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Content and Structure of the Milky Way Galaxy
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At the turn of the century, the Milky Way was our
ENTIRE universe! Then came the
Shapley-Curtis Debate in 1920 that opened up the possibility that
the Milky Way was just one of many galaxies. The "spiral nebulae"
(as the galaxies were called then) were actually galaxies like ours, only
very distant. The question was resolved in the mid-1920's when
Edwin Hubble, using the 100-inch Hooker Telescope at Mt. Wilson, identified
Cepheid variable stars in the Andromeda Galaxy, M31.
The Shapley-Curtis debate makes
interesting reading even today. It is important, not only as a historical document, but also as a glimpse into the reasoning
processes of eminent scientists engaged in a great controversy for which the evidence on both sides is fragmentary and partly
faulty. This debate illustrates forcefully how tricky it is to pick one's way through the treacherous ground that characterizes
research at the frontiers of science. Shu, F., 1982, The Physical Universe, An
Introduction to Astronomy, (University Science Books, Mill Valley, California)
p. 286
Learning ObjectivesAfter listening to the lecture, reading the text and these on-line notes, and completing the lab assigned with this lesson, you should be able to:
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Terms you should know:
disk
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Before we venture out into the rest of our Galaxy, let's pause to look around
at the Sun's nearest neighbors. The majority of our neighbors are low-mass,
dwarfs, too dim to be seen with the naked eye. Check out this list of the 60
stars nearer than 5 pc from the Observer's Handbook of the
Royal Astronomical Society of Canada. Only 4 of these stars are more massive than
our Sun.
Since it is impossible to tell how far away the stars are just by looking
at them, we have measured the parallaxes of these stars and thousands of
others. From these measurements and the location of the stars in the sky,
we are able to put together a 3-D image of where we live. Here are four
different views of our "block":
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| 3-D Sterographic Images |
Link towww.clockwk.com and more images |
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Finally, as we say goodbye, we note recent observations from the Hubble Space Telescope and the Extreme Ultraviolet Explorer: the structure of the interstellar gas cloud surrounding the Sun.
All of our study up to this point has been about the content of the Milky Way and
how we've come to know what we do. Here's a gallery of
some of the wonderful objects we've
seen so far:
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The total inventory of the Galaxy probably includes at least 200 billion other stars (maybe one trillion solar masses) and their planets, thousands of open and globular clusters and nebulae, the gas and dust of the interstellar medium, and perhaps any number of exotic objects like stellar black holes and a supermassive black hole at the Galaxy's center. Its diameter is about 30,000 parsecs (100,000 light years). The galactic disk is separated into complex, and sometimes indistinguishable, layers. The thin disk is about 2000 parsecs thick and contains the dust, gas, and hottest and youngest stars. The thick disk adds about another 700 pc onto the disk, and contains somewhat older (and therefore lower mass, lower metallicity) stars.
Let's get familiar with just how the Milky Way looks to us: a fuzzy, wide swath of light going across the sky. If our galaxy were spherical, with stars distributed uniformly throughout the sphere, the density of stars we would see would be the same everywhere. But, what we notice is that the Milky Way looks like a band: thick in some regions, and thin and barely noticeable in others. This shows that the Galaxy is relatively flat. Farther along in these notes, under The Structure of the Milky Way, we cover how our view changed with the work of Harlow Shapley. Take a look at the linked Gallery of Milky Way Images from astrophotographers. Think if you have seen similar views yourself.
The evidence is mounting and becoming more and more convincing (although controversy may prevail for a while) that the Milky Way is, in fact, a barred spiral galaxy. Research that has sought to explain "expanding arms," carbon monoxide gas kinematics (dealing with motion), large-scale asymmetries in neutral hydrogen distribution, asymptotic giant branch star populations, radial distribution of carbon monoxide, near-infrared counts of carbon stars, and populations of bulge red clump stars all point to the same thing: the Galaxy has a bar that is pointed just off of our line-of-sight to the center.
