One of the more prominent features of the night sky is a thick band of stars that is called the Milky Way. It was noticed in most cultures and nearly all have a special name for it. Its prominence means that stars are not distributed uniformly in all directions of the sky. Since we see them in a band in the sky this means that the distribution of stars is flattened. Herschel the first person to suggest this, put the Sun in the middle of a system of stars that was flattened such that the length was about five times its height. He however had no way of measuring distances to the stars and couldn't tell the extent of the Milky way. He also did not know of obscuration by material in between the stars and so did not account for stars that may have too much material in front and hence become invisible to us.
At the turn of the century the Dutch astronomer J.C. Kapteyn, launched
a massive program to determine stellar brightness and colors,
classifying spectra, measuring radial velocities, counting stars and
mapping the Milky Way. This huge program was a result of cooperative
work of astronomers from many countries. The final conclusion was the
system of stars forming the Milky way was about 6000 parsecs in
diameter with the Sun in the middle. And this was completely
wrong. Both the conclusions ignored the fact that there was
obscuration of light from distant objects by intervening
material. Almost immediately the Kapteyn Universe was challenged by
Harlow Shapley, the American astronomer. Shapley studied globular
clusters, incredibly dense and rich clusters of stars containing upto
hundreds of thousands to a million stars in each cluster. Using the
100 known clusters at the time, he found that the center of the Milky
Way was more likely to be in Sagittarius nearly 10,000 parsecs away
from the Sun and that it was much bigger than that suggested by the
Kapteyn. He put the diameter of the Milky way to be about 30,000
parsecs. The way he measured distances to the globular clusters is
interesting. Henrietta Swan Leavitt was one of the most important
members of the group of astronomers (including Willamina Fleming,
Antonia Maury and Annie Campbell) working in Harvard on classifying
and the understanding of stellar spectra. Leavitt worked with variable
stars, stars that appeared to change their brightness with time. She
found nearly 2000 new variable stars that are called Cepheids after
their prototype, the 
Cephei. She noticed that the brighter
the cepheid the longer it's time period. The stars belonged to two
large clouds of stars called the Magellanic clouds in the Southern
Hemisphere. These are now known to be two tiny galaxies that are
companions to our own Milky Way. Leavitt assumed that the stars were
at the same distance from the Earth and so the period of variation of
the cepheids must be connected to the stars' intrinsic brightness and
not their apparent brightness. So she had found a means of measuring
the intrinsic brightness of a star by looking at the variation in its
brightness over time. It was possible then to tell how distant the
star by comparing this intrinsic brightness with the apparent
brightness. She realized she could establish the distances of distant
star clusters by finding Cepheid stars associated with
them. Unfortunately since she was paid to study stellar spectra she
was not allowed to continue her work on Cepheids and had to return to
measuring brightness of stars from their images on photographic
plates. Regardless her work was and still is instrumental in measuring
distances in the Universe. Had she lived longer she would have
certainly been nominated for the Nobel prize. Shapley used variable
stars he believed to be cepheids in the globular cluster to measure
the distances to them. However the stars he was looking at were RR
Lyrae, a different kind of variable stars that were fainter than the
cepheids. This caused him to over-estimate the distance to the
clusters and hence the size of the galaxy. He had been extremely
fortunate in his choice of objects. Globular clusters are arranged
away from the Milky Way (they are more spherically distributed and not
confined to the disk) and so are less affected by obscuration. They
are also extraordinarily bright and could be seen over large
distances. But it wouldn't be until Trumpler's work with open
clusters in the Milky Way that people would realize the significance
of the effect of obscuration and so reconcile Kapteyn's work with
Shapley's. The currently favored size of the Milky Way is a diameter
of about 50,000 parsecs with the Sun at a distance of 8,500 parsecs
from the center of the galaxy which does lie in the Sagittarius region
of the sky. The size of the galaxy has increased inspite of the
reduction of the scale since Shapley's measurement because we can see
fainter objects. Because of the huge size of the galaxies it is
customary to use kiloparsecs (kpc) as a unit. Clearly, 1000 parsecs
makes 1 kiloparsec. The height of the Milky way is about 100 parsec,
compared to a diameter of 50 kiloparsecs, so it is
a really thin disk.
Around this time there was a debate held called the Shapley-Curtis debate, or the ``Great Debate'' where two issues were discussed. One was the size of the Milky Way and other was the whether some of the blurred non-stellar objects called nebulae were of the same size as the Milky Way and distant or smaller and closer. Curiously each held opinions that we agree with one on one issue and the other on the other. We agree with Shapley of course on the size of the Milky way, when Curtis still stuck to the Kapteyn Universe. Curtis on the other hand argued that the nebulae were outside the Milky Way and as big as it, which we agree with today. Shapley disagreed. In either case it was a matter of accepting how incredibly large the Universe was. Because Shapley believed in a large Milky Way he could not believe there could be anything outside of it. On the other hand Curtis could put the nebulae outside because he thought the Milky Way was small. As it turns out, today we believe the Milky Way to be as immense as Shapley had it, and nebulae are still outside of it, and just as large as it is. Thus we have carried on in the Copernican tradition, expanding our horizons, making the Universe and hence the possibilities immeasurable larger.
