The Milky Way Galaxy- Part II

NGC 4603

Similar to our own Milky Way Galaxy

Chapter Cover Fig.

Halo: Star Populations

Composed of globular clusters

No gas and dust- no recent star formation

Older stars

Redder

Population II

Orbits or globular clusters are largely random

Review of Proper Motion

Pythagorean Theorem gives speed and direction

Halo Defines Galaxy Size

Halo stars define outer reaches of Galaxy

Halo stars are old

RR Lyrae stars are used to find distances

Galactic Bulge

Football shaped

Larger depth than disk

Located at center of galaxy

Gas and dust, star formation in inner regions

Color: yellow-white

Orbits of stars are more random than disk stars, less random than halo stars

View of the Milky Way

What would our Milky Way Galaxy look like to a person on a planet orbiting a halo star?

Motion of Disk

Disk is not a solid disk

Differential rotation

We get velocities from proper motion and doppler shifts of stars

21cm radiation yields radial velocities from cold, neutral hydrogen

Motion of Disk

21cm radiation yields radial velocities from cold, neutral hydrogen

Fig 23.15 is an artists conception of the Galaxy as deduced from 21 cm data and radio data from molecular clouds

Emission comes from areas clumped in spiral arms

Differential Rotation of Disk

Stars close to center take less time to rotate about the center of the Galaxy

Disk is not solid (like Saturn's rings)

What caused spiral pattern and why does the pattern persist

Fig. 23-16 , if arms were tied to disk, then arms would wrap, and destroy pattern

Spiral Density Waves

Density waves are similar to ripples in water

Cars slow down for traffic obstruction, pass, then speed back up

Disturbances (traffic jams) can persist long after obstruction is gone

Stars behave similarly

Fig. Discovery 23-1

Density Wave Theory

Density waves moves through galaxy and causes gas and dust to pile up in a band

Where gas density increases, get increased star formation- spiral arms!

Fig. 23-17

Star formation in Spiral Arms

Successive episodes of star formation broadens arms

Fig. 23-18

Measuring Mass of Milky Way Galaxy

M=a³ / p² M= total mass in solar units, a in AU, p in yrs

Can use Sun: p=225 million yrs, a= 8 kpc

M = 10¹¹ M_M , 100 billion times mass of Sun

But what is M, Galaxy's matter is not concentrated at center of Galaxy

M is the mass within the orbit of the Sun

Want to measure orbital speed of gas in outer edge of galaxy

Rotation Curve

If we measure clouds of gas near center out to edge of disk, get rotation curve

Curve M=a³ / p² does not follow expected Keplerian behavior if Galaxy is composed of a disk

Need extra mass in halo

Dark Matter

What is missing mass?

Red dwarfs

Brown dwarfs

Neutrinos

MACHOs, WIMPs

Dark matter is not normal matter

We cannot "see" it at any wavelength

Fig. 23-20 red dwarfs

Gravitational Lensing

Dark matter interacts gravitationally

Should bend light

Could act like a lens

Fig. 23-21

Center of Galaxy

Radio image (NRL), Visible light: Fig. 23-22

Center of Galaxy: Close Up

Fig. 23-23

Measuring Mass of Black Hole

Sag A^* is the center of the galaxy

Mass = 3 million M_M

Fig. 23-25

Cosmic Rays

One more component of matter in galaxy

Cosmic rays are subatomic particles: protons, electrons and nuclei

Travel near the speed of light, relativistic

90% protons (H nuclei)

Multiple sources responsible, but best candidates are supernovae

Galactic center, neutron stars are also candidates

Summary

Design, size, and regions of Milky Way

Spiral arm structure; nuclear bulge, disk, and halo

What our Galaxy looks like with different viewing perspectives

How variable stars (RR Lyrae and Cepheid) determine size and shape of Galaxy

Period-luminosity relationship

Spiral arms: how formed and maintained

Dark matter: Galactic rotation curve

Phenomena at center of the Galaxy and cosmic rays