Unit 1:  Our place in the Universe                                          1   2   3   4  

3.2. Stars

Stars are huge gaseous spheres of hydrogen and helium. These gases reach so high temperatures that transform the interior of the star in a giant thermonuclear fusion bomb. In this reaction two atoms of hydrogen fuse together to form an atom of Helium and a enormous amount of radiant energy is emitted.

Most of stars is very close to each other forming pairs (binary stars) or groups

more or less closed (stellar clusters). Alone stars like the Sun are scarce.

There are several ways to classify stars. One of them is based on their spectral type. According to this criterion stars are classified into the classes: O, B, A, F, G, K, M.

The spectral class is closed related  with the colour, temperature and size of stars:

Other way to classify stars is according their magnitude. The brighter the star is, the lesser its magnitude. The most faint stars that can be distinguished naked eye are magnitude 6. Polaris, for example, has magnitude 2 and Sirius (the most bright star in the sky) has a magnute -1.4. The Sun has apparent magnitude -26.8.

The magnitude perceived from the Earth is the apparent magnitude and is deceptive. We can see a star as very bright not because it really is but because it is close (the brightness in inversely proportional to the square of the distance). This is why, astronomers define the absolute magnitude which indicates the intrinsic brightness or luminosity of a star. That is to say, it is the magnitude that a star would have if it were at a parsec of distnce from the Earth.

   - Absolute magnitude is the total amount of energy contained in the radiation emitted by the star.

     -  Spectral class indicates the way in which this energy is distributed as wavelength

The graphic representation of both parameters is the Hertzsprung-Russell diagram

(or HR diagram). This diagram shows that the most of stars are located in a line called the Main Sequence.

This diagram is the result of the star evolution. The main sequence is due to the fact that we observe the stars during the state in which they pass the most part of their “lives”.

As we will see, during its life a star displace through the HR diagram as its mass varies.

The life cycle of a star are conditioned by two opposite forces:

When both forces balance the star enters in a period of stability that lasts about 90% of its life. During this time it will go through the main sequence.

The star will stay at the main sequence while it had hydrogen to burn. But, unavoidably, this fuel will be consumed sooner or later.

- Small stars which mass is around 1% of the solar mass (dwarf stars) burns their fuel very slowly and can stay in the main sequence during millions of millions of years.

- Medium-sized stars which mass is similar the solar mass consume their fuel much more quickly (4.5 Tonnes/second) and they deplete it in just some thousands of millions of years (10,000 million years in the case of the Sun).

- Large stars which mass is until 60 times the solar mass (giant and supergaiant stars) consume the hydrogen very quickly  and they stay in the main sequence only few millions of years (between 1 and 100 million years).

Therefore the evolution of the life cycle of a star depends on its mass.

a) Evolution of stars similar in size to the Sun

1º) Birth

       All stars begin their lives with the gravitational collapse of a nebula.

      When this occurs the huge cloud of interstellar gas and dust is fragmented

      in smaller clouds from which proto-stars will form

Each proto-star rotates around its axis and the gravitational collapse continues.

As the mass is compressed, the rotational speed increases. The initial cloud is more and more compact, increasing its density. This makes easier the collision between atoms of hydrogen.

As the frequency of collisions increases, the hydrogen temperature increases too, until it reaches a critical value of 10.10^{6 o}C. This temperature is high enough to allow the thermonuclear reaction of the hydrogen. It starts to form Helium which is accumulated in the nucleus of the star, and a huge amount of energy is released. As a result the star switches on.

If gravity did not act, this energy would make explode the star. Both forces, radiant energy and gravity, balance and the star is stable while it consumes all the hydrogen.

2º) Red Giant

After the exhaustion of the hydrogen, the radiation pressure decreases and gravity start to act over the central part of the star. In this phase, the nucleus contracts and the pressure and the temperature in it increase again. This triggers a new cycle of nuclear combustion, in this case of Helium.

In addition the outer envelopment expands fastly and the star becomes bigger, while its surface temperature decreases. It changes into a Red Giant.

