Short description of stars

• Mass,
• Age,
• Surface tempertaure,
• Luminosity (total power output)

The theory of stars :

Forgetting about composition concerns, only two of the former characteristics suffice to describe the star (they imply the two others). For instance the mass of the star and it's age give the surface temperature and luminosity.

We are making a game, why concern about these characteristics ?

• The mass of the central stars rules the orbits of the planets around it.
• The age does not really matter, but a star about to explode could be interesting in the scenario.
• Surface temperature gives the star's color and thus the color of the light it shines on planets.
• Temperature on the planets depend on the composition of their atmospheres, their distance to the star and also on the luminosity of the star.

What is the so-called "Herzprung Russel diagram" ?

If you take surface temperature an lumisonisty as parameters, they are not completely unlinked. Put for every star a point in a 2D diagram, with one axis giving luminosity and the other the temperature : then a great majority of stars fits within a diagonal band, or whithin two or three patches. The reason why this diagram is important for astronomers is that temperature and luminosity are the easyest star characteristics to measure. Age is not measured and for mass you need binary stars.

Star colors :

Roughly, the color depends on the temperature. Red is the coolest, then it follows the rainbow up to blue, the hottest.

Supernovaes :

Only heavy stars explode. The ones like the sun experience a supergiant phase, and then quietly die and become white dwarves.

Other types :

• Core temperature : whereas the surface temperature is about several thousands of Kelvins, the core temperature is much higher, for instance 12 million Kelvins for the Sun. This is the range of temperatures needed for nuclear fusion.
• Stars evolve : for instance the sun, which is a small yellow star will eventually become a red supergiant in 4 or 5 billion years. Stars first burn their hydrogen, producing helium, by nuclear fusion. Then, they burn the helium they produced, producing . Burning helium is much more intense and quick. The core is much hotter, the Luminosity of the star is much higher, but paradoxically, this heat makes the outer layers (whose composition is mostly unburnt hydrogen) extend considerably, so that the surface temperature ends up lower than before. Once the helium is burnt out, it is the turn of biggers atoms, carbon, oxygen, and so on. It then depends on the mass of the star. If big enough it can reach the iron. Iron does not transform into something else because it is the most stable atom (it has the lowest energy level). Heavier elements are only created during the explosion of supernovaes.
• Brown dwarves : not massive enough to start thermonuclear reaction in their cores.
• White dwarves : core of dead stars (massive enough stars but not too much). Very dense because no thermonuclear reactions sustain the weight : so atoms are compressed. The quantic fuzzyness of electrons provides a pressure that balances gravitational collapse. A white dwarf begins it's life very hot, and cools down indefinitely, unless an other star or object comes and perturbs it.
• Neutron stars : white dwarves cannot sustain themselfs beyond 1.4 times the mass of our sun (this is called the Chandrasekhar limit). After this limit, atoms collapse, electrons mix with protons and become neutrons. All neutrons touch themselves. Neutron stars are heavier that the sun, yet smaller than earth ! Neutron stars are often pulsars : they spin rapidly, have strong magnetic fields and emit periodic radio waves. Neutron stars also have a limit mass (about 1.7 times the mass of the sun). Heavier star remnants collapse to a black hole.
• To be more realistic, composition of regular stars (at birth) and white dwarves play a role in their life and other characteristic.
• Binary stars : more complicated because each one influences each other.
• Many newborn stars are shrouded in a dust cloud.
• Some have dust rings.
• Old masive stars slowly blow their outer layers with a strong "wind" of particles. This gives rise to the beautyful so-called planetary nebulae (which are bad named because they have nothing to do with planets) when this gas is lit by the strong ultraviolet glow of the star.