Our Celestial Home: The Milky Way
If you go outside on a dark summer night, you can look overhead and see a
faint, cloud-like white river running from the southwest corner of the sky to
the northeast corner. To the ancients, this "cloud" looked like a
long splash of milk across the black sky background. Not knowing what is was,
they named the cloud the "Milky Way." As time progressed and astronomical
techniques improved, astronomers discovered that the cloud was actually innumerable
stars that were so crowded together, they gave the perspective of being a giant
cloud. In time, astronomers began to unravel the secrets of the Milky Way and
generate a map of this giant collection of stars. The panorama photograph of
the Milky Way is courtesy of John Gleason and represents what you look at when
you pan the swath of stars running through the summer sky.
those living in the northern hemisphere, every star that you see with your naked
eye is a part of this galactic home we call the Milky Way. If your night sky
is really dark, you might be able to spot the Andromeda Galaxy (M31) (in photograph
to the left) is the late summer and autumn sky, and this object is not part
of our galaxy, but a distant galaxy like ours, situated 2.65 million light years
from here. It is the most distant object you can see with your naked eye. Other
than this object, the rest of the stuff you see in the night sky are all stars
and gas clouds within our own galactic confines.
Those who live in the southern hemisphere can glimpse two large patches of
stars that are actually companion galaxies of ours, and are irregular in shape.
These two places are called the Large
Magellanic Cloud (179,000 light years away) and the Small
Magellanic Cloud (210,000 light years away). The images below, from the
Malin collection show pictures of these neighbors left and right respectively.
Recently, astronomers have discovered a galaxy opposite of our line of sight,
and quite invisible for our naked eyes that the Milky Way may be consuming.
Dwarf Galaxy is about 80,000 light years from us, and is being incorporated
into our Galaxy's structure.
The Milky Way
is a huge group of an estimated 150 to 200 billion stars, all moving in a circular
motion defined by a beautiful spiral shape. In the pair of images below, you
can see a depiction of the structure of our Galaxy. This depiction is based
on a detailed study of the stars in our galaxy. Astronomers have used various
distance measuring techniques to determine the location of many, many stars
relative to our own Sun and relative to each other. The result of this distance
measurement system is the three-dimensional map that you see below.
The sheer dimensions of the Milky Way are staggering. Looking at the galaxy
from an edge-on standpoint, light would require a time of 75,000 - 90,000 years
to go across the diameter of the Galaxy. The central bulge of the Galaxy is
a dense cluster of older stars that are predominantly of spectral class M. Splaying
out from the central bulge is a series of "arms" of younger stars
that are of all spectral classes. Since the Galaxy is spinning, the entire structure
is flattened like a disk. However, the gravitational attraction of the Galaxy
itself holds a spherical influence, and although the majority of the material
is in the disk, there is a "halo" of objects called globular clusters
that independently orbit the central bulge. These small clusters of stars contain
anywhere from a few thousand to a million stars, and act like planets orbiting
the Sun. The difference is that the globular clusters are orbiting independent
of the flattened disk, with some moving in a "north-south" orientation
and others in an "east-west" orientation. Some stay outside of the
spiral arms, while others move right through them. Some orbit in a clockwise
fashion, while others move counterclockwise. And, the stars of these globular
clusters are mostly M class stars with tiny white dwarfs giving evidence to
the advanced age of the clusters.
Our Sun occupies a spot in the so-called "Orion Arm," that is bracketed
by the neighboring Perseus and Centaurus arms. As stated earlier, the entire
Galaxy is rotating, so all of the stars are moving in an elliptical path around
the center of the Galaxy. Along with the other arms of this spiral galaxy, the
Sun orbits the central bulge in a period of 225 million years. Over the estimated
4.5 billion years of our planet, Earth has taken about 20 trips around this
merry-go-round. As you can see from the second image below, the Sun is at an
estimated distance of 25,000-30,000 light years from the galactic center.
It should be emphasized that there are almost as many stars between
the spiral arms as in the spiral arms. The reason why the arms of spiral galaxies
are so prominent is that the brightest stars are found in the spiral arms. Spiral
arms are the major regions of star formation in spiral galaxies and this is
where most of the major nebulae are found.
The few paragraphs in the smaller font below are from the
SEDS (University of Arizona) website that teaches a very nice unit on the
Milky Way. I have brought these words and the following image of the central
bar as well as the image of galaxies similar to our own from that site. You
are encouraged to check out this website for your viewing pleasure and to increase
The Shape of the Milky Way - The Evidence
The first good map of the spiral form of the Milky Way was produced by Oort,
Kerr and Westerhout in 1958 (an early version of their map exists here.) They
mapped the neutral hydrogen (HI) in the plane of the Galaxy. Later, in 1976,
Yvonne and Yvon Georgelin produced a map of many of the major HII regions (bright
nebulae of ionized hydrogen) showing how they are distributed along the spiral
arms. In 1993, Taylor and Cordes produced an updated version. Finally in 1995
J Vallee examined the evidence and concluded that the Galaxy was a four-arm
logarithmic spiral depicted here.
It has long been suspected that the middle of the Milky Way may have a bar,
but the first conclusive evidence was produced by Blitz and Spergel in 1991.
Information on the exact shape of the central bar was published recently in
July 2001 by López-Corredoira, Hammersley, Garzón, Cabrera-Lavers,
Castro-Rodríguez, Schultheis and Mahoney, (available as a postscript
file here.) They conclude that the central bar of the Milky Way looks much like
the bar in the spiral galaxy M95.
If we put all this data together then we get a map like this one below. There
is very little data available about the far side of the Galaxy but spiral galaxies
are usually fairly symmetrical. Features on one side of a galaxy are often repeated
on the other side.