The 2MASS survey's view of the entire Milky Way, seen from our vantage point of Earth.
("Atlas Image obtained as part of the Two Micron All Sky
Survey (2MASS), a joint project of the University of Massachusetts and the Infrared
Processing and Analysis Center/California Institute of Technology, funded by the
National Aeronautics and Space Administration and the National Science Foundation.")
Image Credit:
2MASS/J. Carpenter, M. Skrutskie, R. Hurt
Near-infrared light—which is just beyond the red edge of the visible spectrum—can't penetrate dust as well as X rays can. But observing the Milky Way at these longer wavelengths reveals the more common, cooler stars that make up the bulk of the galaxy's stellar population. Using a select group of 30,000 stars among the 300 million or so cataloged in a huge near-infrared survey called 2MASS (Two Micron All Sky Survey), researchers have produced the first bird's-eye view of our galaxy's spiral disk and the cigar-shape structure, called a bar, at its center.
Of all the stars recorded by 2MASS, the astronomers chose to analyze carbon stars first because of three remarkable properties. First, the stars are so bright that telescopes on Earth can detect them throughout the galaxy. Second, although the Milky Way's dust dims the light from carbon stars, the stars remain easy to recognize and they can't be confused with other stars in the survey. Best of all, carbon stars act as standard candles—all these stars have about the same luminosity, as if they were light bulbs of the same wattage. That property turns them into veritable mileage posts marking off distances throughout the galaxy.
By comparing the luminosity of a carbon star to how bright it appears in the sky, Skrutskie, Weinberg, and their colleagues determined the distance to each of the 30,000 carbon stars in 2MASS. Their preliminary map reveals the outer limits of the Milky Way's stellar disk. At the center of the galaxy, the map shows that the galactic bar measures about 15,000 light-years across.
A comparison of
the galactic center in optical and at infrared wavelengths from the
2 micron all-sky survey at IPAC.
After comparing these two images, it becomes a bit more obvious how important it is
that we observe our universe across the entire electromagnetic spectrum.
So, if we could get a bird's eye view of the Galaxy, what would it look like?
Some think it resembles the barred galaxy NGC 253, shown at the right (click on the
image for a larger view). Observations
have indicated that rather than just two main arms coming off each end of the bar,
the Galaxy has 4 arms. This is not unusual for a barred spiral. Take a look at the
following sketch that has used data to support the artist's rendition:
Why were our ideas regarding the overall structure of the Milky Way Galaxy wrong for so long?
The information and images given above should give you a huge hint. Compare the following two images. The first one is of the galaxy NGC 891, a galaxy thought to closely resemble our Milky Way; the second, another image of the center of the Milky Way, showing the Sagittarius-Scorpio region. Hint: the dark stuff is NOT a hole or an empty lane in the galaxies. What do you think would be the problem? [Another hint: it's a four-letter word that starts with "d"]
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Review Section 18.1 of the text where it discusses how we came to learn that we were not at the center of the Milky Way. This discovery was yet another step in our being misplaced from centers. First, we accepted that we were not the center of the solar system, and in the 1920's, we learned that we were not at the center of our galaxy. What's next? Being told there's a huge universe out there and we (the Milky Way) aren't even the center of it?
What key discovery opened up the true vastness of our galaxy? The variable stars known as RR Lyrae and Cepheids.
| Actual images of an RR Lyrae star in M4 | RR Lyrae Light Curve | Cepheid Light Curve |
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(The negative images are jerky due to misalignment of images. The RR Lyrae
star is at the center--the larger the star, the more luminous it has become.
This star has a period of about 1/3 day.) |
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RR Lyrae and Cepheid variable stars are THE most important step in the Distance Scale of the Universe.