Around this time, Vesto Melvin Slipher, who lead the team that found
Pluto, had found that the nebulae that we believe are galaxies like
the Milky Way, rotated. This lead Bertil Lindblad to conclude that the
Milky Way rotated as well and this is why it was flattened. It now
became important to measure the velocity of stars. As this, through
the use of Newtonian gravity, would lead to the measurement of the
mass of the Galaxy. As proper motions were measured it was found that
stars fell into two categories. One group were the slow stars, i.e.,
had velocities similar to the Sun, where the other were stars moving
fast relative to the Sun. Walter Baade, while studying the Andromeda
galaxy concluded that the Milky Way (like Andromeda) had two
populations of stars. One, he called Population I, were the ones that
made up most of the Milky Way's disk. They were blue, main-sequence
stars that moved in the disk. They were moving slowly relative to the
Sun. The other group of stars were called Population II. These were
red, giant (old) stars that moved in highly elliptical orbits that
took them out of the disk of the Milky Way. They moved fast relative
to the Sun. Now suppose the population II stars are in random motion
about the center of the galaxy and the population I are rotating about
the center. Then the fast motion of the Population II stars is because
of the fast motion of the Sun. This fast motion of the Sun is shared
by the Population I stars which thus appear to move slowly relative to
the Sun. Then from this motion of the stars it is possible to estimate
the mass of the galaxy. The number is about 
solar
masses.
Trumpler's result and the discrepancy between Shapley's result and that of Kapteyn was forcing people to accept the presence of obscuring material in between stars in the Galaxy. The clear presence of dark swathes in the pictures of other galaxies at this time also influenced people into accepting the presence of material in between the stars.
One of Trumpler's results was that stars that were obscured were also redder than unobscured stars. This led people to believe that some of the interstellar material must be dust. The process is essentially the reason why California has such spectacular sunsets. The dustier the air, the redder the Sun gets near the horizon. The same thing is happening to stars because of the interstellar pollution. The dust doesn't just redden and absorb light, they are also warm enough to radiate light. They are too cold to radiate in the visible part of the spectrum, but they are quite bright in the infra-red where they radiate as blackbodies. This radiation from dust has been observed by satellites equipped with infra-red detectors.
J. Hartmann, working from Potsdam University saw dark lines in the
spectra of stars that couldn't be from the binary star
Orionis. The lines associated with the stars moved by Doppler-Fizeau
effect, but these were dark lines that didn't move. He concluded these
had to be lines due to dark material in between the star and us. Now
we know of a huge body of absorption lines in stellar spectra that
inform us about the interstellar gas. These molecules and atoms in the
gas also immediately radiate away the energy they absorb as
photons. If the gas is in between the star and us, this light is not
likely to reach us because the light gets swamped by the star. However
if the gas is to one side, some of this re-radiated light can reach
us. Then the gas can be seen in reflection. These are called
reflection nebulae. Sometimes these reflection nebulae will radiate
not in a few lines but in a lot of colors. They then appear to glow
and are referred to as emission nebulae. These are usually gas clouds
around bright, blue stars. Now it is also possible to study the
emission of colder gas that would normally be seen in absorption. The
molecules in the gas emit radiation in the radio and microwave
wavelengths that can be picked up using modern detectors.
Then in 1944, Hendrik van de Hulst, a student in Utrecht, predicted that hydrogen atoms undergo a very rare transition (once every 10 million years) and radiate a photon of very long wavelength of 21 cm (remember the wavelength of visible light is about 0.00005 cm). This radiation, called 21cm radiation, has turned out to be of profound importance to astronomy. Even though it is so rare, there is so much neutral hydrogen in space that the Milky Way is booming with 21cm radiation.
In the outer parts of the galaxy, even when we run out of stars, there is still neutral hydrogen and 21cm radiation can be observed. This is used to map the rotation of the galaxy at very large distances from the center. And this led to an upset. If the Galaxy runs out of material as the stars peter out, this should be evident in the rotation velocity of the gas there. It should be a lot less than the rotation speeds here near the Sun. Infact the rotation speed hardly changes. This means there is considerable material even in the outer reaches of the Universe where no light of any color, from X-rays to radio waves, is observed. This was studied in great detail by Vera Rubin, and is one of the most important discoveries of this century. Forced by her observation to choose between relinquishing Newton's theory and accepting huge amount of material that isn't visible in any way, astronomers chose the latter as the lesser evil. Thus we were landed into the problem of dark matter often described as the biggest problem in Astronomy.
Using the neutral hydrogen 21cm radiation it has been possible to map out the disk in detail. One of the most prominent features in the disk is the spiral structure. This is present in many external galaxies with lots of gas as well. These can thought of as traffic jams in the Galaxy. Like cars on an interstate traveling in packs, so do the stars and gas in the galaxy. But unlike the cars, the stars and gas collected together in the spiral arms get squeezed by their own gravity. This causes the gas in the spiral arms to fragment and form stars. The high mass blue stars don't survive the long traffic jams and die inside the arms. The longer lived red stars manage to escape the arms. This is why the spiral arms appear bluer than the rest of the galaxy.
The spiral arms are embedded in the rest of the disk, which is the most conspicuous part of the Milky Way. In the middle of the disk is the central bulge, which is nearly spherical rather than disk-like. And right in the center of the disk is the nucleus of the galaxy. Unfortunately the center can't be seen in visible light because of thick obscuring dust. It can however be observed in radio, X-ray and Infra-red. The object believed to be the center of the Galaxy is called Sagittarius A by radio astronomers. The amount of energy released by the object in X-ray, radio and infra-red indicates that the object is most-likely to be a black hole, million times more massive than the Sun, slowly eating away gas that is swirling around it in a little gas disk (a small version of the Milky Way) around it. Above and below the stellar disk of the Milky Way is a thinly populated spherical distribution of stars. These are stars of the Population II, old and red.
And finally there is the massive dark halo. This is the least understood, and perhaps most talked about part of the galaxy. Consisting of the mysterious dark matter that doesn't do anything other than to solve the problem they were invented to solve, it is impossible to characterize the halo with any degree of reliability. And as far as mass goes the frightening thing is that compared to the amount of the dark matter there is believed to be, the rest, all of the things that we can see could just be debris in a mainly dark matter Universe.