23. the milky way galaxy the sun’s location in the milky way galaxy nonvisible milky way galaxy...

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23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter in the Milky Way galaxy Density waves produce spiral arms Infrared & radio galactic nucleus observations

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Page 1: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

23. The Milky Way Galaxy• The Sun’s location in the Milky Way galaxy

• Nonvisible Milky Way galaxy observations

• The Milky Way has spiral arms

• Dark matter in the Milky Way galaxy

• Density waves produce spiral arms

• Infrared & radio galactic nucleus observations

Page 2: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Our View of the Milky Way

Page 3: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

The Sun’s Location in Our Galaxy• William Herschel’s observations

– The Solar System is disk-shaped• Faint stars cluster in a band extending in all directions

– The Solar System is near the galactic center• The number of stars is about the same in all directions

• William Herschel’s nemesis– Interstellar extinction

• Interstellar dust obscures more distant stars• Dark regions in the Milky Way are obscured, not empty

• A telltale phenomenon– Globular clusters surround the galactic center

• Spherical distributions of ~ 106 stars– Globular clusters orbit the galactic center

• Period-luminosity relationship of Cepheid variables

Page 4: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

William Herschel’s Milky Way Map

Page 5: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Cepheid Period-Luminosity Relationship

Page 6: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

RR Lyrae Light Curves

Page 7: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

The Milky Way’s Numbers• Disk

– Flat disk ~ 5.0 . 104 pc in diameter– Earth is ~ 8.0 . 103 pc from the galactic center– Greatest abundance of stars

• Primarily bright young stars, especially OB associations• Abundant new star formation

• Central bulge– Approximately spherical distribution of stars

• Primarily dim old stars• Little new star formation

• Halo– Globular clusters

• Approximately spherical distribution of star clusters

Page 8: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Nonvisible Observations of Our Galaxy• Basic physical processes

– Selective scattering of EMR• Short visible wavelengths are scattered most• Long IR & radio wavelengths are scattered least

– Development of non-optical telescopes• Radio telescopes• Thermal IR telescopes

– Near infrared Wavelengths relatively close to the visible

– Far infrared Wavelengths relatively far from the visible

• Some benefits– Ability to see all of the Milky Way galaxy

• Central bulge• Distribution of interstellar gas clouds

Page 9: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

The Infrared Milky WayFar-Infrared View

(25 µm, 60 µm, 100 µm)

Near-Infrared View(1.2 µm, 2.2 µm, 3.4 µm)

Page 10: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

The Milky Way Galaxy Edge-On

Page 11: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

The NGC 4565 Galaxy Edge-On

Page 12: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Our Galaxy Has Spiral Arms• Observations of other galaxies

– Many disk-shaped galaxies have spiral arms• Number of spiral arms varies• Distinctness of spiral arms varies

– Tentative conclusion• The disk-shaped Milky Way may have spiral arms

• Observations of the Milky Way galaxy– Neutral hydrogen proton-electron spin-flip transitions

• Small energy difference between two possible states• Produces an emission line at the 21 cm radio wavelength

– Neutral hydrogen strongly concentrated in the disk• Doppler shift of various nebulae reveals arm structure

– Four major spiral arms– The Solar System is in the small Orion arm

Page 13: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Our Galaxy Has Spiral Arms

Milky Way rotation

Page 14: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Proton-Electron Spin-Flip in Hydrogen

Page 15: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

The Entire Sky at 21 Centimeters

Page 16: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Neutral Hydrogen in the Milky Way

Page 17: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

M83 Galaxy at Three Wavelengths

Visible Near-Infrared

21 Centimeter

Page 18: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

The Milky Way Galaxy Face-On

Page 19: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

The Milky Way’s Dark Matter• Basic observations

– Stars & nebulae orbit the galactic center• Identical to the pattern in the Solar System

– Orbital mechanics• Keplerian orbits Speed decreases with

distance– Farthest planets in the Solar System have slowest orbital speeds

• Non-Keplerian orbits Speed is almost constant– Milky Way’s rotation curve is nearly constant– Sun’s speed around galactic center is ~ 7.9 . 105 km .

