Summary Of the Structure of the Milky Way The following graphical data is meant to help you understand WHY astronomers believe they know the structure (dimensions, content, etc…) of the Milky Way Galaxy.

This all sky map displays the locations of the most common stars (Main Sequence K & M stars) down to 10’th magnitude. Notice that the distribution is fairly random with equal number as stars in all directions. This suggests that the Sun is in the center of this star distribution. In fact, the Sun only appears to be in the center of the distribution because these K & M stars are so faint that we cannot see them at great enough distances to detect the larger structure of the galaxy. 25 pc 143 pc 55 pc 4 pc 3 pc 25 pc 100 pc 18 pc 21 pc 19 pc The distances that appeared on the right are the distances to 10 randomly chosen stars from this map. They illustrate the point that even though we believe these stars to be most common, they are not good indicators of galactic structure because they cannot be seen at sufficient distances to resolve the structure of the galaxy. Using these stars to map the galaxy would be similar to using the trees nearest you to map a forest. You would only be able to see trees within a few 10’s of meters before they began to obscure each other. So you could only “see” that part of the forest near you, not the larger structure.

20 pc 53 pc 55 pc 41 pc 100 pc 63 pc 32 pc 167 pc 41 pc 44 pc This all sky map displays the locations of some of the most luminous stars (Spectral types O, B & A) down to 10’th magnitude. The distribution is NOT random, but displays a faint, but noticeable, “wavy” pattern, similar in shape to the ecliptic. This “wavy” distribution suggests that these stars form a roughly planar structure in space in which the Sun appears to be near the center. The distances that appeared on the right are the distances to 10 randomly chosen stars from this map. These stars are much more luminous stars but still cannot be seen far away in the disk of the galaxy. However, they can be seen at distances sufficient to reveal the disk-like nature of their distribution. These stars are more useful in revealing the structure of the galaxy, but are not sufficient to fully reveal the dimension s of our galaxy. We must turn to even more luminous objects – star clusters.

The “wavy” distribution is even more apparent in this map indicating the Sun is part of a flat disk of stars. 2,600 pc 5,802 pc 480 pc 1,700 pc 800 pc 2,200 pc 3,000 pc 1,900 pc 2,100 pc 1,250 pc The distances that appeared on the right are the distances to 10 randomly chosen open clusters from this map. They can be seen to a much greater depth into the disk of the Milky Way than single stars. Even so, there are very few open clusters that are visible beyond 3,000 parsecs (about 10,000 light years). This limit is due to the obscuration of light by dust in the plane of the Milky Way. Close study of these open clusters can reveal some details of the galactic structure. The next two slides illustrate this point. This all sky map displays the locations of over 600 open clusters in the Milky Way. These clusters are more luminous than O or B stars since they contain many hundreds of stars, some of which are O & B stars.

Interstellar dust causes extinction of starlight by absorption and scattering. Interstellar dust grains are microscopic particles that effectively scatter short wavelength blue light, but let the longer wavelength red light pass through. The effect called Interstellar Reddening causes stars behind dust to appear dimmer and redder.

Interstellar dust limits our ability to “see” within the disk of our galaxy by the extinction of starlight over large distances. We can only “see” at visible wavelengths to about 10,000 ly within the Milky Way’s Disk

This graph displays the percentage of 600 Open Clusters closer than the distance on the abscissa (x-axis). You can see that 90% of this sample of Open Clusters is within 3,300 parsecs or 10,000 light years. This limit is an artificial limit caused by dust in mid-plane of the Milky Way.

This graph displays the distance of 600 Open Clusters above or below the Sun. The Sun is near the mid-plane of the galaxy. There are many more Open Clusters near the mid-plane of the Galaxy than farther away from it. The width of this distribution could be interpreted to be about ±125 parsecs. Many astronomers call this region the “thin disk” or the “gas layer” within the thicker disk of stars. If you look carefully you can see that the central point of this distribution is slightly below the Sun. This is an indication that the Sun is about 14 parsecs above the galactic mid-plane. The study of these Open Clusters reveals how far the Sun is above the mid-plane of the galaxy!. Now, how cool is that!

Globular Clusters are very large and ancient star clusters

These HR diagrams are for a star cluster of progresively older ages. This HR diagram are is typical for a globular cluster. You can see tht it most cllosely resembles that of a very old star cluster.

A Cepheid is a star that pulsates between a larger, brighter state and a smaller, dimmer one. They are very luminous variable stars, of a class that was especially massive and hot, using up their fuel early, leaving them in this pulsating condition. The term cepheid originates from δ Cephei in the constellation Cepheus, the first star of this type identified, by John Goodricke in At the dimmest part of a Cepheid's cycle, the ionized Helium gas in the outer layers of the star is opaque, and so is heated by the star's radiation, and due to the increased temperature, begins to expand. As it expands, it cools, and so becomes less ionized and therefore more transparent, allowing the radiation to escape. Then the expansion stops, and reverses due to the star's gravitational attraction. The process then repeats.

The luminosity of a Cepheid is found to correlate very closely to its period of oscillation. The longer the oscillation period the greater the peak luminosity of the star. Thus the luminosity of a Cepheid can be determined by measuring its period of oscillation, making Cepheids good standard candles.

