The Size and Structure of the Milky Way Galaxy

We live in a new class of astronomical object

First… an advertisement for 29:52 “Exploration of the Solar System”

The other half of astronomy

Observations indicate the shape

and size of the Milky Way Galaxy

Structure of the Milky Way

• Disk

• Bulge

• Galactic center

• Galactic halo

Question: what simple observationIs consistent with a part of this picture?

A view of the Milky Way with an artist’s touch

Next topic: the motion of the Sun in the Milky Way (leads to something extremely

interesting)

How does the Sun move in the Milky Way?

Next topic: the motion of the Sun in the Milky Way (leads to something extremely

interesting)

How does the Sun move in the Milky Way?

V=220 km/sec

The sun moves in response to the gravitational force of All the rest of the mass in the galaxy

How is the mass distributed?

How could we tell?

Force of Gravity acts on the Sun and all objects in the Galaxy

Gravity has magnitude and direction

Sun responds to net force by moving on circular path around the galactic center

Mass distributions like 1,2, and 3 give circular orbits with

V(R)

A plot of V versus R is called a rotation curve

The Observed Case for the Milky Way

No sign of the “root-R falloff”

Significance of the Rotation Curve for the Milky Way

• Rotation curve stays high outside the bulk of the stars

• Mass indicated by the rotation curve exceeds that in stars

• Most of the mass of the Milky Way is in an unknown form of “Dark Matter”

The New View of the Milky Way

Dadark matter halo

Dark Matter: A Major Problem for Contemporary Physics and

Astronomy• Stars are a small fraction of the mass of

major galaxies• The dark matter problem becomes more

pronounced as you go out in the universe• The form of the dark matter is unknown;

probably not what you studied in chemistry• Possibly/probably an unknown form of

elementary particle

At the Very Center

What do we see at the galactic center (radio and

infrared wavelengths?

• Strong radio source (synchrotron radiation)

• At infrared wavelengths, can see stars orbiting very vast around Sgr A

• http://antwrp.gsfc.nasa.gov/apod/ap001220.html

• Ten years of observations speeded up

The Orbit of Star S2

Star S2 passes about as close to Sagittarius A* as the planet

Pluto passes to the Sun. Indicates supermassive object

in a very small region

Andrea Ghez paper