Next: Galaxy Morphology Up: AY110 lab manual Previous: The Hertzsprung-Russell Diagram

Subsections

Mapping the Galaxy

Introduction

When we look up at the night sky we see many different kinds of objects. Objects you can see with the naked eye are mostly stars and planets, although in the Southern Hemisphere, two nearby galaxies are visible. With a small telescope, one can see "globular clusters'', "open clusters'', "gaseous nebulae'' and "galaxies''. One very prominent feature which can be seen without the use of a telescope (in a dark location), is a swath of light that sweeps across the sky in a broad arc, the Milky Way. We know the Milky Way is a vast collection of stars that orbit about the center of our Galaxy in a flattened distribution resembling a plate or disk. This disk is often called the Milky Way or the Milky Way disk. It is but one of the components that make up our Galaxy.

In this lab, we will try to determine the shape of the Milky Way and our location in it based on observations of the distribution of several different types of objects in the sky.

Goals: to determine the shape of our Galaxy and our place in it by observing patterns in the distributions of star clusters and nebulae
Materials: Galaxy map, colored pencils or markers, scissors, tape, skewer

Getting used to the ideas

We are trying to determine the shape of our galaxy and our location in it. Since the Galaxy is so large, we cannot go outside of it and see what it looks like; instead, we have to infer what it looks like from our vantage point within the Galaxy.

The situation is even more complicated because, when we look at astronomical objects from the Earth, we cannot easily distinguish how far away the objects are. We only see in what direction they appear, but two objects which appear in the same direction are not necessarily at the same distance.

Map Projections

A hard aspect for some students to grasp is the idea of map projections. If you have completed the "Terrestrial Planets'' lab, we discussed the idea of how a spherical object (such as the Earth) can be represented on a flat sheet of paper. In Figure 14.1 is a standard map of the Earth called a "Mercator Projection''. Near the equatorial regions, this type of projection is not too bad, but as you move to the poles, the projection is terrible! Antartica appears to have more land then all of the other continents combined, but in reality is much smaller than North America. This is because you cannot perfectly plot a sphere on a rectangular grid.

**Figure 14.1:** A "Mercator'' projection of the Earth.
$\begin{figure}\centering \leavevmode\epsfysize =0.3\textheight \epsfbox{MAPGALAXY/mercator.eps}\end{figure}$

Map makers have come up with a variety of ways to make better maps that more correctly represent the actual sizes of objects on a sphere. For example, the "Mollweid'' projection seen in Figure 14.2 uses a complex formula to do a better job at rendering the sizes and shapes of the Earth's continents, though it also fails really close to the poles.

**Figure 14.2:** A "Mollweid'' projection of the Earth.
$\begin{figure}\centering \leavevmode\epsfysize =0.3\textheight \epsfbox{MAPGALAXY/mollweid.eps}\end{figure}$

A clever way to make a map of a globe is to mentally start with a spherical globe of paper, and then peel off segments of that sphere (like peeling an orange) so that we end up with a ragged, but more accurate representation of the Earth's surface. An example is shown in Figure 14.3. Here it is slightly more difficult to make out the continents, but we could cut out this map with scissors and tape the pieces back together to construct a spherical globe!

**Figure 14.3:** A map of the Earth made by "peeling'' a sphere apart and plotting the result on a flat sheet of paper.
$\begin{figure}\centering \leavevmode\epsfysize =0.3\textheight \epsfbox{MAPGALAXY/polyglobe.eps}\end{figure}$

On such a map, objects on the far left are actually very near to objects on the far right. This is the type of map we will use in this exercise. Sitting on the Earth's surface, the sky appears to be a spherical entity that surrounds us-it is like a globe surrounding the globe of the Earth (ask your TA to show you the "celestial sphere'' we have in the back room if they haven't brought it out for you to look at).

To develop your intuition about this sort of map, let's each make a map of the directions of all of your fellow students. Let's imagine that we are each standing inside of a globe and you can see all the other students in different directions. Let's all agree that "north'' should be up towards the ceiling, that the left-most panel in the map is looking towards the front of the classroom, and that panels to the right move in a clockwise fashion around the classroom (so the right most panel is almost all the way back to the front again). Remember that we are imagining that you don't know anything about the distance to each student, only what direction they are seen in.

$\begin{figure}\leavevmode \epsfxsize =\textwidth \epsfbox{MAPGALAXY/test.eps}\end{figure}$

Now let's compare the maps. How do the maps differ for students in the middle of the classroom from those of students at the edge of the classroom?

Now let's imagine we can reconfigure the students so they are distributed uniformly throughout the room. This means there are students above and below you as well as around you! However, there may be more students in some directions than in others, depending on where you are located in the classroom. Again, make a map of the directions of your fellow students.

$\begin{figure}\leavevmode \epsfxsize =\textwidth \epsfbox{MAPGALAXY/test.eps}\end{figure}$

Again, let's compare the maps. How do the maps differ for students in the middle of the classroom from those of students at the edge of the classroom?