3º) Death

The accumulated Helium in the nucleus of the star is transformed into Carbon. The nuclear reaction releases an enormous amount of energy that provokes:    

• The external layers of the star detach forming a ring around (Planetary Nebula)

• The nucleus collapses transformed into a White Dwarf which will continue burning Helium and accumulating Carbon in its nucleus. Once the Helium depleted, the nucleosynthesis stops and the star will cool down slowly until switch off completely, as a result it will become a dark and cold star called Black Dwarf.

b) Evolution of stars bigger than the Sun

    (more than 10 solar masses)

This kind of proto-stars becomes into a giant star in a similar way to the one described previously. But they are much bigger and consume hydrogen much more quickly and in more amounts so that they produce more energy and emit an intense blue light. For this reason they receive the name of blue giant stars.

When the hydrogen is totally consumed, the nucleus of the star starts to contract again (because the gravity becomes bigger than the radiation pressure).

This provokes the increment of the temperature and the beginning of a new cycle of nuclear reactions. The star swells and transforms into a red supergiant star. The nucleus of these stars is formed by concentric layers, each one of them contains a different process of thermonuclear fusion that forms a different chemical element (carbon, magnesium, silica, etc.).

All these nucleosynthesis reactions emit energy, but the last one, that provokes the production of iron (Fe), does not release energy, by the contrary it consumes energy. At this point, the nucleus temperature is around 5,000.10⁶ ºC and the end of the star is close.

With its source of energy depleted, after the synthesis of iron, the radiation pressure stops and the gravity produces the collapse of the star. This tremendous implosion generates shock waves that first rebounds in the extremely dense nucleus and then, propagates at high speed, producing a huge explosion.

- As a consequence of the explosion (supernova), enormous amounts of energy and almost all the matter of the star are released.  In this extreme moment the heaviest chemical elements (more than the iron) are produced. All this new matter and the elements originated inside the star during its life are spread through the space. They make up the cosmic dust.

The spreading of these heavy chemical elements through the space pollutes the nearby nebulae, and the expansive wave generated by the explosion can trigger their gravitational collapse that can finish with the formation of new proto-stars. From these proto-stars, richer in heavy chemical elements than their predecessors, new stars will form and may be some of them could have planetary systems.

- As a consequence of the implosion, the nucleus of the supergiant star undergoes a so extraordinary compaction that it changes into:

- A neutron star, if the mass is between 9 y 30 times the solar mass. It is a small remainder and enormously dense that spins very fast. Its magnetic field catches charged particles and provokes the radiation emission in the form of rotational beam. To an observer located in the direction of the beam, this will appear as it were a lighthouse that emits light at regular intervals of time. The star has transformed into a pulsar.

- A black hole, if the mass is more than 30 times the solar mass. The supernova remainders will undergo a gravitational collapse that will transform it in something unimaginably dense: a point of zero volume with a gravitational field almost infinite (singularity) from which even light can escape.

In the 50’s decade, quasars (quasi-stellar radio source) were discovered.

They are stellar objects much samller than galaxies (10⁶ times smaller than the Milky Way) that emit a huge amount of energy (100 times more than a big galaxy) and which brightness fluctuates in periods that go fromfew years to days. They are located very far away from us and they are receding at speeds higher than 90% the speed of light. Its nature is still unknown, but they could be nucleus of very young galaxies.

3.2. In the Hertzprung-Russell diagram (or H-R diagram) has been represented the evolution

      of a star similar in mass to the Sun, since it was born until it die.

a.   What is the luminosity (or brightness) of a star?

b.   What is the difference between the apparent magnitude of a star and its absolute magnitude?

c.   The brightest stars in the diagram, will be the brightest ones seen from the Earth?

     Explain your answer.

d.   Why do they have different colours?

e.   What is the reason of their different sizes?

f.    During its life the star displaces trough the diagram in the direction that the arrow indicates.

    How is this explained?

g.   How does temperature vary along the star life?

h.   How will change the temperature and brightness of the Sun until its death?

3.3. What influence has the mass of a star in its evolution?

3.4. The picture represents a red supergiant star just before to become a supernova.

a.    Indicate which chemical element is synthesising in each layer.

b.    How does temperature vary as depth increases?

c.     How is the end of this kind of stars?

d.    Which are the oldest chemical elements? When were formed the iron and the carbon?

     Were gold or uranium formed during the Big Bang?

e.    The sentence “We are star dust” belongs to the famous astronomer and science

    communicator Carl Sagan. What do you think it is referred to?