Lab9 Exercise

the Milky Way a. -30 degrees Q3: Based on both diagrams , what is the approximate distance to the galactic center? a. RA: 18 hrs, DEC: -30 Degrees P ART II: T HE G ALACTIC D ISK AND H ALO Distribution of Globular Clusters and Young Star Groups on the Sky Young Star Groups Globular Clusters 180 o 180 o o 90 270 150 o o o 120 210 o o +20 o 240 +40 o o +60 o o +80 o +90 o o −80 o −90 o 300 −20 o 60 −40 o 330 −60 o o 0 30 o x x x x x x x x x x x x x x x x x x x x x x x x x x The plot above compares the distributions in the sky of two classes of objects: globular clusters and groups of young stars. This plot represents the whole sky, plotted in a different “latitude-longitude” coordinate system, where the origin of the plot is the direction to the center of the Galaxy (which you just found the coordinates of in the RA and Dec. system). “Longitude” is measured along the plane of the Galaxy, in degrees ranging from 0 ◦ (at the center) to 180 ◦ (opposite from the center) to 360 ◦ (back to the center). “Latitude” is measured above and below the plane of the Galaxy in degrees and ranges from: 0 ◦ on the plane of the galaxy to +90 ◦ directly above the plane, and −90 ◦ below the plane. 3

the Milky Way Vega 8.0 68 ◦ Spica 79.8 317 ◦ Rigel 248.5 209 ◦ Deneb 429.4 85 ◦ Q8: Plot the positions of these 5 stars on the diagram on the previous page, remembering to convert distances from parsecs to kiloparsecs. Deneb is one of the farthest stars that we can see with the unaided eye. Q9: Based on this, how would you describe the location of the individual stars that we can see with the unaided eye in relation to the scale of the galaxy? Where do they seem to be located? a. They are all near the center of the sun With more powerful instruments, we can see groups of stars out to an even further distance. The diagram on the previous page represents the extent to which we can view all stars in the sky. The circle on the accompanying worksheet represents the entire disk of our Galaxy viewed from above. The actual radius of this disk is about 15 kpc. To scale, add and label the following structures to complete this scale model of the Galaxy: 1. the central spherical bulge (radius = 1 kpc), 2. the position of the Sun (you found this earlier), 3. a circle representing the extent of the young star groups near the Sun from the figure on page 92 and the location of the three spiral arms within this circle (NOTE: all of the stars we can see in the sky with a telescope are contained within this circle). Create and label an edge-on view of this model to scale below. Include: the thick- ness of the disk and bulge (the disk is about 0.6 kpc thick and the bulge is about 0.8 kpc thick), and the location of the Sun. Evidence suggests that a supermassive black hole is located at the center of our galaxy. While we cannot see it, we can observe energy being emitted from material falling into it, and also observe the motion of stars orbiting around it. From this we can determine that the mass of the supermassive black hole must be around three million solar masses (3 x 10 6 M  ). While theoretically the entire mass of a black hole is compressed into zero space, we define the extent of a black hole by its event horizon, the distance away from a black hole from which light can barely escape. This is also known as the Schwarzchild radius. The Schwarzchild radius (R) of a black hole (the radius of its “event horizon”) is related to its mass (M) by the formula: 6