Let's summarize by drawing maps for four different idealized situations:

You are at the center of a uniform distribution of objects.

$\begin{figure}\epsfxsize =\textwidth \epsfbox{MAPGALAXY/test.eps}\end{figure}$

You are at the edge of a uniform distribution of objects.

$\begin{figure}\epsfxsize =\textwidth \epsfbox{MAPGALAXY/test.eps}\end{figure}$

You are at the center of a flattened distribution of objects.

$\begin{figure}\epsfxsize =\textwidth \epsfbox{MAPGALAXY/test.eps}\end{figure}$

You are at the edge of a flattened distribution of objects.

$\begin{figure}\epsfxsize =\textwidth \epsfbox{MAPGALAXY/test.eps}\end{figure}$

One final thing to consider for the flattened distribution of objects: how does the appearance of objects on the map depend on the choice of where you put the North Pole?

The Contents of Our Milky Way Galaxy

Before we begin this lab, we should briefly introduce (or re-introduce) you to the various types of objects we will be plotting on our map. The band of light we call the Milky Way is in fact the sum of the light from billions of faint, and distant stars. Stars, and objects containing stars, are what we see with our eye, and those are the types of objects we will be plotting today. As you have/will find out in your lecture sessions, the Milky Way and other galaxies like it contain a number of objects that are not stars (such as molecular gas clouds), but these do not emit light that the human eye can detect (though we can infer their presence since they can absorb light!), and we will not be plotting those types of objects. Besides the band of light called the Milky Way, there are four types of objects we will plot today: Open clusters, Gaseous nebulae, Globular clusters and Galaxies. The first three of these all belong to our Milky Way galaxy, while galaxies are "Milky Ways'' in their own rite, and are located far beyond the boundaries of the Milky Way.

Depending on which labs your professor has chosen, you might have already encountered an "Open'' cluster: the HR Diagram lab deals with the Pleiades, a well known Open cluster. A picture of the Pleiades is shown in Figure 14.4. The Pleiades consists of about 250 stars that are about 100 million years old. All of the stars in the Milky Way form in clusters. This is because they condense-out of cold molecular gas that is found in large clouds. Sometimes the gas cloud is small and produces a handful of stars, sometimes the gas cloud is large (> 10⁶ M _Sun) and produces thousands of stars. Eventually, however, such a cluster will slowly fall apart, and the stars will wander off and circle the galaxy with unique orbits (when in a cluster, however, all of the stars orbit around the galaxy together as a single unit). This is why astronomers call them "Open'', they eventually fall apart. If the were "Closed'', they would not fall apart. Why do they fall apart? Because the gravity from very massive objects (such as molecular clouds) can pull Open clusters apart because they have relatively small (< 5,000 M _Sun) total masses.

**Figure 14.4:** The Pleiades, an "Open Cluster'' of relatively young stars.
$\begin{figure}\centering \leavevmode\epsfysize =0.3\textheight \epsfbox{MAPGALAXY/pleiades.eps}\end{figure}$

In contrast, "Globular clusters'' are "Closed''. Globular star clusters do not fall apart. An example is M15 shown in Figure 14.5. Globular star clusters contain 100,000 stars or more, and thus have large masses, and the gravity from all of these stars keeps them "bound'': Even as they pass by very massive objects, they cannot be pulled apart. Globular clusters are made up of some of the oldest stars found in our galaxy-in fact, most astronomers believe that Globular clusters were the first objects that formed as the large gas cloud that was the "proto-Milky Way'' collapsed.

**Figure 14.5:** M15, a "Globular Cluster'' which contains about 100,000 very old stars.
$\begin{figure}\centering \leavevmode\epsfysize =0.3\textheight \epsfbox{MAPGALAXY/m15.eps}\end{figure}$

"Gaseous nebulae'' are closely related to Open clusters. When a cluster of stars first forms, much of the gas left over from the formation of stars is still present. The hotter stars in the cluster can "ionize'' this gas (see the Spectroscopy lab), and cause it to glow. We see this gas as glowing knots and wisps of material surrounding the stars. The "Lagoon'' Nebula, shown in Figure 14.6, is where several hundred young stars are being born. There is a lot of gas and dust that surrounds the very young open cluster at the center of the Lagoon, and such regions have complex structures.

**Figure 14.6:** The Lagoon nebula, a gaseous nebula that contains several hundred young stars, some of which are so hot they ionize the hydrogen gas, causing it to glow.
$\begin{figure}\centering \leavevmode\epsfysize =0.3\textheight \epsfbox{MAPGALAXY/lagoon.eps}\end{figure}$

As noted earlier, galaxies are large collections of objects mostly composed of stars, star clusters, gas, and dust. They are the hosts of Open clusters, Globular clusters and Gaseous nebulae (as well as molecular clouds, and some other objects we have not mentioned). To see some pretty pictures of galaxies, skip ahead to the "Galaxy Morphology'' lab (the next lab in this manual), where images of several different types of galaxies are presented.