hr–1

– Sun’s trip around galactic center is ~ 2.2 . 108 yrs

• Basic conclusion– Most of the Milky Way’s mass is beyond the Sun

• The visible mass cannot account for this mass• Much of the mass beyond the Sun is “dark matter”

Page 20: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Possible Forms of Dark Matter• Massive compact halo objects MACHOs

– Very dim stars between 0.01 & 1.0 MSun

– Gravitational bending of light has been observed• Tentatively, MACHOs account for < 40% of dark matter

• Known subatomic particles– Neutrinos, now known to have mass

• Weakly interacting massive particles WIMPs– Predicted mathematically but not yet observed

• Masses 10 to 10,000 times the mass of a neutron

Page 21: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

The Milky Way’s Rotation Curve

Page 22: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Microlensing by Halo Dark Matter

Page 23: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Density Waves Produce Spiral Arms• The winding dilemma

– No spiral galaxies revolve like a solid disk• This is not too far from the case• Any difference in rotation rate tends to destroy arms

– All spiral galaxies have persistent arms• Density waves One possible explanation

– Waves are similar to those on ocean surfaces• Propagation in slightly different directions• Constructive & destructive wave interference

– Constructive interference prone to extensive star formation– Destructive interference prone to minimal star formation

– Waves are relatively short-lived• Many stars in spiral arms are OB associations

– Very massive & short-lived at 3 to 15 million years» Only ~ 5% the rotation period of the Milky Way

– Very prone to inducing additional compression & star formation

Page 24: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Problems With the Density Wave Model• A driving mechanism to continue density waves

– Basic issues• A matter of space Huge distances are involved• A matter of time Huge time periods are involved

– One possibility• Barred spirals have asymmetrical gravitational fields

– One problem• Most spiral galaxies are not barred spirals

– Another possibility• Tidal influences of neighboring galaxies

• Types of spiral galaxies– Grand design spirals Classic spiral forms

• The density wave model fits these galaxies well– Flocculent spirals Fuzzy spiral forms

• The density wave model fits these galaxies poorly

Page 25: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Grand-Design & Flocculent Spirals

Page 26: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Infrared & Radio Observations• The Milky Way’s nucleus

– Extremely crowded with stars• One million stars as bright as Sirius• As bright as 200 full moons

– Dominated by a feature named Sagittarius A• Powerful source of synchrotron radiation

– Relativistic electrons spiraling in intense magnetic fields– Contains a feature named Sagittarius A*

• Thought to be the galactic center– Brightest radio source in its vicinity

• The mysterious identity of Sagittarius A*– Not a star Too energetic– Not a pulsar Too energetic– Not a supernova remnant Not expanding– Maybe a small supermassive black hole ~ 106 MSun

Page 27: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Nucleus of the Milky Way Galaxy

Page 28: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

Stars Orbiting the Galactic Center

Page 29: 23. The Milky Way Galaxy The Sun’s location in the Milky Way galaxy Nonvisible Milky Way galaxy observations The Milky Way has spiral arms Dark matter

• An historic perspective– William Herschel’s Milky Way map

• Definite disk shape• Same number of stars in all directions

– The problem: Interstellar extinction• Dust clouds scatter & absorb light

– The solution: Globular clusters• Cepheid variables give distance

• A modern perspective– A thin disk ~ 50 kpc in

diameter• Sun ~ 8 kpc

from center– A central bulge ~ 0.6 kpc

high– A halo dominated by globular clusters

• Non-visible telescopic observations– Radio l’s 21 cm spin-flip

line– Infrared Near- & Far-IR

l’s• Spiral arms

– Confirmed by 21 cm observations– 4 major & several minor arms

• Dark matter– Rotation curve stays nearly constant

• Much unseen mass lies beyond Sun– Three major possibilities

• MACHOs• Neutrinos & other known particles• WIMPs

• Production of the spiral arms– The density wave model

• Constructive & destructive interference– Causal mechanism is unclear

• Asymmetry in barred spirals• Tidal effects from nearby galaxies

• The Milky Way’s nucleus– Viewed in radio & IR l’s– Sagittarius A & Sagittarius A*

• Small supermassive black hole

Important Concepts