GALAXIES Discussion Hello Mission Team members. Today our mission is to learn about galaxies. (Intro slide- 1) Galaxies span a vast range of properties, from dwarf galaxies with a few million stars barely outshining the brightest individual star clusters in our own galaxy, to vast assemblages of a trillion stars in the centers of great clusters. Our own galaxy, the Milky Way, a reasonably bright spiral system, is over one hundred thousand light-years across, and an estimated 400 Billion stars. We know of many galaxies much larger yet. Slide 2 The image above is a small portion of what is called the Hubble Deep Field (HDF), which "was assembled from 342 separate exposures taken with the Wide Field and Planetary Camera 2 (WFPC2) for ten consecutive days between December 18 and 28, 1995." That s actually a really small area of space so from this photograph alone we can see that there is an unbelievable number of galaxies in the universe. Galaxies are concentrations of stars, gasses, and dust. The arrangement of this matter within the galaxy itself is by no means uniform, and we believe no two galaxies are alike. There are, however, characteristics other than basic composition that are common in all galaxies. At the center of most galaxies is a high concentration of older stars. This portion of a galaxy is called the nuclear buldge (or simply buldge). Beyond the nuclear buldge lies a large disc containing young, hot stars. Often times such a disc will rotate around the galaxy's buldge. Such galaxies contain spiral arms that extend light years out into space.. Finally, beyond most discs lies a halo consisting primarily of old stars. While it is possible to view the halo by itself, it is much more instructional to view the galaxy as a whole. In this photo there are also many stars. Stars are massive, gaseous bodies that undergo nuclear reactions and emit light. So how do we tell the difference in this photo? Stars do not really have spikes, even though they appear that way in the Hubble Deep Field. These spikes are caused by scattered light within the telescope's optical assembly.
Slide3 This picture of the Edge-On Galaxy NGC 4013 (meaning we see it from the edge of that galaxy) provides us with a great example of what a galaxy's halo looks like. Galaxy Shapes: Astronomers use shape to classify galaxies. There are three commonly recognized shapes: spiral, elliptical and irregular. Spiral galaxies have two or more "arms'' winding out from a central disk. When viewed from the side, spiral galaxies look like a fried egg. A spiral galaxy is simply a galaxy that contains a set of spiral arms. The Galaxy M51, otherwise known as the Whirlpool Galaxy is an excellent example of such a galaxy. Here you can see very clearly one pair of spiral arms. You will also notice that the arms appear to contain pockets of very bright material. This is star formation. As the spiral arms circle the nuclear bulde, gas and dust are forced into interaction with one another, often times leading to the birth of new stars. (Slide 4 Whirlpool Galaxy) Elliptical galaxies have a smooth and featureless appearance, and are either round or oval in shape. They appear basically the same when viewed from any angle. Irregular galaxies do not have arms or a uniform appearance. Their stars and gas clouds are scattered in random patches. The most difficult part about identifying galaxies by their shape is being able to recognize them when their orientation is unknown. Galaxy Color: Galaxies come in a variety of colors. As a galaxy ages, its color changes. Galaxies with young stars appear blue, while galaxies with old stars appear red. Galaxies with stars of varying ages may appear to be a combination of colors. For example, a galaxy with some old and young stars may appear to be a combination of blue and red. The presence of dust in a galaxy can make it appear more red than it actually is. By studying the light from a galaxy, astronomers can also get information about its chemical composition, its distance from Earth, and the speed at which it is traveling away from us. Galaxy Size/Distance: Determining an object's distance from Earth is a difficult task for astronomers. The size of a galaxy is not useful because objects that are large can appear close next to objects that appear smaller but are extremely large and far away. Astronomers study the light from galaxies to determine their distance, measured in light-years. A light-year is equal to the distance light can travel in a year, approximately 6 trillion miles (a six with 12 zeros). Galaxy Population: Astronomers have long tried to estimate the number of galaxies in the universe. They use a method called ''representative sampling. To obtain their estimate, they first divide the sky into sections of equal size. Astronomers then count the number of galaxies in one section. The count
