Beginning with this lesson, we are now in the part of the course that I am grouping as Unit 3. In Lessons 8 - 10, we will address galaxies and cosmology. Having only three lessons means we again have to refer you to outside resources for more in-depth discussions of certain topics, but we will be moving outward from the Sun and the stars to understand the Milky Way, then the nearby galaxies, and finally the Universe as a whole.
From our location here on Earth, for centuries astronomers have studied the night sky to determine how the Earth fits into the structure of the Universe. The earliest theories that we studied suggested that the Earth was the center of the universe and all other objects revolved around our stationary planet. The first large shift in our understanding came when it was shown that the Sun was the center of our Solar System, and the Earth was simply one of several planets orbiting the Sun. Now that we understand the stars and star clusters, we can take our next step and figure out how the stars are distributed inside the Milky Way Galaxy. This will also allow us to understand how the Milky Way Galaxy fits into the structure of the entire Universe.
By the end of Lesson 8, you should be able to:
Lesson 8 will take us one week to complete.
Please refer to the Calendar in Canvas for specific time frames and due dates.
There are a number of required activities in this lesson. The chart below provides an overview of those activities that must be submitted for Lesson 8. For assignment details, refer to the lesson page noted.
Requirement | Submitting your work |
---|---|
Lesson 8 Quiz | Your score on this quiz will count towards your overall quiz average. |
Capstone Project | During Lesson 8, you will begin work on the Capstone Project and submit Part 1. |
If you have any questions, please post them to the General Questions and Discussion forum (not email). I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.
When you observe the night sky with your eyes, you can see the Moon, perhaps several planets, and many stars. If you are in a particularly dark location and if the moonlight is not too bright, you may also see a faint band of light that stretches from horizon to horizon. This pale, white glow has been called the Milky Way for centuries. The word “Galaxy” actually means Milky Way. If you look galaxy up in the dictionary, you will find that the root of this word comes from the Greek and Latin words for milk. I often like to ask students if they know the scientific name for the sugar in milk -- most have heard of lactose, but fewer have heard of galactose. If someone comes up with galactose, though, I try to make the connection between that sugar and our name for the structure in space in which we live.
Below is a real image of what you might see from a really dark sky site (in this case, Death Valley). For most of us, seeing the Milky Way as bright as it appears in Starry Night or in this image is a rarity, but this used to be a very common site.
There are many images of the Milky Way available on APOD. Some of my favorites all seem to be from Mauna Kea:
With just your eyes, it is difficult to tell that the Milky Way is anything besides a faint, patchy glow. However, with even the smallest telescope or binoculars you can resolve this glow into stars. Below is an image taken as part of the 2MASS infrared survey of the sky [6] that reveals the density of stars in the center of the Milky Way. If you prefer, there is a much higher resolution version of this image [7] available for closer inspection.
This image, and images you have seen previously during our study of the ISM, show you a representative section of the Milky Way, but they also show you why it appears patchy. There are large, dark clouds that obscure some of the stars. Thus, when you see the Milky Way with your unaided eye, you don't see a uniform glow, but a bright glow that is interrupted by dark patches.
The images above show just a sampling of the part of the Milky Way that is visible from a particular site by a typical camera. An interesting question to answer is: How would the Milky Way look if we could see the entire sky all at once? We can use the same techniques that mapmakers use to represent the entire Earth on a flat map to show you how the entire sky looks. For example, here is a projection of the globe of the Earth:
In this map of the Earth, you can see the entire globe and all of the continents and oceans represented inside of the elliptical boundary. Although this particular map projection is not used as much anymore for maps of the Earth, astronomers often use this same projection of the three-dimensional sky onto a two-dimensional picture when they want to represent the whole sky in a single picture. Below are several examples of these Aitoff projections of the whole sky.
There is a common feature in all of these images. The Milky Way is seen as a mostly flat, irregularly-shaped feature that stretches from side to side of every image. This tells us that if we could follow the Milky Way below our personal view of the horizon, it would be seen as a ring completely encircling the Earth. There are obviously some stars outside of this ring, but there are fewer of them (the caption for the map of half a billion stars [11] points out that in some parts of the sky there are 150,000 stars per square degree, while in others there are only 500 stars per square degree). I also included two links to images of the whole sky using different wavelengths of light: infrared and radio waves. The reason for this choice is because I wanted to demonstrate that the Milky Way looks different when observed in different kinds of light. We will study this in more depth in a later section of this lesson.
