Download The Milky Way Galaxy

The Milky Way Galaxy
1/30/03
The Milky Way
• Brief History
• Morphology - what are the components of
the Galaxy?
• The Galactic Rotation
• The Galactic Center
Brief History
What is the Shape of the Milky Way?
• Late 1700s - Herschels counted stars in 683 regions of sky, assumed
all are equally luminous. Concluded that Sun at center of a flattened
system.
• 1920 - Kapteyn used a greater number of star counts and came to
roughly the same conclusion
Star Counts:
If stars are distributed uniformly in space, then in any patch of sky, the total
number of stars with flux less than a limiting flux, f is:
N(f '>f) = A f-3/2; where A = constant
Log(N(m’<m)) = C + 0.6m; where C = constant
(Note: These formulae were derived on the board in class)
logN
Actual star counts are
less than would be
predicted by this
relationship at fainter
flux levels (or larger
magnitudes)
m
Conclusion: stellar density not uniform but decreases with distance
from Sun; faster in direction perpendicular to Milky Way and slower
in the direction of the Milky Way
Milky Way is a highly flattened disk
• 1919 - Shapley studied globular clusters; used distance derived from
pulsating stars to determine that Sun is not at center of Milky Way.
These were found at great distances above and below the plane of
the Galaxy, where extinction effects are much less than that found
along the Milky Way
Globular clusters
•Definitely bound by gravity
•Contain large numbers of stars in a very small volume:
20,000-1,000,000 stars in a volume 20 pc in diameter
•very round and symmetrical in shape - very old -among the first stellar complexes formed in the galaxy
Distances from Variable Stars
Morphology of Galaxy
Disk
• Young thin disk
• Old thin disk
• Thick disk
Thin disk
• Diameter ~ 50 kpc
• Young thin disk scale height = 50 pc
• Old thin disk scale height = 325 pc
• Contains youngest stars, dust, and gas
• Contains Sun, which is 30 pc above midplane
• M* = 6  1010 Msun
• Mdust+gas = 0.5  106 Msun (scale height 0.16)
• Average stellar mass ~ 0.7 Msun
• LB ~ 1.8  1010 Lsun
• Population I stars in the Galactic plane
• Contains ~ 95% of the disk stars
• [Fe/H] ~ -0.5 - +0.3
• Age ~ < 12 Gyr
• Spiral structure seen in neutral H, HII regions, young O and B stars
Thick disk
• Diameter ~ 50 kpc
• Scale height = 1.4 kpc
•M* = 2-4  109 Msun
•LB ~ 2  108 Lsun
• [Fe/H] ~ -1.6 - -0.4 (less metal rich than thin disk)
• Age ~14-17 Gyrs
Gas and Dust
Spheroidal Components
• Central bulge
• Stellar Halo
• Dark Matter Halo
Central Bulge
• Diameter ~ 2 kpc
• Scale height = 0.4 kpc
•M* = 1  1010 Msun
•LB ~ 0.3  1010 Lsun
• [Fe/H] ~ -1.0 - +1.0 (less metal rich than thick and thin disk)
• Age ~10-17 Gyrs
Stellar Halo
• Diameter ~ 100 kpc
• Scale height = 3 kpc
• number density distribution ~  r-3.5
•M* = 0.1  1010 Msun
•LB ~ 0.1  1010 Lsun
• [Fe/H] ~ -4.5 - -0.5 (metal poor)
• Age ~14-17 Gyrs
Dark Matter Halo?
Rotation of Galaxy implies that there is a lot of mass in
our Galaxy that we don’t see (ie, if we count up the mass
from the stars that emit visible light, it’s much less than
that implied by observing the motion of stars as a function
of radius from the center of the Galaxy.
How do we know that the stars in the disk rotate around
the center of the Galaxy? How do we know the rotational
velocity of the Sun? How do we know the rotation curve?
(rotational velocity as a function of radius from the
Galactic center?)
Determining the rotation when we are inside the disk rotating ourselves
23.5°
39.1°
To determine the rotation curve of the Galaxy, we will introduce a more convenient coordinate system, called the
Galactic coordinate system. Note that the plane of the solar system is not the same as the plane of the Milky Way
disk, and the Earth itself is tipped with respect to the plane of the solar system. The Galactic midplane is inclined at
an angle of 62.6 degrees from the celestial equator, as shown above.
The Galactic midplane is inclined
62.6° with the plane of the
celestial equator. We will
introduce the Galactic coordinate
system.
l=0°
Galactic longitute
(l) is shown here
l
l=90°
l=270°
l=180°
Galactic latitude(b) is shown here
b
Galactic Coordinate System:
b
l
Derivation of radial and
tangential velocities for a rotating
disk was done on the board.