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Touching the stars

Created: 2024-09-05

Author(s):
Cristina Mercante (Circolo Astrofili Veronesi), Raffaele Belligoli (Circolo Astrofili Veronesi)

In this activity, you will learn about the dimensions of stars in comparison to the Sun with a set of polystyrene spheres. By using spheres of different colours, you will also learn about different types of stars and their evolution: from blue spheres, representing hot and young stars, to red spheres, symbolising cold and old stars.
IMPORTANT: This activity can also be performed with blind students, to teach them about the sizes of stars and stellar evolution through an interactive, fun, and hands-on approach. In this case, the teacher may need to adapt the parts about the colours of stars, based on the specific student group he/she is working with.

Materials

Eight polystyrene spheres with diameters: 1mm, 2mm, 4cm, 5cm, 6cm, 7cm, 8cm, and 20 cm
https://www.amazon.it/sfere-polistirolo/s?k=sfere+di+polistirolo
Tempera colours: yellow, white, orange, blue, and red
The printed cards associated with the stars (see attachment: you can print the cards in A4 format or smaller)

NOTE: The scale used in this activity is 1 mm:1,392,000 km (average diameter of the Sun). This makes it easier to use the other spheres. It is possible to use another scale. In this case, you will need to recalculate the relative sizes of all spheres.

Goals

To introduce astronomy as a tactile and exciting, hands-on experience.
To understand that stars are very different from one another.
To understand that stars are not all similar to our Sun, the only star we can observe closely.

Learning Objectives

Learning the size of stars in comparison to the Sun.
Learning about the relationship between a star's mass, color and its life cycle.
Learning that stars above a certain mass end their life as supernovae.

Background

What’s a star?

A star is an enormous sphere composed primarily of hydrogen plasma, where nuclear fusion reactions, occurring at millions of degrees in its core, transform hydrogen into helium.

More: https://en.wikipedia.org/wiki/Star

How does a star evolve?

Nuclear fusion powers a star for most of its existence. Starting from hydrogen, chemical elements of the Periodic Table up to iron are generated. Depending on the mass of the star, its lifetime can vary from a few million years for the most massive ones, to billions of years for the least massive ones. During the various phases of their lives, stars change their colour, temperature and size.

How does a star end?

Once a star like the Sun has exhausted its nuclear fuel, it expands to become a red giant. The outer layers are then expelled as a planetary nebula, while the core collapses into a dense white dwarf. Stars with around ten or more times the mass of the Sun explode as supernovae and their inert iron cores collapse into an extremely dense neutron star or black hole.

More: https://en.wikipedia.org/wiki/Stellar_evolution

Stellar classification: colour vs temperature

Stars can be classified based on their surface temperature (which is linked to their colour and mass), using the letters O, B, A, F, G, K and M: type O stars are the hottest, while the other letters refer to gradually cooler stars.

It is customary to describe class O and B stars as "blue", those of class A as "white", those of class F as "white-yellow", those of class G as "yellow", class K as "orange" and class M as "red".

https://en.wikipedia.org/wiki/Stellar_classification

The 8 stars

In this activity, you will present 8 stars of different sizes and colours. For specific infromation about these 8 stars, please refer to the printed cards (see attachment).

Full Description

Before the activity

Print the eight cards associated with the stars.

Prepare the eight spheres, colouring them with the tempera colours corresponding to the star's temperature.

The reference stars are:
Sun (1mm sphere, yellow)
Sirius (2mm sphere, white)
Aldebaran (4cm sphere, orange)
Polaris (5cm sphere, yellow)
Mirfak (6cm sphere, yellow)
Rigel (7cm sphere, light blue)
Almak (8cm sphere, orange)
Y Canum Venaticorum (20cm sphere, red).

Place the spheres on a table, each with its card.

Image: the spheres representing the eight stars, together with the corresponding cards (Credits: Raffaele Belligoli)

Activity description

Step 1: Let the participant hold the first ball of 1 mm, the Sun, and explain what a star is.

Step 2: Explain that at this scale, the size of a planet, such as the Earth, is imperceptible. Introduce the final evolution phase of Sun-like stars (white dwarf)

Step 3: While the participant holds the Sun in one hand, give him/her the next sphere (star Sirius), letting him/her compare the sizes of the two stars. Introduce the fact that stars have different sizes, while talking about Sirius.

Step 4: The participant continues to hold the Sun in one hand. Give him/her the next sphere (star Aldebaran). Compare the sizes of the two stars talking about Aldebaran. You can introduce the fact that stars also have different colours, and how this is related to temperature.

Step 5: The participant continues to hold the Sun in one hand. Give him/her the next sphere (Polaris) and compare it to the size of the Sun. While talking about Polaris, you can introduce the concept of double stars.

Step 6: The participant continues to hold the Sun in one hand. Give the participant the next ball (Mirfak), and compare its size to the Sun. While talking about Mirfak, you can introduce the final evolutionary steps of this type of stars (supernovae).

Step 7: The participant continues to hold the Sun in one hand. Give him/her the next sphere (Rigel). While comparing the sizes of the two stars, the participant is introduced to blue supergiants.

Step 8: The participant continues to hold the Sun in one hand. Give him/her the next sphere (Almak). While comparing the sizes of the two stars, you can talk about this kind of stars and multiple systems.

Step 9: The participant continues to hold the Sun in one hand, while you pass him/her the next ball (Y Canum Venaticorum). While comparing their sizes, you can briefly talk about this star, named "La Superba" by astronomer Angelo Secchi.

Step 10: The participant holds the Sun in one hand. Explain him/her that the largest known star is UY Scuti, with a diameter of 1708 million km. If placed at the center of our solar system, it would expand beyond Jupiter's orbit, coming within 1 AU of Saturn. At the scale used for the activity, it would be a sphere measuring 1.7 metres in diameter.

Performing "Touching the stars" with the general public (Credits: Raffaele Belligoli)

Evaluation

To evaluate students' understanding, you can have them repeat the activity and describe the stars, and record how many “key concepts” they remember.

You can also use a questionnaire like the one below.

A star is...

a massive ball of gas that generates light and heat through chemical reactions in its core. (right answer)
a massive ball of gas that generates light and heat through chemical reactions on its surface.
a massive ball of very hot lava.

Do stars evolve during their life?

they are transformed into a planet
a star can change color over time, never changing its size
a star changes color and size over time (right answer)

How do stars die?

Stars never die
As white dwarfs or supernovae, depending on their masses (right answer)
They always explode as supernovae

Which colour indicates the hottest stars?

Blue stars are the hottest ones (right answer)
Red stars are the hottest ones
the color of a star is not related to its temperature

The Sun...

is the biggest star in the Universe
is the biggest star in our Galaxy
is a medium size star of our Galaxy (right answer)