The Sun is a star. We see many hundreds of stars with our naked eyes, and with telescopes, we can see that the band of the Milky Way is made up of the combined light from many stars. Historically, astronomers have used several methods to understand how the Sun, the bright stars, and the Milky Way all fit together into a coherent picture of the layout of the universe. In the next section, we are going to study one of the early simple methods for doing this: counting stars.
Additional reading from www.astronomynotes.com [15]
One of the most famous astronomers of the 18th Century was Sir William Herschel. His fame comes mostly from the discovery of the planet Uranus, but another very important work of his was his picture of the Milky Way derived from a technique that he called “star gauging.” Herschel observed the sky in more than 600 different locations and counted every star he could see to the apparent brightness limit of his telescope. He assumed that all stars had the same intrinsic luminosity so that he could estimate the distance to each star from the Earth (this work was done before the HR diagram was created and before we understood the true range of luminosities of stars). Using his data, he created a map of the whole sky that incorporated the number of stars he counted and their distances from the Earth. This is reproduced in the image below.
Note in the image above that Herschel has provided the location of the Sun—it is the darker star near the center of the image. The conclusion you can draw from Herschel's work is that the Sun is located at, or is near, the center of the Milky Way.
In the early 20th century, the astronomer Jacobus Kapteyn orchestrated an even more impressive experiment than Herschel's to map out the Milky Way. Relying on the help of astronomers from all over the world, Kapteyn acquired photographs of regions on the sky he called “Selected Areas,” and he studied, in detail, the apparent brightness of the stars on the photographs and supplemented this with studies of their velocities. He used all of this data to construct a higher precision map of the Milky Way than Herschel. Below is Kapteyn's map.
In the map above, you can see that Kapteyn represents the Milky Way as a flattened disk with a radius of approximately 8500 parsecs. The Sun is offset from the center a bit, but is closer to the center than the edge. So, both Herschel and Kapteyn agreed roughly about the characteristics of the Milky Way. They both found that the Sun lies close to the center of the distribution of Milky Way stars and that the system is a flattened structure several times longer than it is thick. Unfortunately, neither Herschel nor Kapteyn knew enough to account for one extremely important effect that skewed their results. Thus, even though both undertook impressive experiments to study the Milky Way, they were both wrong in their conclusions! Light gets blocked by dust even in regions that are not as obvious as the dark nebulae. Therefore, extinguishing of light by material between the stars is more important than either Herschel or Kapteyn were aware, and it is this effect that caused them both to get incorrect results for the size and shape of the Milky Way.
We have already discussed some of the material found between the stars, called the interstellar medium or ISM. There is an abundance of gas in the ISM, and we can directly observe this gas when it emits light, creating a nebula. There is also an abundance of dust in the ISM, which is observable because it affects the light reaching us from distant stars. This dust is made up of microscopic, solid particles. Recall from Lesson 5 (page 2) that when a photon of light encounters a particle of dust, it can be scattered, which causes the light from a distant source behind a cloud of dust to appear dimmer. Recall also that dust makes the light from distant sources appear more red than it was when it was emitted.
The combined effects of interstellar extinction and reddening by dust makes it impossible for us to observe stars that are located behind too much dust. Because of this, our view of the Milky Way is blocked in many directions. A popular analogy here is to picture yourself driving in a thick fog. You can only see the headlights of other cars when they get very close to you. Also, if the fog is pretty uniform, you will only be able to see to the same distance in each direction, for example, you might only be able to see all headlights within a 20' radius. Because of the dust in our galaxy, in optical light we can only see nearby stars well. This means that we are not able to see all the way to the edges of the distribution of stars in space. Because we can see roughly the same distance in each direction we look in the sky because of the dust, this gives the illusion that the Sun is in the center of the distribution of stars.
There are a few methods that we can use to get around this limitation of observing through dust, and those revealed a Milky Way much larger than Herschel or Kapteyn inferred.
Additional reading from www.astronomynotes.com [15]
In the lesson on star clusters, you learned that these objects are excellent tracers of the size and shape of our galaxy. What this means is that since these groups of stars are part of our galaxy, their distribution in space helps define the boundaries of our galaxy. Star clusters are big and bright, so they stand out above the background, making them easy to spot even at large distances. An analogy in this case may be to think of them like the tallest skyscrapers in a city. From a distance, you can see these tall buildings very easily, allowing you to determine roughly your distance from the city. You can also estimate how big the city is by how many skyscrapers it contains and how spread out they are.
