1. Astronomy
Chapter VII
Stars

2. 7.1. Classification of Stars
Chapter 7 The Stars
7.1. Classification of Stars
The properties of a star include its brightness, luminosity, temperature, size, magnetic field, density, age, and many other properties.
All these properties, however, depend upon its initial mass and chemical composition.
The chemical composition of a star changes with time depending on the nuclear reactions which occur inside it. One of these nuclear reactions is the proton-proton chain reaction.
STARS
Sun like stars
Massive stars
Properties of stars depend upon their initial mass. Thus stars are classified according to their initial masses:
Stars which have masses close to the mass of the Sun (Sun-like stars),
Stars which are at least a few times more massive than the Sun (Massive Stars),

3. 7.2. Lives of Stars
We have grouped the stars into two categories according to their initial masses. Each group has a different life cycle.
We call a star 'living', if there are nuclear reactions occurring to inside the star and its life ends when its fuel is totally expended.
We are interested in how the stars use these fuels.
How much time it takes for a star to burn its fuel, what happens after this, and what the properties of the remnants(remaining) are.

4. All stars initially contain only hydrogen
All stars initially contain only hydrogen. At the beginning no other material in the chemical make-up of a star can be found.
The amount of matter (hydrogen) determines the strength of the gravitational force.
The molecules attract each other inwards due to gravity thus the star is compressed and so gets smaller.
As the star contracts its temperature increases under the influence of the gravitational force, the gravitational force is equal to the force due to the gas pressure.
An increase in temperature and a decrease in volume causes a massive pressure rise within the star (recall that PV=nRT).
An equilibrium process occurs due to these phenomena (hydrostatic equilibrium).
The temperature and the volume reach a value such that the gravitational force and the force due to the thermal gas pressure are equal to each other.

6. Nuclear reactions cause small nuclei to combine to form a heavier nucleus.
In the proton-proton chain reaction 4 hydrogen nuclei combine to form 1 helium nucleus.
Further nuclear reactions can take place under higher temperatures and pressures.
These reactions cause heavier nuclei to combine further (Table 7.1).
The required temperature increases with increasing mass of the fuel at each step of the reaction.
Now let's take a look at how each group of stars evolve, which reactions take place and what happens after the evolution ends.

8. 7.2.a. Sun-like Stars All stars in this group are similiar to the Sun.
Suitable conditions lead to the proton-proton chain reaction occurring and hydrogen is converted into helium. During this process the temperature and pressure increases.
It takes about 10 billion years for hydrogen in the core to be expended. The star spends this time interval in the main sequence (Figure 7.1).

9. After the hydrogen in the core is used up; the hydrogen in the shell outside the core starts to react.
The core contracts while the hydrogen shell expands due to these nuclear reactions.
The star moves to the upper-right of the H-R diagram. That is, the star expands into a red giant (Figure 7.2).

10. The star gets hotter; hot enough for the triple-alpha reaction to occur.
At this point helium undergoes nuclear reactions and is converted into carbon.
After the reactions in the core begin again, the outer shell contracts and the star moves to the left of the main sequence in the H-R diagram (Figure 7.3).

11. When the helium is expended in the core the shell reactions become important.
Helium undergoes reactions in a shell just outside the core and hydrogen undergoes reactions in a shell outside the helium reaction shell.
The outer layers start to expand and the star moves to the upper-right once more in the H-R diagram
(Figure 7.4).

12. The star loses its outer layers due to external effects.
The core continues shrinking while losing its outer layers until it becomes degenerate(decline).
Thus, the outer layers have been loosened leaving a hot, degenerate core which is called a white dwarf (Figure 7.5).
We can conclude that the remnant of a Sun-like star is a white dwarf.

13. 7.2.b. Massive Stars
STARS
Sun like stars
Massive stars
Here the first stage is again hydrogen burning and the star is in the main sequence during hydrogen burning. These two properties are the same as Sun-like stars.
The first difference is that reactions take place faster. However, there are differences with the evolution of Sun-like stars.
More mass causes greater gravitational forces and thus higher temperatures and much faster reactions.
As a result the star does not spend as much time in the main sequence.
It may take only several million years for a massive star to burn all of its hydrogen.
The time interval for the hydrogen burning stage decreases with increasing mass.

14. Another difference is that hydrogen is converted into helium by a different nuclear reaction called the CNO cycle, in massive stars (while it is converted via the proton-proton chain in Sun-like stars).
In Sun-like stars only hydrogen and helium burning occurs while additional reactions occur in massive stars.
These additional reactions cause heavier elements to be formed.
Sun-like stars can burn hydrogen to form helium and can burn helium to form carbon while massive stars can burn carbon and even heavier elements like oxygen, and silicon.

15. Hydrogen burning will continue in an outer shell after the hydrogen source is expended in the core.
Once the hydrogen is gone from the core it burns helium into carbon and an outer shell burns hydrogen into helium.
When helium is also finished in the core, the core will start to burn carbon into oxygen while a shell just outside the core burns helium and another outer shell burns hydrogen and so on.
Each reaction in the core will continue in an outer shell and a new shell is produced at each step (Figure 7.6).

16. Although the star stays in the main sequence for the hydrogen burning stage at each of the following reaction stages the star moves to the left in the H-R diagram, then whenever the fuel is completely burned in an outer shell the star moves to the upper right of the H-R diagram (Figure 7.7).
The number of steps is determined by the initial mass of the star.
The more massive the star, the more reaction stages there are. More stages mean that heavier elements are formed.

18. 7.3. Remnants of Stars
White Dwarfs
White dwarfs are remnants(remainings) of Sun-like stars consisting of degenerate matter inside (helium and carbon generally) and a thin and dense gas layer outside.
A typical diameter of a white dwarf is similar to that of Earth while its mass is similar to that of the Sun; 1.4 solar masses at most.
That is, the density of a white dwarf is very high (about 106 g/cm3) compared with a living star. Since there are no nuclear reactions occurring inside white dwarfs, they are faint(weak, poor) objects in the night sky.
One more striking property of white dwarfs is that they cannot expand by gaining mass, instead they contract, thus their density increases.

19. Neutron Stars
Neutron stars are created when giant stars die in supernovas and their cores collapse, with the protons and electrons essentially melting into each other to form neutrons.
Neutron stars are the smallest and densest stars known to exist.
They are smaller than white dwarfs (with dimensions of about 10 km) but they are much heavier.
Although neutron stars are very small compared with living stars they are still giant objects which are capable of completing a whole revolution in 100 seconds.
Compare this with the rotation period of the earth which is 24 hours i.e seconds. A neutron star, depending on its mass, can reach incredibly high angular speeds and some neutron stars can rotate 1000 times per second.

20. Black Holes
The most massive stars, i.e. initially more than 20 solar masses; complete their life as a black hole. After nuclear reactions finish burning all the fuel in the star's core and shells, the huge amount of mass collapses continuously since nothing can balance its gravitational force.
The gravitational force of a black hole is so great that even the natural opposing forces from degenerate neutrons cannot overcome it, and this leads to the collapse of the star. The gravitational pull of a black hole is so great that even light (photons) cannot escape.
The massive gravity of the black hole not only traps the mass inside but also sucks in the material surrounding it. The space and time around a black hole is distorted.

23. Questions: 1. Which property of a star is dominant?
2. How does the initial mass of a star affect its life cycle?
3. Which reactions take place on a sun-like star?
4. Which reactions take place on massive stars?
5. How does the initial mass of a star affect the type of remnant the star leaves behind?