Summary Of the Structure of the Milky Way

1. 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.

2. 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. 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.

3. 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.

4. 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. The “wavy” distribution is even more apparent in this map indicating the Sun is part of a flat disk of stars. 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.

5. 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.

6. 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! 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. This graph displays the distance of 600 Open Clusters above or below the Sun.

7. Globular Clusters are very large and ancient star clusters

8. 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. These HR diagrams are for a star cluster of progresively older ages.

9. 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. 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. 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. 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.

10. 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. This graph displays the distance from the Sun of 105 Globular Clusters located in the constellations of Sagittarius, Scorpio and Ophiuchus. 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

11. 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. This graph displays the distance from the galactic center of 145 Globular Clusters. Thus the Globular Clusters define the size of the galaxy by delimiting the edge of the galactic halo. STOP Outer Boundary of Galactic Halo

12. 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.

13. 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.

14. 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

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

16. Disk Halo Bulge Bar