LET S MAKE A PORTRAIT OF A GALAXY

Transcription

1 LET S MAKE A PORTRAIT OF A GALAXY Veselka S. Radeva EAAE Summer School Working Group (Bulgaria) Abstract The goal of the exercise is for students to learn about the different types of galaxies, about the Hubble diagram for galaxies, and about some of the characteristics of galaxies. Students will determine the level of flattening and the size of several elliptical galaxies, and the size of several spiral galaxies. Students will have to make measurements and calculate the size of galaxies using very simple formulae. In this exercise students will use the astronomical software and the images provided by the Discovery Space project and the National Astronomical Observatory, Rozhen, Bulgaria. INTRODUCTION Definition of the term Galaxy A galaxy (coming from the Greek kyklos galaktikos, representing the Milky Way) is a large gravitationally bound system of stars, interstellar gas and dust, unseen dark matter, and possibly dark energy. Typical galaxies contain 10 million to one trillion, or more stars, all orbiting around a common centre of gravity. Most galaxies are several thousand to several hundred thousand light years in diameter and are usually separated from one another by distances on the order of millions of light years. The Hubble Sequence In 1925 Edwin Hubble created a system of galaxy classifications called the Hubble Sequence or Tuning Fork Diagram. The classification separates galaxies into: - Elliptical Galaxies - Spiral Galaxies - Barred Spiral Galaxies - Irregulars And then divides each one up by shape. Figure 1. The Hubble Sequence Elliptical Galaxies These galaxies are very different from spiral galaxies in shape and stellar population, and range from objects that are almost spherical, to those which are almost flat. Those 1

2 which are spherical are classed as E0 galaxies, and those which are flat are classified as E7, with the types in between being given the numbers E1 to E6. Elliptical galaxies contain mostly Population II population stars and little or no gas or dust. Star formation is not taking place in them. Elliptical galaxies can be the largest galaxies in the known universe (called giant elliptical) or can be very small galaxies (called dwarf elliptical). Lenticular Galaxies Lenticular galaxies look like spiral ones, thus their abbreviation is S0. However, their structure is quite different: stellar formation stopped long ago, because the interstellar matter was used up. Therefore, they consist of old population II stars only, or at least chiefly. From their appearance and stellar contents, often they can hardly be distinguished from ellipticals observationally. Spiral Galaxies Spiral galaxies have a flat, disk-like shape. At the centre of the disk is a ball-shaped distribution of stars referred to as the nucleus. The nucleus of the spiral galaxy and the halo of globular clusters surrounding the galaxy contain mostly old stars - called Population II type stars. The spiral arms contain a mixture of old and new stars - Population I type stars. There is also gas and dust within the spiral arms. And it is within this gas and dust that new star formation is taking place. Spiral galaxies are further classified as Sa, Sb, and Sc. This classification is based on the appearance of the galaxy. The Sa type galaxies have a large central nucleus and tightly wound spiral arms. In contrast the Sc type galaxies have a small nucleus and loosely wound spiral arms. Barred Spiral Galaxies The second type of galaxy classified by Hubble are barred spiral galaxies. Their characteristics are very much like spiral galaxies except that instead of the spiral arms emanating from a spherical nucleus, the spiral arms emanate from a bar. This bar is an extension emanating from the spherical nucleus. The stellar populations are predominantly the same as those within spiral galaxies. Once again barred spirals are labelled according to the appearance of the arms and the central bulge, and so we see labels such as SBa, SBb, and SBc, as well as intermediate types such as SBbc, and so on. Irregular Galaxies As the name suggests, irregular galaxies are those galaxies which have no specific form, and so the group contains a very diverse selection of objects. In fact, there are two types of irregular galaxies. Type I are usually single galaxies of peculiar appearance. They contain a large fraction of young stars, and show the luminous nebulae which are also visible in spiral galaxies. Type II irregulars include the group known as interacting or disrupting galaxies, in which the strange appearance is due to two or more galaxies colliding, merging or otherwise interacting gravitationally. Type II galaxies appear to contain a large amount of dust. 2

3 Main characteristics of galaxies The main characteristics of galaxies are very different and can vary within large boundaries. The characteristics are very difficult to determine and many galaxies have their characteristics undetermined. Table 1 shows the limits of variation of galaxy characteristics compared to the same characteristics of the Sun. Table 1. Galaxy characteristics Characteristics Type according to Hubble Elliptical Spiral Irregular Mass М Sun М Sun М Sun Diameter 1/10 kpc 100 kpc 5 kpc 100 kpc 1 kpc 10 kpc Luminosity L Sun L Sun L Sun Speed of rotation (km/s) < Spectral class K F - K A - G Contents of the star population old Old and young Old and young Contents of interstellar matter <0.1 % 1 10% % Figure 2. Images of the galaxy NGC1032 3

