Discover the Colours of the Stars

Students learn that stars have different properties that can be measured directly (brightness, colors) or inferred indirectly (temperature). They also learn by analogy to relate the colours of the stars with another important property: their temperature.

After this activity, the children will be able to:

• distinguish the different colours of the stars.
• explain why different stars have different colours.

• During the activity, the children identify different colours of stars. The teacher can check the results of the exercise. In addition, teacher can ask the children for the range of colours in stars.
• The teacher may show pictures or illustrations (artist impressions) of stars having a red, a yellow and a red colour. The children should indicate the main difference one can infer from those colours (namly: the temperature of the stars).
• Afterwards, the children may be asked to indicate, what a “cool” and what a “hot” colour of a star is.

• Set of images for this activity
• Constellation viewer

When Al-Sufi started to observe the stars in Isfahan, he noticed the colours of some stars differed from others. This difference in colour had never been noted or recorded by Greek, Babylonian or Egyptian astronomers! Al-Sufi thought that the colour is an important property of the stars and included it his catalogue together with their brightnesses and positions on the night sky.

Now we know that stars are hot due to nuclear fusion in their central regions: due to the high pressure, cores of atoms (like hydrogen) collide and fuse into heavier elements (helium, carbon, oxygen, etc.) creating the elements that we know in chemistry, up to the element iron. The colour is indicative of the surface temperature of stars.

The names of the stars

(a)(Optional for students who have done activity 5.1.1 "Help Al-Sufi match the Greek and Arabic constellations") Look closely at the Orion constellation: Do you remember the name of the corresponding Arabic constellation?

(b) Do you remember the name of the brightest star in the Orion constellation and what it means?

(c) Do you notice any difference between the colours of the stars in this constellation? What star colours do you see?

Fig. 10 Orion constellation (Credits: Hubble/ESA)

(d) Al-Sufi kept and used the Arabic names of stars in the Orion constellation in his “Book of the Constellations”. You can find the meaning in the table below:

Fig. 11 Orion constellation (Credits: Hubble/ESA)

(Credits: Scorza)

Cover the above picture of Orion with a piece of paper. Try to fill in the names of the four stars from the table in the picture below and their meanings:

Fig. 12 Orion constellation (credits: Hubble/ESA)

The colours and temperature of the stars Have you ever seen an iron bar heated on the stove? In case you haven't, just take a look at the picture below. When the iron bar is heated and becomes hotter and hotter, it turns red, then yellow, then white and finally (before it melts) blue!

Fig. 13 Iron bar (Credits: Blender3D)

(e) Look at the picture of stars taken with the NASA/ESA Hubble Space Telescope and try to answer the following questions:

Fig. 14 Credits: HST image

Which are the hottest stars? Which are the coldest ones? Which stars have a medium temperature?
Do you think that our Sun has a high, low or medium temperature?

We now know that stars have different colours because they are made up of hot glowing gas of different temperatures. Just as in the case of the iron bar, the star colours depend on how hot they are. The temperature of stars can sometimes reach 40 Million degrees in their inner cores! However the surface temperatures of stars range between approximately 3,000 to 40,000°C. Al-Sufi didn't know that the colours of stars are related to their temperatures, but he would certainly have loved to discover it!

(f) Take the constellation viewer and insert one after the other the cards with the following constellations:

Orion Great Bear Lyon Lyra Taurus Scorpio

Fig. 15 The constellation viewer (Credits: Scorza)

For each constellation and with the help of the Card-set 2, complete the table of Fig. 16 by filling in the names of the brightest stars of these constellations and their colours (according their temperature and the table 17a):

Fig. 16 Constellation table (Credits: Scorza)

_Fig. 17a Table of temperature of the stars (Credits: Scorza)

Modification of the constellation viewer to watch the colours of the stars You can modify the cards of the constellation viewer such that the brightest stars shine in the viewer in their corresponding colours. You can do this by colouring small pieces of a transparent sheet with permanent markers. Glue these small pieces at the back side of the corresponding stars of the constellation figures before gluing the figures on the blue cards). When you insert these modified cards, you will be able to see not only the constellation, but now also the colours of the brightest stars!

Fig. 17.b By inserting small pieces of coloured transparent sheets, you can reproduce in the constellation viewer the colours of Betelgeuse and Rigel (Credits: Scorza)

Space Awareness curricula topics (EU and South Africa)

The journey of ideas, Stars