show less than 10 A.U. across 10 33 W output - more than a million times the sun 3 million M Event horizon less than 0.05 A.U.
Figure 14.23 Orbits Near the Galactic Center
Figure 14.24a-c Galactic Center Zoom
Figure 14.24d-f Galactic Center Zoom