from that one section is then multiplied by the total number of sections in the sky. Astronomers have estimated the number of galaxies in the universe to be between 50 and 100 billion. Some elliptical galaxies show no evidence of having formed stars since a brilliant epoch early in cosmic history, while spiral and irregular galaxies have been making stars briskly over their entire lifetimes. Some galaxies produce most of their energy deep in the infrared, and some are so diffuse and faint as to be barely detectable against the faint glow of the Earth's night sky. Galaxy nomenclature and catalogs: Only catalog numbers generally denote Galaxies. Only a handful of galaxies are well-known or unusual enough to rate distinctive names (such as the Whirlpool, Antennae, Pinwheel, and Cartwheel). A given galaxy may sport numbers from several catalogs. The most cited sources are: Messier number - from a list compiled visually by Charles Messier and several colleagues during the eighteenth century. Many of the brightest and most conspicuous galaxies (as well as gaseous nebulae and star clusters) appear in the Messier lists. NGC/IC (New General Catalog and Index Catalog) - compilations by J.L.E. Dreyer from the 1860s- 1880s. Galaxy Clusters Early surveys of galaxies on the sky showed that certain regions have more than their share of galaxies; such concentrations as the Virgo cluster were known long before the nature of galaxies was understood. More complete statistics have shown that the distribution of galaxies in space is far from the uniform "sea" first envisaged, with many (perhaps most) galaxies arrayed in groups, clusters with thousands of members, superclusters, and even larger sheets and fingers stretching as far across the Universe as we can map. Questions for Mission Team members What is a galaxy? What types of galaxies are there? How many stars in the Milky Way? If you could cross the Milky Way at the speed of light how many years would it take? How many miles in a light year? Talk about how far that really is, trying to understand space-distance. Will humans ever venture into the galaxy? Activity for Mission Team members Hubble Deep Field Galaxy exercise. - Identify types of galaxies on overhead projector How Many Objects Are There?
In this activity Mission Team members will practice estimation skills as they begin to explore the Hubble Deep Field image. They will first give a rough "ball park" estimate of the number of objects in the image. They will then follow several steps, using representative sampling techniques to improve on their original estimates. Finally, they will calculate the number of similar objects in the universe, based on their own estimate. Prior to beginning the lesson, the Mission Team Leader and Mission Team members must agree on a protocol for counting the objects. They must agree on how to count objects that appear in more than one section and address other issues that could lead to inconsistent counting. Representative Sampling Representative sampling is a technique used for counting large quantities. First, astronomers develop an estimate based on a small section. They then multiply the estimate by the number of sections in the entire area being counted. The Hubble Deep Field is an excellent image to use for representative sampling. The Hubble Deep Field is divided into three areas or cameras called A. B. and C. Each camera is further divided into 12 sections labeled by rows 1-4 and columns a-c. Instructions 1. Ask Mission Team members to brainstorm and write down several situations in which a scientist might best use a rough estimate rather than an exact count. 2. Ask Mission Team members to estimate the number of objects in the Hubble Deep and record their estimate on the worksheet. 3. Assign each group one camera (A, B, or C) from the Hubble Deep Field to use for representative sampling. Reproducible copies of the three cameras are provided. Astronomers prefer to use negative or reversed images - dark objects on a light field - because they are easier to distinguish between objects. Each Mission Team member can count a different section of the camera (12 sections in each). Mission Team members then can use this information to calculate a group as well as a class average. 4. The Mission Team Leader should decide whether to use individual or group estimates to do the following calculations: x 12 = X / X x 3 = Y / Y x3 x 10 7 th = Z (Mission Team member's count from one section of one camera) X = objects in one camera section Y = objects in Hubble Deep Field Z = objects in the universe 5. The Mission Team Leader should share with Mission Team members the astronomers' estimates for the number of objects in the Hubble Deep Field (-3.000) and in the universe (50-100 billion). Mission Team members should compare their estimates with those of astronomers and attempt to explain the wide range of numbers that are possible
Mission Team Leader s Note: Over 1000 galaxies are identifiable in this picture. There is no way you could find them all. Let me try to give you a perspective about how much of the sky this pictured covered. If I held a dime 25 feet away from you and you took my picture. All these galaxies would fit in the eye on the dime.