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RR Lyrae stars are old, low mass, stars occupying part of the horizontal branch. They are primarily found in the halo of galaxies, particularly the globular clusters. The average apparent magnitude of Star #42 in the globular cluster M4 is 13.5. If we assume an absolute magnitude for all RR Lyrae stars is 0.75, what is the distance to Star #42 and thus M4? What familiar formula would you use to calculate this? Did you get about 3500 pc? The distance to M4 is difficult to determine for the simple reason that it lies in a part of the Galaxy that is heavily obscured by dust. Click on the images to the left to see a larger view of each one. The globular cluster M4 is marked on the close-up image. Note that the close-up has been rotated clockwise by about 90 degrees. It is easy to see the long pillars of dust in the region. Astronomers estimate that the stars in M4 are dimmed by about 1 magnitude. The bright, yellowish star in these images is the red supergiant Antares. |
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Here are two actual light curves of RR Lyrae stars. The one of the left is of the prototype star: RR Lyrae itself, with a period of 0.5668 days. The one shown on the right is a newly discovered one (2000), the discovery made by Dr. Scott Anderson and then graduate student Armin Rest using imaging data from the Sloan Digital Sky Survey. The plot was made from data taken by undergraduates in astronomy using the Monashtash Observatory in Ellensburg. The period for this star is 0.48588 days, an extremely precise determination. Note that the light curves measure the change in the apparent magnitude of the star and note also the similarity in the way the light curves look. (The data are not shown over time; rather, astronomers do a bit of an "accordion" calculation that essentially folds all of the data so that 1.5 periods are emphasized.)
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Cepheid stars are massive stars that are transversing the top
of the H-R diagram as they evolve. They are usually found in
the disks of galaxies, in regions of active star formation.
Although Cepheid variable stars are intrinsically much brighter
than RR Lyrae stars, they are often hard to observe because they
lie in crowded regions. RR Lyrae stars are in uncrowded regions,
and thus more ideal for observing; however, they are intrinsically
a lot less luminous than Cepheids. Thus, there is a limitation as to
how far we can actually observe these stars -- galaxies in the local group
are fine, but not much farther than approximately 2 million light
years, and even at that distance the stars are about 25th magnitude!
(You can use the magnitude equation to calculate this yourself.)
| The Period-Luminosity Relationships for RR Lyrae and Cepheid Variable Stars. |  |
Once the secrets of the RR Lyrae stars were revealed and that information
used to calculate the distances to the globular clusters of our galaxy,
we found out that the Sun, most definitely, was NOT at the
center. In the lab,
RR Lyrae and the Distance to M4,
you used an RR Lyrae variable, Star No. 42 in the
globular cluster M4 to
find the distance to that cluster. M4 is one of the nearest globular clusters
in the sky.
| Table 1: Overall Properties of the Galactic Disk, Halo, and Bulge | 
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| Property | Disk | Halo | Bulge |
| Shape | highly flattened | roughly spherical; mildly flattened | football shaped |
| Star Types | young and old | only old stars | young and old stars; more old stars at greater distances from center |
| ISM | gas and dust | no gas or dust | gas and dust, especially in inner regions | New Stars? | ongoing star formation | no star formation for past 10 billion years | ongoing star formation in inner regions |
| Kinematics | contents move in circular orbits in the Galactic plane | stars have random orbits in three dimensions | largely random orbits with some net rotation about the center |
| Substructure | spiral arms | no obvious substructure | nucleus; ring of gas and dust near center; bar |
| Color | overall white color with blue spiral arms | stars reddish in color due to old age and cool temperatures | yellow-white due to mix of stars |
Galaxies rotate. The stars, their planets, the interstellar medium, all revolve around the center of our galaxy. At one time it was thought that the spiral arms of the Milky Way and other spiral galaxies were the result of originally straight arms "winding up" due to this rotation. The Sun orbits the center every 250 million years and has gone around about 60 times. We know that the stars inward of the Sun orbit faster, and stars outward from the Sun orbit slower. As the following cartoon shows, that over the course of the Milky Way's lifetime, 15 billion years or so, the spiral arm structure would be destroyed.
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We just have to look at our neighborhood to notice that young stellar
objects, emission nebulae, O and B stars, and recently formed open clusters
are distributed in a spiral pattern. Measurements of the 21-cm
emission of atomic hydrogen also suggests a spiral structure. From
this evidence, we conclude that the spiral arms trace young,
massive star formation.
