ARCturus

The Milky Way cannot be seen from space because our own solar system is inside this galaxy. All stars and clusters of stars that can be seen belong to the Milky Way. The neighbouring galaxy, Andromeda, is a similar kind of a spiral galaxy as the Milky Way. By observing Andromeda, it is possible to reach the essence of the Milky Way. In a dark and clear night, there is an option to realize the huge immensity of our own galaxy (picture from The Star Maps).

 

Aim: To prepare a mini-size model of the Milky Way with the help of the pictures of Andromeda and by using the information on the dimensions of the Milky Way.

Explorations: In literature and on the Internet, there are diagrams of the Milky Way as well as information on its dimensions. The diametre of the galaxy is 100 000 light years, whereas the thickness of the bulge only is one tenth of it, i.e. 10 000 light years. Discuss with the students the essence of a galaxy and ponder the forms it can take. In case the topic ‘Galaxy types’ (ARCIV Galaxy types) is already familiar, this discussion may be a recall of the issues the students have learned earlier.

1)  Using a free hand, sketch the shape of a spiral galaxy both face-on and sideways. Discuss the shapes and the amount of the spiral arms. Ponder the position of the Sun in the galaxy.

2)  Familiarize more thoroughly with the concept ’light year’, which is the distance the light travels in a year (at the speed of 300 000 km per second). Convert a light year to kilometres or metres.

In common language one light year means a distance of 9 500 billion kilometres.

3) Let’s adjust the diametre of the Milky Way and the thickness of its bulge in a correct scale to fit to the paper used. Draw the Milky Way both face-on and sideways in a chosen scale. Define and mark the Sun’s position in the spiral arm.

4)  * It is even easy to build a 3D-model of the Milky Way. Choose a large piece of thick cardboard and sketch on it the spiral pattern of the Milky Way face-on and in scale. Cut the figure off. The spiral arms are curves that become thinner from the centre to the end. The edges of the spiral arms may need some extra elbows to meet the reality. The students need to be explained the real structure of the spiral arm.

5)  * A three-dimensional impression can be reached by fixing paper mass or cotton wool mass in scale on both sides of the cut cardboard. The mass illustrates the bulge of the galaxy. To be precise, the thickness of the bulge is one tenth of the diametre. The bulge gets thinner where the spirals begin. The spiral arms are thicker near the bulge and get thinner at their end. Use paper mass to prepare the Sun and paint it yellow. It needs to be emphasized that the scale of the Sun is not correct. Fix the Sun in its position in the spiral arm. The distance to the centre of the galaxy is 25 000 light years in scale.

6)  * In addition to gas and dim stars, there are spherical clusters of stars outside the Milky Way disk. They can be illustrated in the model by fixing pearls or squeezed paper balls in various sizes in thin metal threads or wooden sticks. Students with a vivid imagination may paint the disk and the balls in fancy colours.

7)  * The Milky Way is no stationary solar system. It rotates around its centre once in about 250 million years. The rotational velocity of the Milky Way at the distance of the Sun is 250 kilometres per second. When revolving around the centre of the Milky Way, our Solar system moves towards a certain point, a solar apex (see ARCIV Milky Way), which is in the constellation of Hercules. The stars with their solar systems move at various speeds in the spiral arms. This is why the mutual positions of the stars change in the long run.

The students can ponder and solve simple mathematical problems of the motions of the Sun and the Milky Way.

Methods: Drawing a two-dimensional picture and preparing a three-dimensional model. Using the model to illustrate the motions of the Milky Way. Discussing the constitution and motions of the galaxy.

Materials: Ordinary craft materials, cardboard, filling material, wooden sticks, threads, material for fixing, colours.

Pondering: How does the shape of the Milky Way change in the long run, when the heavenly bodies move at various speeds in various parts of the Milky Way disk? Is there a black hole in the centre of the Milky Way? If there were, how would it change the shape of the bulge and the distribution of matter in the Milky Way?

Evaluation of results: Ponder the meaning of the prepared model in illustrating the galaxy. What parts of the Milky Way lack in the model and how could they be illustrated?

Hints: Hang the completed model with a thin yarn, a little inclined, on the ceiling of the classroom. It rotates slowly, but not about its centre. The best case would be if it could be hung in a way that really makes it rotate about its centre – and even in the correct direction!

Keywords: galaxy, solar system, spiral arm, spherical cluster of stars, apex.

 

 

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