In 1917, Harlow Shapley used the globular clusters in the Milky Way to gain a better understanding of the Milky Way Galaxy. He measured their positions and distances and plotted their locations on a two-dimensional chart. I have reproduced his work in the two-dimensional plot below, but using modern data for the distances and locations of all known globular clusters.
In this figure, the X and Y axes are in units of distances in kiloparsecs (1 kiloparsec = 1,000 parsecs). The hatched region represents the plane of the Milky Way (that is, roughly the part of the sky where the Milky Way is visible to the eye), and the X located at (0,0) marks the location of the Sun in the plane of the Milky Way. The Galaxy fills a 3D region in space, so this 2D plot only shows a slice from the top to the bottom through the plane of the Milky Way.
Shapley's data was not as extensive as in the plot shown above, nor was it as accurate. Similar to Herschel and Kapteyn, dust extinction and reddening affected his distance measurements to the clusters, and thus his conclusions as well. Because dust makes stars appear fainter than they truly are, if you do not account for the amount of extinction, you will overestimate the distances to these objects. This is just what Shapley did. However, the data he did have allowed him to make two very important discoveries:
There appears to be a significant discrepancy between the work of Shapley using the globular clusters and much of our early discussion about the appearance of the Milky Way in the sky. Why are the globular clusters tracing out a round, spherical distribution on the sky if the Milky Way itself appears to be a flattened plane? The answer is that the Milky Way is actually a little of both!
If we use a different type of object to trace the structure of the Milky Way, we find a different size and shape. If we use open clusters as tracers, they do match well the shape of the visible band of the Milky Way. Open clusters are predominantly young objects, so if we select other objects that also trace out the regions in the Milky Way that contain newly forming or newly formed stars (e.g., giant molecular clouds, O and B stars, emission nebulae), they also show that the Milky Way is a flattened, disk-shaped object. So the globular clusters (which contain very old stars) reveal a spherical component of the Milky Way, while the open clusters and other young stars and star-forming regions reveal a disk-shaped component of the Milky Way.
Again, we can use modern data and plot the locations of both open clusters and globular clusters to compare and contrast their locations. Below are two different views of this data. The first is a top-down view and the second is an edge-on view. In both images, the green dots represent open clusters, and the yellow dots represent the globular clusters. The scale is approximately the same as the plot above.
Viewing the Milky Way from the top down in the plot below, you can see that the open clusters and the globular clusters are not observed in the same location. This is misleading because it is a selection effect. What this means is that there are more open clusters in the Milky Way than the ones shown in green, we just are not able to observe the more distant ones given the current state of telescope and detector technology. In this image, the Sun is located in the center of the dense group of open clusters, and like with Herschel's map of the Milky Way, the only reason the Sun is in the center is because that is the location from where we are observing and we are prevented from observing the objects far beyond the edge of that group. In the case of the globular clusters, you can see the Sun is offset from their center (just as you can in the plot above), and in this case, we are able to observe the majority of these objects in the Milky Way, so that offset is real.
If you take the top view image above and picture rotating it by 90 degrees, you will get the edge-on view of the Milky Way below:
Now when we look at the Milky Way edge-on, we can see how the globular clusters still describe a circular region, but the open clusters do not. This shows you how you can use the distribution of the open clusters to measure the thickness of the Milky Way, but the globular clusters to measure its radius.
Additional reading from www.astronomynotes.com [15]
Defining the Milky Way is a bit difficult, because it is not one single coherent, solid object. Instead, The Milky Way is considered to be the sum of all the individual objects (stars, planets, nebulae, dust particles, etc.) that are gravitationally bound to each other. That is, if an object like a star or a star cluster feels a strong enough gravitational pull from the rest of the objects in the Milky Way that it cannot escape, it is considered to be part of the Milky Way. If we draw borders that enclose all of these objects, we can roughly define the shape of the Milky Way. In general, the word galaxy refers to a collection of gravitationally bound stars and associated material that is above some minimum size (to differentiate galaxies from massive star clusters).
Using this working definition, we can show that the Milky Way contains many billions of individual stars. Astronomers have found that these stars are not part of one single, homogeneous structure, but instead different populations of stars form somewhat distinct structures with different properties. For this reason, the study of an object like the Milky Way often is described as studying stellar populations. The figure below is a model for the different populations in the Milky Way.
The artist's conception above shows an image of the Milky Way disk with its spiral arm structure represented (labeled in blue). Also shown are the central bulge region (labeled in red) and the globular clusters (labeled in yellow). The wireframe in the background represents the halo.