A leading theory is that the spiral arms represent density waves,
coiled waves of gas compression moving through the disk. These
density waves, although rotating around the center of the Galaxy, do
so much more slowly than
the individual stars. As the interstellar medium is compressed,
star formation, with its subsequent luminous O and B star formation,
is initiated. In this case, the density waves drive star formation.
An
alternative explanation has star formation driving the density waves.
We've talked about the effect massive stars have on their surroundings.
These stars compress the interstellar medium around them, perhaps
initiating additional star formation. Massive stars live for a very
short time before going supernova. The subsequent shock waves could also
create even more star formation. This theory is known as
self-propagating star formation.
An analogy of the density wave theory (befitting the problems
we are having these days) is that of traffic on the freeway in the
presence of an accident off to the side of the road. Drivers all slow
down as a caution and to "look". The density of cars increases
momentarily, but all cars eventually move through the area and resume
normal speed beyond the accident. From above, one would see the area
where there was the slow-down -- even more so if all cars turned on their
headlights.
Where do these spirals
come from? What was responsible for generating the density wave in the
first place or for creating the line of newborn stars whose evolution
drives the advancing spiral arm? We don't really know.
Scientists speculate that (1)
instabilities in the gas near the Galactic bulge, (2) the gravitational
effects of nearby galaxies, or (3) the possible barlike asymmetry within
the bulge itself may have had a big enough influence on the disk to get
the process going.
Return to the lecture notes on the color magnitude diagrams of various open clusters in our galaxy, especially the use of open clusters as distance measures. Here is a method we have used not only to find out distances to parts of the Galaxy but also to map out regions of past star formation.
The luminous objects in the halo are low-mass, old, metal-poor field
stars or low-mass, old, metal-poor stars in globular clusters. The
halo has no gas or dust at all. Even the globular clusters, which must
have had stars massive enough to go supernova when they were first
formed, have little or no gas and dust. Only an occasional planetary
nebula is seen. Globular clusters are, therefore, devoid of any new
star formation.
On the other hand, the bulge, because of its crowded nature, must be
full of activity. The bulge has a mixture of stars: old and new,
low mass and high mass, metal poor and metal rich. Unfortunately,
because of the heavy obscuration by dust, we cannot see into most
of the bulge region.
Take a look at an
infrared image of our galaxy.
Analysis of infrared and radio observations indicate
that the heart of our Galaxy harbors roughly 50,000 stars per cubic
parsec. That's a stellar density about a million times greater than
in our solar neighborhood, high enough that stars must experience
frequent close encounters and even collisions. Infrared radiation has
also been detected from what appear to be huge clouds rich in dust.
In addition, radio observations indicate a ring of molecular gas
nearly 400 pc across, containing some 30,000 solar masses of
material and rotating at about 100 km/s.
When we study our planets, we note that as we get farther away
from the Sun (which contains almost all of the mass of the solar
system) the planets orbit more and more slowly. Remember Kepler's
Third Law that relates period and distance from the Sun? One would
expect, based on Kepler's laws and Newton's physics, that if most
of the mass of our Galaxy were located towards its center (where there
is most of the light), that as we moved farther and farther from the
center, objects would also orbit more and more slowly. This is
definitely not the case.
The Galactic rotation curve indicates that as we move away from the
center, we never reach the edge of the matter. The rotation curve
must stay high as the amount of mass within that radius continues
to increase.
The mass accounted for by the luminous objects in our galaxy is not
enough to cause the rise as we go to the outer regions of the Galaxy.
We can only conclude that there is a large amount of
dark matter (non-luminous material) that we have not detected, and
that we really do not know much about. Computer simulations indicate
that the luminous regions of the Galaxy must be surrounded by an
extensive, invisible dark halo.
We have been able to probe deeper and deeper into our galaxy's heart,
and what a remarkable place it is!
