The Shape of the Milky Way
Introduction
A Simulation (the outside movie)
VR (the inside movie)
Given all the millennia that humans have been observing the night sky, one
might expect that the structure of our own galaxy, the Milky Way, is well known.
However, it turns out that our understanding of our own sun's place among other stars
of the Milky Way has been uncovered only relatively recently, and is still somewhat
incomplete.
Lacking city lights (and hence,
light pollution), ancient cultures were treated to far better views of the night sky
than most of us
experience today. They saw thousands of stars in black night skies, that were split into
two halves by a diffuse band of light. People in some parts of the world saw this band
as a great river, perhaps the source of a great terrestrial river in their own homelands.
Others saw a celestial path followed by gods, either across the heavens or
between heaven
and earth. To the ancient Greeks the band had a milky appearance. Our English word
"galaxy" is
derived from this Greek root "gala" meaning "milk". The name "Milky Way" is a
literal translation of the Latin "Via Galactica", and is now used to denote the band of
light observed in the sky, as well as to
distinguish our home galaxy from the billions of other galaxies scattered across the
universe. Astronomers also distinguish our home galaxy from others by referring to
it as the Galaxy (with a capital G). (You still can see
the Milky Way yourself, but it may require a little effort.)
As the first astronomer equipped with a telescope,
Galileo Galilei discovered (among
many other things) that the Milky Way was actually
the integrated light of countless stars that are individually too faint to see with the
unaided eye. That was back in 1610. Nearly 200 years later,
William Herschel used a
much more powerful
telescope to count faint stars in different directions. From this work he believed that the
stars forming our galaxy were grouped in a flattened distribution, with our sun near
the center and the Milky Way marking the directions of the Galaxy's greatest extent.
Another jump of over 100 years brings us to 1917, when
Harlow Shapley
discovered
that the globular star clusters are distributed over a roughly spherical volume that was
not centered on the sun. The center of the globular cluster distribution, and thus the true
center of the Galaxy, was estimated to be about 10 kiloparsecs away in the direction of
Sagittarius.
In the following decades, further research revealed that the "spiral nebulae" noted by
earlier observers were not diffuse gas, but actually enormous and distant collections of
stars much like our own galaxy. At this same time, it was also found that our own
galaxy lacks the beautiful symmetries seen in the distant galaxies
because interstellar dust blocks our view of most of the Galaxy. Some denser clouds of
this dust are evident to the naked eye as the "Great Rift" which splits the Milky Way in
two through the constellations of Cygnus and Aquila, and the
"Coal Sack"
which is visible from southern latitudes.
More advances starting in the 1940s and continuing today, have allowed astronomers to
observe the Galaxy in portions of the spectrum that are invisible to the human eye. In
particular radio waves and infrared radiation can now be employed for the task of
surveying our galaxy. Both of these forms of radiation can penetrate through
interstellar dust clouds much more easily than visible light, and can reach us from most
anywhere in the Galaxy. Radio waves
generally tell us where clouds of gas are located. Infrared radiation, (not too much
redder, or longer wavelength, than visible red light) traces the locations of the stellar
population of the Galaxy.
The following depiction of the Milky Way is a simple illustration of current ideas about
the shape of the Galaxy. It is intended to represent the stellar content of the Milky Way
as seen in infrared light. However, this view is not too different from what you would
see in visible light if you could step outside the Galaxy and look back.
This section illustrates what our galaxy may look like if we were free to move around it
at will. It also shows how a disk of stars appears as a thin path of light across the sky
when viewed from within the plane of the disk.
The simulation is presented as a MOVIE
(730k - in QuickTime
format). (Click here for the
movie in AVI format.) The following pictures are frames from the movie.
The movie starts with an
image from the COBE spacecraft. (Click here to explore other
images from COBE.) The image is taken in the near-infrared portion of the
spectrum, where most of light is from red giant stars and the obscuration of dust is
much
weaker than in the visible portion of the spectrum. The light from the distant galactic
bulge
(~8500 parsecs away) is pretty much blocked at visible wavelengths, but can be clearly
detected at these wavelengths. By examining the near-infrared emission of the Milky
Way
we get a much better impression of the overall shape of our galaxy. Its shape
resembles
that of distant edge-on spiral galaxies, such as
NGC 891
(or here or
here),
NGC 5746,
NGC4565, or
NGC 3628.
The image then fades into a simulated version of the Galaxy. Unlike the real Galaxy,
the simulated version can be rotated and viewed from different angles. The colors don't
represent the actual colors of the stars, but rather different structural components of the
galaxy. (The bulge is yellow, the disk is white, the spiral
arms are cyan, the halo is red, and the molecular ring is
green.)
The Galaxy proceeds to rotate, first to present the face-on view, and then about its
rotational axis. Notice that the bulge of the Milky Way is elliptical in shape. The shape
is somewhat difficult to see from our actual location in the disk, but is obvious from the
bird's-eye perspective. The model illustrated here may exaggerate the length of the
bulge. This is the most extreme of several different bulges which all have the same
appearance when seen from our location in the galactic disk.
Halfway through the movie the colors drop out intentionally to show the appearance of
the Galaxy as it might look in a black & white photo taken by astronomers in another
distant galaxy. The spiral arms stand out better in black and white, without the colors
distracting your eye. As an example, the inner portions of
NGC 1232
(or here) resemble this
model of our own Milky Way. The bulge of NGC 1232 appears
to be slightly elongated, but not to the degree of a bar in a typical barred spiral galaxy.
At the end of the movie we zoom into the Milky Way to get a close-up of
the location of our own Sun, which is about 2/3 of the way from the center to the
edge of the disk. Once we get close enough, we can see our sun (marked by a cross)
and its 500 nearest neighbors (marked
in green). All these stars are within about 13 parsecs (or 42 light-years) of our own
sun. Notice how crowded it really is. The whole Galaxy is really this crowded, but
the simulation really only shows about 1 star out of every 10 million in the Galaxy!
If you would like to go on a self-guided spin about the Milky Way, get the (highly recommended) Rotater application for the Macintosh, or the emulative (though less impressive)
Rotate for DOS.
Then grab
my Milky Way data file (which can be expanded using
Stuffit Expander for either Macs or Windows).
A different representation of the data (thanks to Richard Wolton) is available
for DOS as 3dmw.zip in the Astronomy
section of the SIMTEL archive.
Rick Arendt -- March 17, 2004
Credits
Go Home!
A VR view of the Galaxy!
An Astronomical Alphabet