We can describe these structures and their properties in more detail:
Because we are embedded in the Milky Way's disk, it is quite difficult for us to discern the substructures in the disk. So our understanding of the Milky Way's structure continues to evolve as we study it in more depth. Below is a labeled image of the structure of the Milky Way's disk and bulge, which includes the latest updates for the location and densities of the Galaxy's spiral arms determined by observations with the Spitzer Space Telescope.
The whole sky or large fractions of the whole sky have been surveyed with telescopes operating in different wavelength regions. The maps that have been created from these surveys show the Milky Way disk in detail. NASA Goddard Space Flight Center has collected these images and made a poster, shown here, and an interactive website for users to investigate the images.
Depending on which version of Starry Night you are using, you may also be able to study the multiwavelength Milky Way within Starry Night. Under the Options Menu, Under the Stars... menu item, there may be an option to select "Milky Way." Choosing this option gives you a menu that lets you choose which wavelength to view the Milky Way and it should look just like the maps at the NASA Multiwavelength Milky Way website above. This feature is available in Starry Night Enthusiast 7, the version of the software recommended for this course.
There are three observations that the NASA set of images makes clear:
These images help synthesize some of the material we studied in previous lessons. For example, we know that the molecular clouds that form stars are dense, dark clouds that also contain dust. We also know that the dust is what obscures the optical light from reaching us, so we should expect that wherever there is a giant molecular cloud in the map that shows the molecular gas in the Galaxy, we should see that is where the obscuration in the optical image is the strongest. If you compare the two maps labeled “molecular hydrogen” and “optical,” you will see that the brightest parts of the molecular hydrogen map correspond almost exactly to the darkest locations in the optical map.
The near-infrared (which means wavelengths of light just larger than the red part of the optical spectrum, approximately 10,000 Angstroms) image of the sky also reinforces what we learned about the properties of dust. That is, dust scatters blue light very efficiently, but red light is not affected as strongly. This also applies to near-infrared photons; most of these pass through the dust without being scattered. In the disk of our galaxy, we expect to find that by number, K and M Main Sequence stars will be the most prevalent, and we also know that in all populations older than about a few hundred million years, red giant stars (also types K and M) will be very common. Since the spectrum of a K and M star will peak in the red or near-infrared part of the spectrum, the majority of stars in the disk of our galaxy should emit strongly in the near-infrared. Thus, the near infrared map should trace out very well the distribution of the K and M stars in the disk, and most of this light should be visible to us because it is not heavily extinguished by dust.
In most of the images of the sky, several point sources are visible, and some of these are labeled in the finder chart underneath the maps. Many of the point sources visible in x-rays and radio waves are either supernova remnants or pulsars. However, the point source at the very center of our Galaxy is a different type of object altogether, and is the first example of a type of object we have not yet discussed.
Additional reading from www.astronomynotes.com [15]
The very center of our Galaxy in the core of the bulge is located in the direction of the constellation Sagittarius. The dust gets thicker and thicker as we look into the center of the Galaxy, so the best options for observing the Galactic center are in radio waves and in infrared light. Researchers from the Naval Research Laboratory used data taken by the National Radio Astronomy Observatory's Very Large Array to produce an image of the Galactic Center in radio waves, shown here.
Several of the objects visible in this region are labeled Sagittarius (or Sgr for short) A, B, C, etc., which were the simple names given to bright features apparent in lower resolution images. The objects with SNR in their names are supernova remnants, which should indicate that this is a region of the galaxy where there are young, massive stars forming. Clearly, this is a very complex region of the Milky Way with many overlapping structures. However, astronomers have used many different types of observations at different wavelengths in an effort to reveal even more about the Galactic Center. For example, the below image from the Chandra X-Ray Observatory, taken by Penn State Professor Emeritus Gordon Garmire's team, reveals that Sgr A can be further reduced to a few sources, including a bright, small source called Sgr A*:
If we return to radio observations of Sgr A, the image below is from the Very Large Array, and it shows that in the central few parsecs there is a spiral structure made up of gas that is surrounding the central point source (Sgr A*). The gas appears to be rotating around Sgr A*, which is a clue about the nature of this object.
Now let us consider the nature of Sgr A* in particular. This object emits a large amount of radiation in IR, X-rays, and gamma-rays. It appears to be motionless, but we see gas apparently orbiting the source. Recently, observations of stars also found to be orbiting Sgr A* have given us significant new insight into the nature of this object. See UCLA's Galactic Center Group Animation of the Stars Orbiting Sgr A* [33].
