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The Milky Way
The Milky Way
The Milky Way is the great band of light that is best seen in
the summer from a dark site.
The name "Milky Way" is derived from its appearance as a
dim un-resolved "milky" glowing band arching across the
night sky.
The term is a translation of the Classical Latin: via lactea.
The Greek philosophers Anaxagoras (ca. 500–428 BC) and
Democritus (450–370 BC) proposed the Milky Way might
consist of distant stars.
Actual proof of the Milky Way consisting of many stars
came in 1610 when Galileo Galilei used a telescope to study
the Milky Way and discovered that it was composed of a
huge number of faint stars
The Milky Way
In the mid-eighteenth century, Thomas Wright and
Immanuel Kant suggested that it could be explained if we
were located within a flattened disk of stars.
Kant even went on to propose that other fuzzy patches of
light were similar distant galaxies coining the phrase
“Island Universes”
Gemini Observatory
Trying to see the Milkyway
The view in visible light show reflection, emission nebula
as well as dark dust.
Trying to decide the size and shape is hard when you are
embedded in it.
Neutral Hydrogen
Using the 21-cm band, the regions of neutral hydrogen
may be mapped.
The Radio View
Infrared View
The first attempt
In 1785, William
Herschel counted
all the stars he
could see and
produced a drawing
of the band of light
in the night sky
From this, Herschel
decided that the Sun
was embedded in a
thin band of stars
He wasn’t too far off
The Milky Way
Around 1900, Jacobus Kapteyn started a large
project measuring motions of stars to determine
3D structure of Galaxy. By 1920, he had
formulated a model in which:
 the Sun was 650 pc from center of an
oblate spheroid, about 5 times wider than
it was thick, which had a radius of order
3000 pc.
Globular Clusters
•At about the same time, Shapley
had formulated a very different
picture from observations of
globular clusters:
the galaxy was 100 Kpc
across.
the Sun was 15 Kpc from
center of the globular cluster
distribution.
The Milky Way
The modern picture is closer to Shapley's than Kapteyn's. The
two were only reconciled when it was realized than
interstellar extinction is very significant. This has the effect
of making the distribution of stars appear more centered
upon us than it really is (Kapteyn's model) as well as rather
small. At the same time, it makes the globular clusters
appear fainter and more distant so makes Shapley's model
too large. Interstellar extinction is less important for the
globular clusters than stars in the disk, and so Shapley was
closest. Kapteyn realized that this was possible but could
find no evidence for extinction; the phenomenon was only
proved beyond doubt in the 1950s from studies of star
clusters.
The Milky Way
In the modern picture of our Galaxy, the Sun is 8 kpc from the center
of the Galaxy. The Sun resides in a flattened circular disk which is
of order 200 pc thick and extends out to about 15 Kpc. Thus the
disk is of order 100,000 light-years across. At the center of the
disk is the spheroid or bulge. This is probably elongated into a bar,
making the Milky Way a barred spiral. The bulge does not have a
well defined size, but dominates the inner kiloparsec of the Galaxy.
This whole structure is surrounded by the halo, a fairly spherical
distribution again centered upon the center of the Galaxy. The halo
contains stars, globular clusters and dark matter. It is possible
that the Galaxy is only a minor component located at a dense clump
in a much larger distribution of dark matter that permeates the
whole Universe.
Finally at the center of the Galaxy is a region some 30 pc or so across
known as the nucleus. Star densities in the nucleus are greater than
106 stars/pc3, over a million times the density near the Sun.
Shape and Structure
The Milky Way: The Disk
•
With a radius of 15 Kpc and a thickness of a few hundred parsecs,
the disk is very flattened.
•
Stars, gas and dust in the disk execute circular orbits around the
center of the Galaxy under the influence of the gravitational
attraction of the mass of the Galaxy.
– At the Sun, the orbital speed is about 200 Km/sec, which means that
one orbit takes 250 million years.
•
The disk is where most of the action now takes place, with giant
molecular clouds, spiral arms, dust clouds, stars, star forming
regions and core-collapse supernovae. The disk is about 1010 years
old. There are of order 1011 stars in the disk.
•
The disk probably formed from the collapse of gas clouds with
their spin or `angular momentum' more-or-less preserved during
collapse. This is in contrast to the formation of the halo.
The Disk Neighborhood
Of course we see individual
(sometimes multiple) star
systems --- Many of them!
From the 'normal'
To the spectacular
The Disk Neighborhood
Then there are the vast Star
Clouds --- Open clusters both
older (Hyades) and newer
(Pleades) and those still being
born (Orion and Eagle
nebulae)
The Disk Neighborhood
There is a lot of gas - both
hot (glowing) and cold (dark)
There is a lot of dust
And, probably, a great deal
of 'Dark Matter'
The Milky Way: The Bulge
• Hidden behind the galactic plane to a large
extent, the bulge is less well understood
than the disk.
• Stellar orbits there are very elongated,
there are many cool old stars there.
• It appears to have an elongated bar shape.
The Milky Way: The Halo
•
The halo extends out to 25 Kpc. It is probably a flattened sphere
in shape, although it is hard to tell. It contains relatively few stars
- of order 108 - but a great deal of unseen or dark matter. It has
little rotation and stars are on very elongated orbits. Thus nearby
halo stars appear to move backwards relative to the motion of the
disk.
•
The halo probably formed very early in the history of the Galaxy
from the merger of large star clusters.
– This is a complex process, but means that stars is the halo are in
general very old and therefore lacking in heavy elements cooked up in
later supernovae.
– Thus they can be spotted as stars with low element abundance as well
as high speed (of order 200 km/s) motion.
•
The globular clusters are part of the halo and may be remnants of
the early clusters that formed it.
The Halo
The halo has many
globular clusters
in orbit about
the galactic
center
The Milky Way: The Nucleus
• At the center of the Galaxy sits the nucleus. This is best
seen at infrared and radio wavelengths which are less
affected by dust than visual wavelengths.
• In recent years high resolution observations of stars near
the Galactic center at IR wavelengths have revealed high
speed motions.
– The interpretation of these motions is that there is
concentration of over 2 million solar masses within the inner
few light-weeks.
– It is thought that this might be a black-hole as no star cluster
can survive for long at such high density.
– It is not an active one in that gas is not accreting onto it.
The Galactic Nucleus
• These are nearinfrared images of
the galactic center
Our Current View:
You are here
A Roadmap to Home
www.anzwers.org/free/universe
The Warp in the Milky Way
Choosing distant stars from
Hipparcos data, astronomers at
the Turin Observatory and Oxford
University confirm that the disk is
not really flat. Outside the Sun's
orbit, stars follow tilted orbits.
•
The result is the curved shape of
the disk shown here, resembling
the brim of a hat.
For clarity's sake, the extent of the warping is exaggerated here (X10).
The Warp in the Milky Way
In January 2006, researchers reported that the
heretofore unexplained warp in the disk of the Milky
Way has now been mapped and found to be a ripple or
vibration set up by the Large and Small Magellanic
Clouds as they circle the Galaxy, causing vibrations at
certain frequencies when they pass through its edges.
Note: The Magellanic galaxies have too small a mass by
themselves to do the job, but using Dark Matter as an
amplifier seems to account for everything.