Best estimates of the statistics of the Milky Way
Diameter of Galaxy
75,000 - 90,000 light years
Number of stars
Mass of Galaxy
600 billion solar masses
Length of central bar
25,000 light years
Distance from Sun to the center
26,000 light years
Thickness of Galaxy at Sun location
2000 light years
Rotation period of the Sun around the Galaxy
225 million years
Below - four galaxies which look like the Milky Way. NGC 3953 (top left) is
55 million light years away and 95,000 light years in diameter. NGC 5970 (top
right) is 105 million light years away and 85,000 light years in diameter. NGC
7329 (bottom right) is even further at a distance of 140 million light years
but it is larger with a diameter of 140,000 light years. NGC 7723 (bottom right)
is 80 million light years away with a diameter of 90,000 light years.
end of SEDS material
To better understand our place in space, astronomers utilize
telescopes that are attenuated to wavelengths of the electromagnetic spectrum
other than the visible band. A stunning comparison of these photographs taken
from NASA is shown below, and by click on the image
itself, you can see it in a larger format.
Mystery of the Milky Way Structure
What this image set above demonstrates is that more than visible
light is pouring out of our Galaxy. There are large x-ray and gamma ray sources,
and bright gaseous regions glowing in the Infrared. Scientists are still trying
to determine just what holds our Galaxy together. The visible light spectrum
indicates the presence of 150 to 200 billion stars, but the structure holds
a mystery. The spiral arms do not have enough visible mass to maintain their
structure. Although the entire galaxy rotates every 225 million years, it is
actually rotating so fast that the stars should be flung off into intergalactic
space. The fact that these stars remain in their arms alludes to the presence
of additional mass of a kind not yet identified. This so-called "Dark Matter"
is theorized to exist, but has yet to be found or described.
Star Types Within Our Galaxy
Our galaxy has two different kinds of stars. The central bulge
and halo globular clusters are dominated by M class stars burning Hydrogen into
Helium, and are as old as the Galaxy itself. These stars are reddish in color
and are depicted in the drawings according to that color. Meanwhile, the spiral
arms are composed of these same M class stars, but also hold a lot of bright,
hot, blue and white stars. Spiral galaxies are unusual in that they are regions
of intense starburst activity. The term "starburst" means new stars
are forming from old stars. Stars that are formed from recycled dead star material
are called Population I stars, while those
that are original to the galaxy are Population
II stars. The Pop II stars are all rich in Hydrogen, poor in metals, and
long-lived. Pop I stars are more rich in metals and short-lived.
Starburst activity occurs due to an interesting invisible structure
in the spiral arms called a Density Wave, depicted in the drawings above. Orbiting
the central bulge of the Milky Way, and other spiral galaxies like ours, are
these dense arms of gas and dust that orbit at a slower rate than the visible
star component arms. As a spiral arm of stars and gas catches and moves through
a density wave, the gas clouds are affected in such a manner that new stars
collapse from them. Therefore, looking at a typical spiral arm of the galaxy,
the trailing part of the arm that has just passed through a density wave will
hold the youngest stars that were newly formed, while the leading edge that
has yet to move through the next density wave will hold older stars that were
formed at a more distant time. In this manner, the spiral arms continually recycle
stellar material. It is this very recycling of material that possibly gave rise
to our own Sun ... a second generation Population I star rich in metals, and
with a planet rich in metals and now life.
In conclusion, the picture below shows the Andromeda Galaxy with
the subtle differences between colors of stars. The core of the galaxy is primary
composed of Population II stars which are older, more evolved, and less-frequently
recycled. They appear reddish in color. The arms of the galaxy have recycled
stars that are younger, often hotter, and therefore blueish in color. All of
the other stars in this image are actually stars in our own galaxy that are
in front of this much more distant object. The two larger bright spots are companion
galaxies that are gravitational associated with Andromeda.
Formation of Our Galaxy
The key to understand how our Galaxy formed is the location, ages and chemical
composition of the various stellar populations. The oldest stars are in the
halo and bulge. The most metal rich stars are in the disk and bulge. From this
we deduce that the halo formed first, followed by the bulge then disk. All the
gas is located in the disk (which is rotating) because gas clouds can undergo
Inelastic collisions occur when two objects collide and share momentum as
a single body. Stars are too small to collide within the Galaxy (their cross
section is very, very low). But gas clouds are large and can 'stick' together.
Given the above facts, it is theorized that the formation and evolution of
our Galaxy must have taken place through a series of continuous stages. First,
the Galaxy began as a large single gas cloud a few hundred of thousand light-years
across. Passage near other proto-galaxies caused this large cloud to spin. This
rotation was far from organized as currents and smaller clouds formed within
Spheres of gas containing about a million solar masses of material collapsed
first, and these became the halo globular clusters. These first clouds were
very weak in chemical abundance, but the first supernovae in the halo stars
began to enrich the interstellar medium.
Cloud-cloud collisions steadily eliminated those clouds with the greatest
inclination and those moving in the opposite directions until the distribution
of gas clouds became flatter and flatter. Most of the gas was directed to the
bulge regions where the high densities produced a highly dense core region.
Lastly, the remaining gas settled into the disk where the rotation slowed
the formation of new stars until spiral density wave formed to dominate the
appearance of the Galaxy today. This is the dominant theory for the formation
of our galaxy. To learn more details about the formation
of other galaxies, please go to Galaxy Formation
in the Universe for a more detailed explanation.
Later, you should move forward to
Galaxy Structure, and then to various
Galaxy Images and an attempt to unravel the
mysteries of the Cosmos itself, or return to the Galaxy
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