RR Lyrae variables are periodic variable stars, commonly found in globular clusters, and often used as standard candles to measure galactic distances. This type of variable is named after the prototype, the variable star RR Lyrae in the constellation Lyra. RR Lyraes are pulsating horizontal branch stars of spectral class A (and sometimes F), with a mass of around half the Sun's. RR Lyrae stars pulse in a manner similar to Cepheid variables. They are much more common than Cepheids, but also much less luminous. The average absolute magnitude of an RR Lyrae is about 0.75, only 40 or 50 times brighter than our Sun. Their period is shorter, typically less than one day, sometimes ranging down to seven hours.

Movie of RR Lyrae Variables Please notice in the movie below that all of the variable stars (i.e. the blinking stars) all have about the same maximum brightness. Since these stars are all at the same distance (because they are part of the globular cluster), they then must have the same luminosity. So these RR Lyrae stars are good standard candles because they all have the same luminosity.

The distribution of globular clusters is very different from the open clusters. Globular clusters appear to be concentrated in one area of the sky towards the constellations of Sagittarius, Scorpio and Ophiuchus. 7,100 pc 24,500 pc 9,700 pc 6,700 pc 8,500 pc 8,200 pc 8,600 pc 10,500 pc 4,700 pc 3,800 pc The distances that appeared on the right are the distances to 10 randomly chosen globular clusters near the center of their distribution on this map. They are seen to be much farther away that the open clusters or the single stars we examined earlier. The average distance of just these ten randomly selected globular clusters is 9,230 parsecs and is close to the accepted distance to the center of the galaxy of between 8,000 and 10,000 parsecs. This all sky map displays the locations of about 150 globular clusters in the Milky Way. These clusters are more luminous than open clusters because they typically contain 500,000 stars many of which are red giants. Also, they are not (generally) buried in the dust of the Milky Way's disk, but are seen in the dust-free halo above and below the disk. A close examination of the distances and distribution of globular clusters tells astronomers the location of the Sun and the true extent of the Milky Way. The next two slides illustrate these points.

This graph displays the distance from the Sun of 105 Globular Clusters located in the constellations of Sagittarius, Scorpio and Ophiuchus. There are no Globular Clusters closer than 3,000 parsecs (almost 10,000 light years) from the Sun. The number of clusters begins to drop between 8,000 and 10,000 parsecs from the Sun indicating that the center of this distribution must be in that range. Thus the Globular Clusters reveal the location of the center of the galaxy even though we cannot see it directly because of dust in the galactic mid- plane. STOP Galactic Center

This graph displays the distance from the galactic center of 145 Globular Clusters. About half of the Globular Clusters are located within a sphere of 5,000 parsec radius from the center. The number drops rapidly at increasing distances. 90% of all Globular Clusters are within 25,000 parsecs from the galactic center. Thus the Globular Clusters define the size of the galaxy by delimiting the edge of the galactic halo. STOP Outer Boundary of Galactic Halo

Where is M13: Open Clusters On the web, search for and open the program called “Where is M13”. Using the preset filters, explore the distribution of 1.Nearby Stars 2.Open Clusters 3.Globular Clusters The following three images show parts of the galaxy with the Sun (the Orange dot) and the distribution of the three sets of objects listed above.

What does each distribution tell us of our place in the galaxy? The distribution of nearby stars suggests that we inhabit a small spherical distribution of stars with the Sun at the center.

What does each distribution tell us of our place in the galaxy? The distribution of open clusters suggests that the Sun inhabits a thin disk of stars with a radius of a ten thousand light years and a thickness of a few thousand light years (like a hockey puck) with the Sun very near the center of the distribution.

What does each distribution tell us of our place in the galaxy? The distribution of globular clusters suggests that the Sun is far from the center of the galaxy, about 28,000 light years (equivalently 9,000 pc).

Summary So far… The Distribution of stars can reveal part of the disk-like nature of the Milky Way galaxy, but are not “deep” enough probes to fully reveal the structure of the Milky Way. Open clusters can define the thickness of the Milky Way’s thin disk where star formation is active. Globular clusters allow astronomers to know the direction to the center of our galaxy, in spite of the obscuring dust that prevents us from seeing it directly, and tells astronomers the distance of the Sun from the galactic center and the largest dimension of the Milky Way – the Halo. In addition, more complex analysis of globular clusters has yielded the orbital speed of the Sun as it orbits the galactic center and the age of the Milky Way. Finally, when knowledge of the orbital speed of Sun is combined with its distance from the galactic center an estimate of the mass of the galaxy can be obtained.

33,000,000 pc 50,000,000 pc 53,000,000 pc 76,000,000 pc 62,000,000 pc 42,000,000 pc 75,000,000 pc 4,200,000 pc 16,400,000 pc 92,000,000 pc This all sky map displays the locations of very many distant galaxies beyond the Milky Way. The distances that appeared on the right are the distances to 10 randomly chosen galaxies on this map. As you can see, galaxies are visible many millions of light years away except in the Zone of Avoidance located near the galactic mid-plane. This zone where almost no external galaxies can be seen is due to the obscuring dust in the Milky Way. We believe there are many galaxies in that zone but we cannot see them through the dust of our own galaxy. Further, the larger circular region of the zone near the center is a silhouette of the bulge of our own galaxy.

These final two slides present images of other galaxies thought to be similar to the Milky Way in Structure You should be able to attach numbers or dimensions to each of the labeled objects on the following images that reflect the dimensions of the Milky Way and the position of the Sun

Spiral galaxy NGC 4565 Gas Layer Bulge Disk Halo Boundary Sun Analogs to the Milky Way Galaxy

Disk Halo Bulge Bar

This is the current model of our Milky Way Galaxy with a central bulge, a bar, two dominant spiral arms and several smaller spiral arm fragments.