4 PEDAGOGICAL INSTRUCTIONS Educational goal: To introduce students to the main types of galaxies, to the Hubble s galaxy diagram, and to the galaxies main characteristics. Activity: 1. Students will learn how to process astronomical images of elliptical and spiral galaxies with two astronomical computer programs: LTImage and Avisfitsviewer ( library.htm). 2. Students will process the images of elliptical galaxies in order to calculate the level of flattening of the galaxies. 3. Students will process astronomical images of elliptical and spiral galaxies in order to calculate the diameter of the galaxies. Practical preparation: 1. The teacher chooses astronomical images of 5 elliptical and 5 spiral galaxies ( The teacher finds information about the distance to these galaxies from the Internet. 3. The teacher learns how to use the astronomical programs for processing images. Organization of the lesson: Steps: 1. The teacher introduces the different types of galaxies, the Hubble diagram and the galaxy characteristics. As a supplement to the lesson for galaxies, the teacher can use the information in the Introduction section. 2. The teacher divides the class into 6 groups. 5 groups are going to process the images of different types of galaxies. The 6 th group will look for information in Internet about the characteristics of every galaxy. 3. At the end of the lesson students fill in Table 2 together Table 2. Results from the processing of the images Galaxy Distance Diameter Flattening (for elliptical galaxies) Type Image 4

5 Instructions for students: o Problem 1. Calculating the flattening of a galaxy We will calculate the flattening of an elliptical galaxy using the following formula: f e = = a where а the large semi-major axis of the ellipse b the small semi-minor axis of the ellipse f focal length 2 2 ( a b ) a Figure 3. Elements of an ellipse If the eccentricity of the ellipse is е = 0, the ellipse is a circle with radius r = а = b. If е = 1, the ellipse is flat and becomes a line with length 2а. When e is close to 0, the galaxy is of type Е0, while if e is close to 1, the galaxy is of type Е7. o Problem 2. Calculating the size of a galaxy 1.- Calculating the diameter of a galaxy in pixels. When you know how many arcseconds there are in one pixel, you can calculate the diameter of a galaxy in arcseconds (you can obtain the necessary information from the astronomical program that is used for processing the image). The thickness of the galaxy NGC1032 is 180 pixels Figure Calculating the diameter of a galaxy: Step 1. Using the image data library from skywatch.org, we can easily calculate any of the visible dimensions of the galaxy. Here, we will measure the thickness of NGC Step 2.1. With the help of the LTImage software, the required dimension is measured in pixels. Figure 4. The diameter of a galaxy in pixels 5

6 Knowing the pixel scale of the image (0.28 arcseconds per pixel), we are able to work out how thick NGC 1032 is in arcseconds: 180 pixels 0.28 arcseconds/pixel 50 arcseconds Step 2.2. Now all we need is to turn these 50 arcseconds into light years. It is an easy task as we know r, the distance to NGC 1032: ly. Our circumference s perimeter is: C = ly π = π ly An arcminute is 1/60 th of a degree (remember that there are 360 degrees in a circle). An arcsecond is 1/60 th of an arc minute. Therefore, there are 1,296,000 arcseconds in a circle! (1,296,000 = 360 degrees 60 arcminutes 60 arcseconds). And if x is the required dimension, then: 50 / = x / π, and x = ly We have just calculated that the diameter of the galaxy NGC 2032 is light years. Conclusion: During this lesson students gain knowledge of galaxies and obtain skills for working with astronomical programs for processing images. Thus, they calculate some of the most important characteristics of galaxies (their size) and the way professional astronomers determine these characteristics. Students will find information in Internet about each of the galaxies and will fill in Table 2. Together students will make a portrait of each galaxy. References Николов Н., Радева В., Илиева Е., Астрономия, Педагог Images from Zsolt Frei s Galaxy Catalog - 6

7 APPENDIX 1 IMAGES OF GALAXIES STUDENTS ARE GOING TO PROCESS Galaxy Distance Ly Diameter ly Flattening (for elliptical galaxies) Type Image NGC NGC NGC NGC , NGC , NGC NGC , NGC NGC NGC NGC M M M