ARCturus

All galaxies are huge star systems. The amount of stars in them can vary on a scale between tens of millions and thousands of billions. According to their constitution, there are two groups of galaxies: spiral and elliptical. Both groups have several subgroups. A third group, a non-constructional group, includes all galaxies with an irregular shape.

Aim: Get acquainted with the classification of galaxies. Using star lists, find in each group a galaxy with features typical of the group.

 

Explorations: Discuss with the students and recall the characteristics of a galaxy. Explain the concepts of ‘galaxy’ and ‘star’ in detail. In this connection, it is worthwhile to concentrate only on the common types of galaxies with a clear and simple structure. This helps to gain a better understanding of the classification with its numerous oddities.  Sometimes it is very difficult to find a group for a galaxy. That may be the case if the shape of the galaxy is very irregular and difficult to recognize.

 

1)  Use pictures and get to know the spiral galaxies. There are two types of them: the ordinary spiral galaxies are denoted by a letter S, and the barred-spirals by SB. What is common to the spiral galaxies is that in their centre they all have a bulge, of which two or three spiral arms start to wrap. The tightness of the spiral pattern is the basis on which the galaxies are subdivided into even smaller categories.

The Milky Way is a typical spiral galaxy. A radio telescope and hydrogen gas have been used to find out its shape. The results indicate that the Milky Way is a spiral galaxy of the Sb-type. The hub of matter is mainly gathered in a flat disk. Its diametre is 100 000 light years and the thickest part of the bulge is 10 000 light years. In addition to the bulge and the spiral arms, there is interstellar matter and matter in clusters of stars outside the disk. The most familiar object to observe in this group of galaxies is Andromeda, a galaxy similar to our Milky Way. It is a dim light blotch, which can be found with the help of binoculars or a small telescope. However, the spiral pattern can only be distinguished with major telescopes.

2)  Use source material to explore the classification further in detail: get acquainted with the barred-spirals. In the barred-spiral galaxies the spiral arms are not connected with the center of the disk but start wrapping from the ends of a bar-shaped centre. There are many kinds of constructional differences in these galaxies as well. The amount of spirals may vary; there can be many spirals or only a few. The spirals can also be divided like branches of a tree. The thickness of a barred-spiral galaxy and the shape of the fluffs wrapping in the ends of the bar also vary a lot. One of the well-known barred-spiral galaxies is the Large Magellanic Cloud in the southern hemisphere.

3) Next, get to know the other main group of galaxies, i.e. the elliptical galaxies. Their shape may vary; it can resemble a ball or be very oval. Their difference from the spiral galaxies is that they have no interstellar matter. These galaxies are denoted by the letter E, plus a subgroup number to indicate how oval the galaxy is. For instance, E0 is a spherical and E7 a very flat galaxy.  Sometimes it is difficult to know what group would be correct for the galaxy because its typical features are common in many groups.

4)  The students can draw examples of various galaxy types. The pictures can be cut off and fixed on a common table of galaxies in the classroom. With the joint collection at hand, discuss the validity of the classification. Correct the possible mistakes in classification, i.e. position the pictures joined in a wrong group back to the group they belong to.

5)  Working in teams, use craft cotton wool and first prepare elliptical galaxies. Each team may choose how oval they want their galaxy to be. Collect the completed models in the table made for the previous task. The models can also hang from a thin yarn which is tied to a string stretched up across the classroom. Discuss the classification and ponder how illustrative the models are. The instructions for preparing a model of a spiral galaxy can be found in the text ‘The Milky Way’. Extra models can be prepared for the subgroups of the spiral galaxies or barred-spiral galaxies.

6) * Find in the star lists the most well-known galaxies of each galaxy group. Discuss the history of the Messier list as well as the ways to denote galaxies.  Write down the Messier code and the special name of the picked-up galaxy. Also define the constellation in which the galaxy – as seen from Earth – seems to appear.

7) * Get to know the irregular galaxies. They are star systems which are asymmetric in relation to their axis. They do not have a precise centre. One of the most well-known irregular galaxies is the Small Magellanic Cloud in the southern hemisphere. Find in the star lists a few irregular galaxies with their codes. Discuss some other galaxy types, such as giant galaxies, lens galaxies, circle galaxies and dwarf galaxies. It is assumed that they have been formed in galactic accidents, such as: collisions, fusions or captures of matter.

Methods: Getting acquainted with literature, finding and classifying pictures of galaxies and preparing models of them.

 

Materials: Picture materials, craft cotton wool, literary material.

 

Pondering: Discuss the complexity of the classification, limit definitions and recognition problems. Ponder the possibility for using some other kind of classification.

 

Evaluation of results: Ponder the prevalence of the galaxies on the basis of the galaxy types the students have found. Discuss the modelling of the irregular galaxies.

 

Hints: It is great fun to prepare with the students a collection of various galaxy types and hang it up on the ceiling. The collection does not need to be all-inclusive, but merely introduce models of various galaxy types and their subtypes.  This helps in getting a bigger picture of the topic and keeping it in mind.

 

Keywords: Spiral galaxy, elliptical galaxy, irregular galaxy, Messier, spiral arm, bulge.

 

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