Using the highest resolution IR cameras available, astronomers have repeatedly observed the stars orbiting around Sgr A*. They have measured the orbit of a star that comes within 17 light-hours of the object in the core of our Galaxy, which is a distance that is only a few times larger than the orbit of Pluto around the Sun. Using Kepler's laws, if we measure the period and semi-major axis of this star's orbit around Sgr A*, we can calculate the mass of this object. The mass that results from the study of this star and other nearby stars is 4 million solar masses! The only type of object that astronomers believe can have a mass of approximately 4 million stars, but a radius of about 100 AU, is a black hole. Clearly the supernova explosion of one star could never produce a single black hole with a mass so large, so this object must have formed in a different manner. Sgr A* is one example of a class of objects called Super-Massive Black Holes, or SMBHs.
In the context of the Milky Way as a whole, Sgr A* is considered to be the very center of the Galaxy. However, you must keep in mind that this object is found in the central ~100 AU of a galaxy that is something like 30 kiloparsecs or more in radius, so in every image you have seen so far of the Milky Way Galaxy, Sgr A* would be much smaller than the single pixel in the center of the image.
Additional reading from www.astronomynotes.com [15]
Now that we have a concept of the size, stellar populations, and an overall understanding of the Milky Way as a galaxy, let us consider another property that we can determine for the Milky Way: its mass. In most instances, when we intend to calculate the mass of an astronomical object, we return to Newton's version of Kepler's third law:
The Sun is orbiting around the Galactic center, so in principle, if we can measure the Sun's distance from the Galactic Center and its orbital period, this means we can estimate the sum of the masses of the Sun and the Galaxy (at least the portion of the Galaxy that is interior to the Sun's orbit). Since we anticipate the Galaxy's mass to far exceed the Sun's mass, we can take the value that we calculate to be the Galaxy's mass. So, what is the answer? How massive is our galaxy?
The distance from the Sun to the Galactic Center can be measured using a few different techniques, but it is a difficult measurement to make. It is still the case that researchers disagree about the exact value, but it is approximately 8 kpc (that is, 8,000 parsecs). There is a related, but also difficult measurement to make, and that is the velocity of the Sun with respect to the Galactic Center. It is approximately 200 km/sec, which allows us to estimate the period of the Sun's orbit around the Galactic Center in the following way:
If you take the semi-major axis of the Sun's orbit to be 8 kiloparsecs and the orbital period to be 250 million years, you can determine that the Milky Way's mass interior to the Sun's orbit is approximately 1011 solar masses, or 100 billion times the mass of the Sun.
Now, let us compare and contrast motions in the Solar System of the planets and motions in the Galaxy of the stars. What we did above to calculate the period of the Sun's orbit was to use the equation:
We can rearrange this equation and calculate orbital velocity for any object given its period and semi-major axis. If we apply this to the planets in the Solar System, you find that as you get more distant from the Sun, the orbital velocity of the object is slower. Below is a two-dimensional plot that I created for the orbital velocities of the planets (and Pluto) as a function of their distance from the Sun. Each point is labeled with the first letter of the object's name (e.g., V = Venus). This type of plot (orbital velocity as a function of distance from the center) is referred to as a rotation curve.
The behavior of the planets in the Solar System as exhibited in this plot is often referred to as Keplerian Rotation. Clearly, the Milky Way Galaxy is more complicated than the Solar System. There are at least 100 billion objects, gas clouds, and dust, and there is not one single dominant mass in the center. However, astronomers expected that as you got more distant from the center of the Galaxy, the velocities of the stars should fall off in a manner similar to the Keplerian rotation exhibited by the planets in the Solar System. However, astronomers have observed that there is a significant difference between the predicted shape of the Milky Way's rotation curve and what is actually measured. See the image below.
The solid line labeled B is a schematic rotation curve similar to what is measured for the Milky Way. The dashed line labeled A is the predicted rotation curve displaying Keplerian rotation. What the rotation curve B tells us is that our model of the Milky Way so far is missing something. In order for objects far from the center of the Galaxy to be moving faster than predicted, there must be significant additional mass far from the Galactic Center exerting gravitational pulls on those stars. This means that the Milky Way must include a component that is very massive and much larger than the visible disk of the Galaxy. We do not see any component in visible light or any other part of the electromagnetic spectrum, so this massive halo must be dark. Today, we refer to this as the "dark matter halo" of the Galaxy, and we will discuss dark matter more in our lesson on cosmology.
