Lab #7: Galaxies, Location of the galactic center

Lab 7: Galaxies, Location of the galactic center.

Part I: Galaxies

You have a photograph showing a portion of a rich cluster of galaxies located in the constellation Hercules about 175 million parsecs from earth. The picture also contains a number of foreground stars belonging to our own galaxy. Distinguishing these foreground stars from roundish galaxies sometimes takes a bit of care; anything noticeably out-of-round or fuzzy looking is a galaxy. Stars are circles with fairly sharply defined edges. Brighter stars may show four points, pointing in the same directions on every star; these are from optical effects taking place in the telescope.

Please take care not to damage the photograph. Do not write on it, and do not write on paper that is on top of it. (The impression could come through and indent the photograph.)

Study the details of this photograph for several minutes. Ask for a magnifying glass if you think it would help you. Then, answer the questions below on the answer sheet.

1. Notice the variety of types which are present. A rather poor quality copy of the photo is attached to this sheet. (It is printed as a negative.) On this copy, label a galaxy that is each of the following types (Label one galaxy each with A, B, C, and D). Your answers will show better if you make them some color other than black.

A. Normal spiral

B. Barred spiral

C. Elliptical

D. Irregular

2. Collisions between galaxies are fairly common in rich clusters because so many galaxies are so close together.

A. Near the center of the picture are two large galaxies which look very near each other. Would you say they are colliding, or just a chance alignment along the same line of sight? (Remember, a collision would disrupt the galaxy's structure.)

B. Can you find somewhere else where galaxies seem to have passed close enough to cause disruption? If so, label them with a 2B on your copy.

3. It's worth reflecting for a while on the vastness which you are looking at. Make crude estimates of the total number of galaxies and number of stars shown in this picture as follows:

A. A giant elliptical galaxy might typically contain 1000 billion (one trillion) stars and have the same diameter as a good size spiral. How many of these can you find?

Multiply this number by 1000 billion to obtain the number of stars.

B. The larger spirals similar to our own Milky Way contain about 100 billion stars. Count these and multiply to obtain the number of stars. (In judging the size of a spiral, go by how long it is not how wide. Narrow just means you are seeing it edge on.)

C. Galaxies one-third to one half the diameter of those you counted in A and B contain about 10 billion stars. A typical example is indicated on the answer sheet. (Again, be careful not to underestimate the size of a spiral which you're seeing edge-on.) Count these and multiply.

D. The faintest galaxies here contain maybe one billion stars. Look closely, many of these are faint little patches of light that you might not even notice at a casual glance. For example, in the area indicated on the answer sheet you should see at least five of these on the original picture. Rather than trying to count all of them, count the number in one-tenth of the picture, and multiply by 10. Since the picture is 8 by 10, this would be a strip one inch wide along one of the eight inch sides. When you have the total number for the picture as a whole, multiply it by one billion.

E. Total up your numbers from A through D. How many galaxies did you find altogether? How many stars altogether? As if these numbers weren't large enough, the cluster extends well beyond the edges of the picture, and also, the smallest dwarf galaxies are too faint to show up.

Part II: Location of the Galactic Center.

Background: Beyond the stars which are apparent to the naked eye are more distant stars which fade into a general milky glow. (Galileo discovered that this "Milky Way" is many very faint stars when he first turned his telescope on it in 1610.) Since this background glow is seen only in the direction of the Milky Way, there must be no distant background stars in other directions. Thus, the stars are collected into a system with some kind of very flattened shape, like a disk.

Where are we located in this disk? The center? The edge? Somewhere between? Until recently this question could not be answered by direct observation since visible light from much of this system, the galaxy, is blocked by interstellar dust clouds. About 1915, Harlow Shapley suggested that the answer could be found from the distribution of globular clusters. Shapley showed that these clusters are distributed not in a disk like most stars, but in a more or less spherical volume. Shapley guessed correctly that this sphere of globular clusters is centered on the same point as the galaxy's main disk, and thus located the center of the galaxy.

Data:

Your data was compiled by Charles Messier in 1787. This catalog was compiled as a list of objects to avoid looking at. Hunting for new comets was a "hot topic" at this time, and these fuzzy objects were easy to mistake for a comet. Ironically, they are now recognized as far more important than any comet. The 29 globular clusters on it makes a list which is incomplete, but a manageable length. The Messier catalog is attached.

Right Ascension and Declination are a system for locating points on the sky much like latitude and longitude are used to locate points on the earth. Right Ascension is given in hours (h) and minutes (m) of time (The time taken by the sun to move the given distance. 1 hour = 1/24 of the sky's circumference, or 1 hour = 15°.). Declination is in degrees (° ) and minutes ( ^‘) of arc.

Analysis:

Plot the location of each globular cluster in the catalog on the chart of the sky provided. (Skip over the other objects listed. For example, skip M1, plot M2 thru M5, skip M6 thru M8, etc.) Your points will show better if you make them some color other than black.

If we were at the center of the galaxy, then the clusters would be all around us and your map of the sky would show roughly equal numbers of them in all directions. Since your map doesn’t look like that, we must be off to one side. The picture at right is from the viewpoint of some distant alien looking at our galaxy from outside. The dots are globular clusters. The galaxy’s main disk has been left out. Notice that the closer your line of sight passes to the center of the galaxy, the more globular clusters there will be in a patch of sky a given size.

Study your graph to see how these clusters are distributed around the sky. Bear in mind that the southernmost part of the sky is not visible from the latitudes where Messier was observing, so that part of the distribution is missing. Also bear in mind that the Milky Way is the plane of the galaxy, and therefore should contain the galaxy's center. So, look for the part of the Milky Way surrounded by the most clusters the closest together.

Conclusions:

1. What would you say are the coordinates of the galaxy's center?

2. What constellation does this lie in?

3. Roughly, how far away is the galactic center? To find out, say the average globular cluster has an absolute magnitude between -7 and -8. Pick half a dozen clusters which have coordinates very close to those of the galactic center. Some of these will be closer to us than the center is, others will be farther away. Taking the average of their apparent magnitudes will give roughly the magnitude of a cluster at the galactic center. From the apparent and absolute magnitudes, find the distance. (Show how you got your answer in the space on the answer sheet.)

Messier 1-50.JPG