ARCturus

The stars twinkling in the sky are a constant source of mystery and inspiration for mankind. In ancient times, when the essence and construction of the stars were not known, the stars were considered as tokens of the supernatural world. They inspired human imagination in many amazing ways. The stars are suns shining their own light. Our Sun is also a star, our nearest star. In the picture, the Sun is just about to set below the horizon (picture IH, Finland).

 

Aim: To get to know some stars and their life cycles. To discuss their phases of origin and explore the most recent observations of how the stars are born.

 

Explorations: Recall orally and using pictures of the characteristics of the Sun and their impact on the Earth. The Sun is a middle-aged star, the age of which is five billion years. Its origin and life cycle resemble the ordinary life cycle of myriad stars of the same kind.

 

The stars are born mainly from cold and shrinking clouds of hydrogen and helium. The same cloud may produce tens or hundreds of stars. The gravity makes the gas shrink. In that process the temperature of the gas rises gradually and reaches a level in which the nuclear reactions begin. In a temperature of many million degrees, hydrogen starts to transform to helium. This fusion reaction releases simultaneously a huge amount of energy that moves from the centre outwards and bursts into space. We say that the star starts to shine. Every star has a unique life cycle, the essence of which depends on the mass of the star. The end of the life cycle may be peaceful and waning or it can be restless and exploding.

 

1)  Get to know the brightest and most famous stars in the northern sky, such as Vega, Sirius, Capella, Arcturus, Rigel and Betelgeuze. To make the comparison easier, the students can use lists of stars and make a table of some features, such as degree of brightness in the northern and southern hemisphere, temperature, luminous intensity and size compared with the size of the Sun, star type as well as distance. Also other qualities may be included in the table if wanted. Compare the stars included and discuss their essence.

 

2)  Continue the familiarization further with the help of star lists. The students are asked to find special kinds of “stars”, i.e. stars that - when using more efficient devices for observation - turn out to be double stars or multiple stars. In many cases the companion is much smaller and dimmer than the main star. That is why the ordinary observation devices at schools cannot find them. However, the students can use source material and include a few of these special stars in the table they made in the previous task. For instance, the alfa star Castor in the constellation Gemini is a six-fold star with three binary stars revolving around each other. This star is also the famous eclipsing variable star Algol, the brightness of which varies regularly.

 

3)  By using binoculars and a telescope observe the stars mentioned above in nature. Interesting discoveries can be observed and discussed together. If there is more than one binocular available, it is worthwhile to divide the students into small groups, so the members are able to observe together. This may even arouse some competition between the groups. If the school has a telescope with a diametre of the lens more than 5 cm, the teacher can ask students to use star maps and search the beta star Albireo in the constellation Cygnus. It is considered to be the most beautiful double star in the sky.

 

4)  Using source material, explore various kinds of stars in the classroom. In order to speed up the process, divide the class in groups and provide each group with a selected topic. For instance, the following kinds of stars may be explored: white dwarf, red giant, brown dwarf, neutron star, pulsar, X-ray pulsar, black hole, planetary nebula and giant star. The results can be presented as a mini poster on the classroom wall and introduced to the others.

 

5) * Explore the life cycle of a light star. A star of about the same mass as that of the Sun swells first and becomes a red giant. In the following phase it may become a planetary nebula. That happens when the core of the star has shrunk and becomes dense and hot, and the radiation pressure makes the outer parts scatter to a spherical nebula in the universe. Finally, the core shrinks and becomes a white dwarf with a diametre of one per cent from that of the Sun. Process this phenomenon to a large life-cycle map on the classroom wall. Add in the map a brief illustration of every concept.

 

6) * Continue by exploring the life cycle of a heavy star. A star with a mass minimum of a few times bigger than the Sun swells at first and becomes a red giant. When the core of the star collapses, the outer parts may explode and produce a supernova. Depending on the mass, the star can either explode totally and be scattered into the universe or it can collapse further and become a neutron star or a black hole. A star may end up becoming a black hole only if the mass of the star in the beginning of the process is at least as big as that of three Suns. Also this process needs to be gathered on a life-cycle map on the classroom wall.  Introduce the parts to the class.

 

Methods: Getting acquainted with the life of a star, observing the brightest stars, finding special stars, with the help of source material getting acquainted with the final phases of a star. Making life-cycle maps.

 

Materials: Binoculars, telescope, star map, list of stars, craft material.

 

Pondering: What are the factors that have had an effect on the type of a life-cycle a star has had? What is the result of a life cycle? Can there be other fates for a star, at least in theory?

 

Evaluation of results: Explore similarities and differences in the life cycles, discuss observations and star types, ponder the amount of each type in the sky measured in percents.

 

Hints: It is worthwhile to find in advance some observable objects of double stars in the star list. In the outdoors, find  these objects first with the help of a star map. Searching on a blind basis can also be used but the results may be scarce.

 

Keywords: Star, double star, binary star, optical binary star, spectroscopic binary star, physical binary star, red giant, neutron star, black hole, white dwarf, black dwarf, fusion.

 

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