**Perhaps we are children of two!**

# **Introduction**

The chemical elements were created by nuclear fusion reactions in the
hot interior of remote and long vanished stars over many billions of
years. During their evolution, stars synthesize all the elements (apart
from the cosmological Hydrogen and Helium) which compose the planets,
the earth and our body. This process is named *stellar nucleosynthesis*
and it can be slow (up to 10 Billions of years) or explosive: in this
case we have a *supernova*. During a supernova's event there is a great
energy involved, which allows the synthesis of almost all atomic nuclei
and the dissemination of heavy elements in the interstellar medium.
Though supernovae are extremely rare[^1], they are important to
understand the element's distribution in the Universe.

A single star becomes a supernova only if its mass exceeds ten Solar
masses (SN type II) and such massive stars aren't common in a galaxy.
Other supernovae (SN type Ia) involve a couple of small stars, forming a
binary system. We are children of the stars since elements forming our
body are produced in the stars and thus, perhaps, we are sons and
daughters of two.

The study of this matter, which we hope can be fascinating and exciting
for young people, it the target of this activity. The students are
stimulated to investigate the dynamics and the evolution of a binary
system through the creation of a model which permits them to visualize
the abstract mathematical concept.

A model isn't the reality, but a logic structure that permits us to
interpret the reality: we think that it's very important to face a deep
reflection around the model, all together, teacher and student. The goal
of the hand-on activity isn't to make easier the arguments, but to
educate students to the criticism and to the analysis.

The didactic unit is composed by 4 "lessons", that can be developed with
high school students. Every lesson is divided in three parts: *the goals
of activity*, *the main items*, in which we present the teacher the
essential concepts of the activity, *the development in classroom*, in
which we suggest some strategy for the communication to the students.
The development in classroom is a fundamental moment during which it's
possible to educate student to the scientific method, using reasoning
based on the analysis and synthesis.

Sometimes students feel the research about all this astrophysical
arguments is too distant: in order to avoid this problem, we have tried
to make reference to the recent literature, every time in which it is
possible.

## Unit's plan

1.  *Understanding the stellar evolution.*

> In this first part, the students discover the star's evolution,
> recognizing the main physics parameters: mass, time, spectral class
> and luminosity. It's important that students understand these
> concepts, so that they can ideate, build and criticize the models.

2.  *Star's Binary system.*

> This lesson focuses on the rule of gravitation in the dynamic and
> evolution of a star's binary system: the student gets acquainted with
> concepts as equipotential lines and Roche lobes.

3.  *Building the model.*

> The students are stimulated to ideate and realize a model, considering
> all the parameters studied in the previous lessons. The model, derived
> by simple mathematical tools, permits an easy visualization of an
> abstract idea, but also a deeper insight about the physical concepts
> which lie behind it.

4.  *Discussing about the model.*

> After the realization of the model, the student faces the moment of
> the analysis: some phenomena are expected, but others are not
> realistic. After a deep criticism, they discover that the model is too
> simple: the model has some defects because it doesn't take into
> account the relative motion of the stars (static model) and of the
> third dimension. Is it possible to realize a more complete model? With
> a pc software? The unit ends with a challenge!

[^1]: In our galaxy (10^11^ stars) there is one supernova every 50
    years: there should be 4 + 1 Type Ia, 2 + 1 Type Ib/c and 12 + 6
    Type II observable supernovae per millennium. (E. Cappellaro, M.
    Turatto, D. Y. Tsvetkov, O.S. Bartunov, C. Pollas, R. Evans & M.
    Hamuy, \"The Rate of Supernovae from the combined sample of five
    searches\", Astronomy and Astrophysics 322, June 1996.)