Returning to the image of the Milky Way that we studied before, the wire frame halo is actually meant to represent the extent of the dark matter halo. In the image below, compare the scale of the disk to the scale of the dark matter halo.
Below are some resources related to the Milky Way Galaxy:
Have another website or printed piece on this topic that you have found useful? Share it in our Comment space below!
I do like to keep the historical perspective in mind as we are progressing through the lessons. We began this lesson by considering a few of the early attempts by observers to determine how the Solar System fits in to the Universe. Then, we saw that, over time, by studying stars and star clusters, we have been able to better determine the overall structure and our place within the Milky Way. Historically, astronomers were studying at the same time how the Milky Way fits in to the Universe. Now that we understand the Milky Way, we are going to look into the history of the determination of the Milky Way's place in the Universe.
First, please take the Web-based Lesson 8 quiz.
Good luck!
You will see a link to the Capstone Project in its own module below the weekly lesson modules in Canvas.
Upload your document for Part 1 to the Capstone Project drop box in the Canvas module for the capstone project.
See the grading rubric [39]for specifics for how the capstone project will be graded.
You have finished the reading for Lesson 8. Double-check the list of requirements on the Lesson 8 Overview page to make sure you have completed all of the activities listed there before beginning the next lesson.
Links
[1] http://www.nps.gov/deva/naturescience/lightscape.htm
[2] http://apod.gsfc.nasa.gov/apod/ap070508.html
[3] http://apod.gsfc.nasa.gov/apod/ap090219.html
[4] http://apod.gsfc.nasa.gov/apod/ap090127.html
[5] http://apod.gsfc.nasa.gov/apod/ap010627.html
[6] http://www.ipac.caltech.edu/2mass/
[7] http://www.ipac.caltech.edu/2mass/gallery/showcase/galcen/Galactic_Cntr_full.jpg
[8] http://www.ipac.caltech.edu/2mass/gallery/showcase/galcen/index.html
[9] http://en.wikipedia.org/wiki/File:Aitoff-projection.jpg
[10] http://apod.gsfc.nasa.gov/apod/ap980523.html
[11] http://apod.gsfc.nasa.gov/apod/ap990426.html
[12] http://apod.gsfc.nasa.gov/apod/ap010202.html
[13] http://apod.gsfc.nasa.gov/apod/ap011020.html
[14] http://www.ipac.caltech.edu/2mass/gallery/showcase/allsky/index.html
[15] http://www.astronomynotes.com
[16] http://www.astronomynotes.com/ismnotes/s2.htm#A1.1.1
[17] http://articles.adsabs.harvard.edu/full/1922ApJ....55..302K
[18] http://hubblesite.org/newscenter/archive/releases/2002/01/image/a/
[19] http://www.astronomynotes.com/ismnotes/s6.htm
[20] http://www.haydenplanetarium.org/universe
[21] http://www.astronomynotes.com/ismnotes/s4.htm
[22] http://www.astronomynotes.com/ismnotes/s9.htm
[23] http://www.spitzer.caltech.edu/images/1925-ssc2008-10b-A-Roadmap-to-the-Milky-Way-Annotated-
[24] http://mwmw.gsfc.nasa.gov/mmw_edu.html
[25] http:=
[26] http://mwmw.gsfc.nasa.gov/mmw_MW.html
[27] http://mwmw.gsfc.nasa.gov/mmw_maps.html
[28] http://www.astronomynotes.com/ismnotes/s9.htm#A2.7
[29] http://www.nrl.navy.mil/media/news-releases/1998/wide-field-radio-imaging-of-the-galactic-center
[30] http://apod.gsfc.nasa.gov/apod/ap020803.html
[31] http://chandra.harvard.edu/press/01_releases/press_020101.html
[32] http://images.nrao.edu/Galactic_Sources/Galactic_Center/407
[33] http://www.astro.ucla.edu/~ghezgroup/gc/animations.html
[34] http://www.astronomynotes.com/ismnotes/s7.htm
[35] http://en.wikipedia.org/wiki/Galaxy_rotation_curve
[36] http://chandra.harvard.edu/xray_astro/navigation.html
[37] http://chandra.harvard.edu/resources/illustrations/milkyWay.html
[38] http://chandra.harvard.edu/photo/2003/0203long/animations.html
[39] https://www.e-education.psu.edu/astro801/node/2078