Exercises

Now we will create maps which show the distribution of different types of celestial objects in the sky. In Table 14.1 is a list of constellations within which bright objects can be observed using a small telescope. The list is separated into four sections, one for each type of object: globular clusters, open cluster, gaseous nebulae and galaxies. For each object type we have listed names of constellations and the number of objects of each type which can be found within the constellations.

**Table 14.1:** Location of different objects in the sky
Constellation Name	Number of	Number of	Number of	Number of
	Globular Clusters	Galaxies	Open Clusters	Gaseous Nebulae
Cepheus			3	6
Pegasus	4	2
Aquarius	1
Capricornus	4
Grus		2
Draco		5
Cygnus			7	6
Vulpecula				4
Aquila			3	2
Sagittarius	20		2	3
Telescopium	8
Hercules	4
Ophiuchus	8
Scorpius			6
Lupus			2	1
Norma			4
Ursa Majoris		16
Canes Venatici		10
Bootes	2
Coma Bernaices	3	12
Virgo	2	11
Centaurus	2	5	1	4
Leo		8
Hydra	1	2
Camelopardalis		4
Monoceros				8
Canis Major			6	3
Puppis			5	1
Perseus			4	5
Taurus			3	3
Orion			3	6
Eriadanus		8
Dorado	3	4
Cassiopeia			8	7
Andromeda		4
Triangulus		7
Pisces		3
Cetus		7
Fornax		2
Sculptor	1	5

Figure 14.7 has a blank map of the sky with the boundaries of all the constellations marked on it. This map is a bit like a map of the world that shows the borders between countries but nothing else. Remember however, when we are looking at the sky we are looking up at the inside of a sphere that completely surrounds us, not down onto the outside of the Earth. The continuous band of stars already marked on the map is the Milky Way.

For each type of object, your group will construct a separate celestial sphere. In the end, you should have four celestial spheres, each with one type of object marked on it.

Complete the following steps to construct each sphere:

Mark the location of each object in your set on the celestial sphere map. As an approximation, just place the marks near the center of the constellation the objects are in. The marks should be dark and large enough so that you can see the marks on the other side of the sheet.
Use a different color for each type of object. This will help you distinguish the different types of celestial spheres. (Remember that each sphere will have only one type of object marked on it.)
Cut out the map by cutting along the outside bold-face lines. Be sure it NOT to cut off the tabs at the top and bottom of each of the map sections. These are essential for the proper construction of the globes.
A celestial sphere can be constructed using tape and skewers (be careful!). Your TA will give you more detailed instructions. Be sure that the skewers go through the small circles on the tabs at the top and bottom of each of the sections of the map.
Be sure to construct your celestial sphere inside out because the Earth is really at the center of this "globe''. The objects we see are above and around us so when we construct these spheres, the marks should be on the inside surface of the sphere.

Describe the distribution of each type of object. Work as a group with each celestial globe and describe the distribution you observe for each type of object. Be as detailed as possible and make references to the plane of the Milky Way which has already been marked on each map and also the center of the Milky Way Galaxy which is toward the constellation Sagittarius. This information will be used to complete the questions so make sure you take good notes! (20 points)

Milky Way:

Open Clusters:

Gaseous Nebulae:

Globular Clusters:

Galaxies:

Questions

The following questions are intended to help you reconstruct the shape of our galaxy based on the exercises done in the lab. Be sure to answer all parts of the questions and use complete sentences as always.

Do any of the descriptions of the idealized distributions of objects discussed in section 14.2 (i.e. center of flattened distribution, edge of flattened distribution, center of uniform distribution, edge of uniform distribution) match the descriptions of the distributions of the celestial objects? If so, which ones? Explain your reasoning. (10 points)
Considering the answer to the previous question, where do you think we are located with respect to the system of globular clusters? open clusters? gaseous nebulae? Explain your reasoning. (10 points)
Draw a sketch of the Galaxy based on your answers to Questions 1 and 2. Draw your picture showing our location relative to the distributions of the four types of objects. Be sure to include all four types of objects and label them. (15 points)
What do you think the distribution of galaxies, as compared with the distributions of the other objects, tells you about where galaxies are located? (10 points)

Summary

(35 points) Please summarize the concepts you learned from this lab. You might wish to discuss:

Describe each of the 4 types of objects discussed in the lab.
When you look up into the sky and see a band of stars, what does that tell you about the structure of the Galaxy you live in?
Describe the difference (in terms of the 4 objects discussed in lab) in what you would see if you were a) standing in the center of our Galaxy, and b) standing at the edge of our Galaxy.
What conclusions can you draw about the distribution of objects in our galaxy and beyond?

Use complete sentences, and proofread your summary before handing in the lab.

**Figure 14.7:**
$\begin{figure}\centering\leavevmode \epsfxsize =0.8\textwidth \epsfbox{MAPGALAXY/constellations.ps}\end{figure}$

Next: Galaxy Morphology Up: AY110 lab manual Previous: The Hertzsprung-Russell Diagram

Tom Harrison 2008-07-09