Absolutely the most comprehensive summary of what is at the center of
the Milky Way from the
Naval
Research Laboratory
A Few
Images of the Galactic Center
An abstract concerning the possibility of a massive black hole at the
heart:
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Session 97 - Galactic Structure, Galactic Center. Oral session, Friday, January 09, Georgetown [97.07]The Galactic Center Black Hole A. Eckart, R. Genzel (MPE Garching Germany) High spatial resolution, near-infrared imaging and spectroscopy of the nuclear star cluster obtained in the last few years have given key new insights about the mass distribution in the Milky Way Center. Measurements of radial and proper motions for more than 200 stars show that stellar velocities increase with a Kepler law down to a scale of a light week from the compact radio source Sgr A. The data make a compelling case for the presence of a compact, c entral dark mass of about 2.6 x 106 solar Mass. Simple physical considerations show that this dark mass cannot consist of a stable cluster of stars, stellar remnants or substellar condensations. Energy equipartition requires that at least five percent of the dark mass (105 solar Mass) must be associated with Sgr A* itself and likely is enclosed within less than 8 light minutes. If one accepts these arguments it is hard to escape the conclusion that Sgr A* is indeed a massive black hole at the core of the Milky Way. |
Observing the Galaxy at different wavelengths (different energies) gives
us far more information than we could get from just the optical. Many
parts of the Galaxy, including the nucleus, are hidden from view due to
dust. Viewing at radio wavelengths allows us to probe the very depths
of the center. The 21-cm line of atomic hydrogen traces "warm" hydrogen in
the interstellar medium and
carbon monoxide (thought to always be associated with molecular hydrogen)
traces the cool interstellar medium. Infrared radiation is given off by
dust. Optical wavelengths shows us the stellar distribution,
RR Lyrae and Cepheid variables, and traces
hot O and B stars across the Galaxy. X-ray radiation indicates regions
of hot, shocked gas. Gamma ray radiation points out objects such as
the Crab, Vela, and Geminga pulsars.
Take a look at
our Milky Way at multiwavelengths. Examine the
images closely and read the accompanying descriptions. Many of the descriptions
are quite technical, but you should be able to pick out how the radiation is
produced and the kinds of events that radiation at that frequency traces.
The explanations given as links under the
"education" portion of the
Multiwavelength Milky Way Project are quite a bit easier to understand.
This link was
adapted from one created by NASA called the
Multiwavelength Milky Way. Check out this NASA site as well.
Let's take a look specifically at how we detect regions of atomic, molecular, and ionized hydrogen.
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| If the proton and electron have parallel spins they are at a slightly higher energy state than if they have anti-parallel spins. The hydrogen atoms will move to this lower-energy state and emit a photon having a wavelength of 21-cm. The transition is not very probable, but there are SOOO many hydrogen atoms in the interstellar medium that we see strong 21-cm emission in cool (temperatures of around 120 Kelvin) regions. This is the lowest energy transition of the hydrogen atom. Usually the atom is excited to this slightly higher energy level through collisions. | ||||
We have also seen excited hydrogen when we've looked at images where star formation is occurring, such as the Orion Nebula and the Rosette Nebula. This radiation has a wavelength of 656.3 nm, or in the red part of the spectrum. Visible light is obscured by interstellar dust, and so we cannot trace these regions much farther than a few thousand light years.
Practice Multiple Choice Exam
on the basic
knowledge from the text. Instant feedback provided.
For a really spectacular set of images of the Galaxy and all that it has in it,
by season if you like, go to
Catching the Light, astrophotography
by Jerry Lodriguss. Check it out!
Harvard's Addition to the Literature on the Black Hole at the Center of the Milky Way
ASTRONOMY PICTURES OF THE DAY
A Frothy Milky Way
Galactic Center in the Infrared
High Energy Picture of Center
A Galactic Cloud of Antimatter
Milky Way Molecular Map
COBE Image of Center
Lensing Through Baade's Window
Gamma Ray Halo
A Galactic Fountain
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