The Milky Way

PHYS390 Astrophysics

Professor Lee Carkner

Lecture 19

The Milky Way

We can see the band of the Milky Way on a dark night

Nature of galaxy not known until early 20th century

Basic structure Central dense bulge

Old halo with dark matter

Disk Most visible area of the MW

Diameter Sun is ~8 kpc from center

Two components Thin disk of younger stars Site of current star formation

Thick disk of older stars Fainter and has fewer stars

(few % of thin disk)

Metallicity We use metal abundance as a proxy for age

Normally use the iron to hydrogen ratio

compared to the sun[Fe/H] = log [(NFe/NH)star / (NFe/NH)sun]

Range: - 0 (exactly like the sun)

Not perfectly reliable Iron comes from Type Ia supernovae and may vary

with region Not completely mixed

Age of Disk

Thin disk has broad range of metallicities Started forming stars 8 Gya and still going on

today

-0.6 to -0.4 Formed from episode of star formation

between 10 and 11 Gya

Spiral Arms

Gas, dust, young stars, bright stars, blue stars all concentrated in arms

Hard to map in our galaxy Form via density waves

As clouds orbit the Milky Way, they get stuck in areas of greater density

The Bulge

The central part of the MW is a thickened bar-shaped bulge Hard for us to see due to extinction

Due to several waves of star formation

Region within which ½ of the light is emitted

Halo Above and below the disk are

the globular clusters About 150 total Metallicity around -0.8

May be associated with thick disk

Or else would have broken up over the last ~12 Gyr

Rotation Curve

period of sun ~ 230 million years

Luminous mass looks concentrated at the core

Instead galaxy has flat rotation curve Rotational velocity constant

with increasing distance from center

Dark Matter

However, orbits of stars exterior to the sun

indicate that there must be a total of about 1012 Msun

Dark matter is about 95% of total galactic mass Cannot be dust, gas or stars

Mass to Light

Ratio of mass in solar masses to light in solar luminosities

For total Milky Way ~ 60

Dark Matter Candidates MACHOs

White dwarfs, neutron stars,

black holes, red dwarfs, brown dwarfs

Not enough detected in microlensing surveys

WIMPs Weakly Interacting Massive

Particles

Should be able to detect in very large isolated detector arrays

Galactic Center

Galactic center is 8 kpc from the sun in the constellation of Sagittarius Can find from distribution of halo globular clusters

Best data from radio, IR and X-ray (not visible)

stars are “isothermal”

Radio Observations

A complex series of thermal and non-thermal sources

At the center is a very bright, unresolved source, Sgr A* Less than ~2 AU in size

X-ray Observations

Sgr A* corresponds to a bright X-ray source

Explosions of material must have occurred in the past

IR Observations

The K band at 2.2 m is used to observe stars close to Sgr A*

Can use Kepler’s third law to find mass of Sgr A*

The Core

Sgr A* has a mass of 3.7X106 Msun in a space less than 2 AU in size

Destroys near-by stars to provide mass for accretion disk and outflows

Black hole is fairly quiescent

Next Time

Read 25.1-25.4 Homework: 24.2, 24.30, 24.33, 25.2b,

25.8a, 25.8b