Download The Size and Structure of the Milky Way Galaxy

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
observation
Is 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)
V=220 km/sec
How does the Sun move in the
Milky Way?
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/ap001
220.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