Galaxy Classification and Their Properties Group Member Names: You may work in groups of two, or you may work alone. Due November 20 in Class! Learning Objectives Classify a collection of galaxies based on visible-light pictures. Be able to explain why and how you can tell from a picture how much gas and dust are in a galaxy, how much star formation is occurring, and the ages of the stars in the galaxy. Introduction You will need the galaxy photos on Page 1 of the Powerpoint at the lab web site. When individual stars are viewed through a telescope, they typically appear as bright points of light without any apparent size or structure. However, some objects in the sky look fuzzy and cloud-like through a telescope. Some of these objects, like in this Hubble Space Telescope image, are actually galaxies: gravitationally-bound collections of billions, or even hundreds of billions of stars. Galaxies are much much farther away from us than the individual stars we see in the night sky. (This image, containing roughly 10,000 galaxies, is only about one thirteen-millionth of the total area of the sky. Each side is about as long on the sky as half the thickness of a fingernail with your arm stretched out. Only a few of the things here are stars in our own Milky Way Galaxy.) Part 1 - Applying Hubble s Classification Scheme Page 1
In the 1930 s, Edwin Hubble developed a way to classify galaxies according to their appearance (or, morphology), which we still use today. The broad categories of galaxy types are elliptical, spiral, barred spiral, lenticular, and irregular. Within those, there are subtypes, depending on how round or elongated the elliptical galaxy appears, and how open or tightly wound the spirals are. Using the images we provide, complete the following table, using a simplified Hubble classification. Classify each galaxy as simply either spiral or elliptical. Try to find patterns in terms of size, shape, color, or any other distinct features that help in sorting the galaxies. Category Galaxy ID Numbers Defining Characteristics (describe the characteristics that you used to distinguish one class of galaxy from the other) Elliptical Page 2
Category Galaxy ID Numbers Defining Characteristics (describe the characteristics that you used to distinguish one class of galaxy from the other) Spiral Part 2 - Understanding the Types of Galaxies In Part 1 you classified the galaxies into different categories according to their appearance, or morphology. We will now investigate what a galaxy s morphology can tell us about its physical characteristics. These physical characteristic can include: (a) the ages of stars in the galaxy; (b) the presence or absence of dust in the galaxy; and (c) the presence or absence of gas and star formation in the galaxy. Keep in mind that these properties are linked together in a physical way. The objective of this activity is for you to learn how these characteristics are related to galaxy classification and morphology. The Ages of Stars 1 - Which of the galaxies appear to be mostly red? (Note: the word red in this context also includes the colors orange and yellow.) Record the number and Hubble classification (elliptical or spiral) of each galaxy. Why do you think these galaxies appear red? Page 3
2 - Which of the galaxies appear to be mostly blue? Record the number and classification (elliptical or spiral) of each galaxy. Why do you think these galaxies appear blue? 3 - Which types of galaxies appear to have many young stars: elliptical, spiral, or both? Explain your reasoning. 4 - Do the galaxies that you identified in Question 3 also contain old stars? Explain your reasoning. Dust and Gas in Galaxies Besides stars, galaxies also contain dust and gas. This absorbs visible light, and can produce dark bands across, or patches in, the galaxy. Over time, this material can form stars under the right conditions. 5 - Which of the galaxies show evidence of dust? Record the number and classification (elliptical or spiral) of each galaxy. Page 4
6 - Would you say that a galaxy that is experiencing active star formation contains little or abundant gas? Explain your reasoning. 7 - Which type of galaxy (elliptical or spiral) would have abundant gas available? Explain your reasoning. Part 2b - Galaxy Collisions, or So, The Thing About Irregular Galaxies... Galaxies are not unmoving, static things in the universe! They are giant swirling masses of gas, dust, and stars, and they are attracted to each other through gravity. Even if it takes a very long time (to human lifetime standards), they will often interact and collide and merge with each other. The merging of orderly spiral or smooth elliptical galaxies can, for a time, produce a very odd-looking, jumbled intermediate stage -- an irregular galaxy. Individual stars in colliding galaxies will almost never directly collide with each other; the space between stars is still vastly enormous compared to the size of the average star. However, if there is gas and dust in one of the colliding galaxies, these will feel the effects of the galaxy collision, and this can trigger a brief flood of star formation. Page 5
8 - Two elliptical galaxies are colliding/merging. Will the merger show a lot of star formation? What type of galaxy (elliptical, spiral, or either) do you think will eventually emerge? Explain your reasoning. 9 - Two spiral galaxies are colliding/merging. Will the merger show a lot of star formation? What type of galaxy (elliptical, spiral, or either) do you think will eventually emerge? Explain your reasoning. Part 3 - A Student Discussion Consider this discussion among three students about a galaxy that appears red: Student 1: Because there is mainly red light in this galaxy and no blue light, I think that only small, red stars formed in this galaxy, and not any big, blue ones. Student 2: I disagree. It s just that blue stars don t last very long. I think the blue stars that may have been there in the past have already evolved into red giants, so the galaxy looks red due to the light from all the red giants. Student 3: Wait a minute...i think you are both wrong. I thought that both blue stars and red giants live short lives, so they should both be gone. I think that all the blue stars that formed early on have evolved into the red stars that are there now. So the galaxy appears red because it s full of a lot of old, red stars that used to be the blue stars. 10 - Do you agree or disagree with any or all of the students? Explain your answer. Page 6