The Sun and Other Stars

The Sun is a star. It is no different from the pinpricks of light in the night sky except that it is much closer to us. In principle, a star is nothing more exotic than a vast collection of hydrogen and helium gas held together by the force of its own gravity. It contains so much matter, however, that the pressure in its central region is enormous and the temperature soars to millions of Kelvin.

The combination of these factors allows nuclear fusion to take place. This process produces the energy which causes a star to shine and, in the vast majority of stars, the chemical element which is undergoing fusion is hydrogen, the simplest atom which can exist. In stars, hydrogen atoms in the core are fused into helium, the sophomore element of nature.

The fusion of hydrogen takes place via the proton-proton chain. The conditions at the core of a star are so intense that atomic nuclei are stripped of their surrounding electrons. In the case of hydrogen, its nucleus is a single proton. The proton-proton chain represents the collisions which must take place between the hydrogen and other nuclei in order to build a helium nucleus.

In step one, two hydrogen nuclei (protons) collide to form a heavy hydrogen nucleus (known as deuterium), a positron and a neutrino. In step two, another proton collides with the deuterium nucleus to form a light helium nucleus and a gamma ray. The final step of the reaction involves the collision of two tritiums which forms a helium nucleus and releases two protons.

A few, more unlikely routes to form helium are also possible, but in a star such as the Sun the above reaction route is the dominant one. The energy given out by these reactions is produced because the reactions convert a small amount of mass into energy. In all cases the summed mass of the reactants (those particles on the left-hand side of the arrows) is less than the summed mass of the products (those particles on the right-hand side of the arrows). This difference in mass is known as the mass defect, Δm, and is turned into energy. E, in accordance with Einstein’s famous equation,

The constant of proportionality in this equation is the square of the speed of light.

Study has shown that there are a myriad different types of star strewn through space. An excellent way in which to classify them initially is by their colour, since that instantly yields the star’s surface temperature. For example, yellow stars such as the Sun have surface temperatures of 6,000 K. Cool red stars have surface temperatures of about 4,000 K, whilst the hottest stars in the Universe are blue with surface temperatures in excess of 20.000 K.

The precise classifications depend upon other characteristics such as luminosity, radius and a complete analysis of the star’s spectrum. These properties help determine the mass of the star in question and the fusion reactions taking place in its core. To help in the classification a diagram, know as the Hertzsprung-Russell diagram after its inventors, has been developed (see Fig. 1. 2).

Fig. 1.2. Every star in the Universe has a place on the Hertzsprung-Russell (H-R) diagram. The sweeping band of stars running front top left to bottom right across the centre of the plot is the main sequence, where the majority of stars can be found. It represents those stars, such as the Sun, which are in the stable hydrogen-fusing stage of their existence. The numbers along the main sequence are stellar masses, in units of the Sun’s mass

Upon this diagram every known star has a place which is determined by its brightness and surface temperature. As a star evolves, the nuclear reactions taking place within it change in accordance with which elements are available for fusion, and the star changes its classification. The changes can be charted on the H-R diagram and produce evolutionary tracks which represent the different classifications a star occupies during its lifetime.

Just like an Earth plant or creature, a star has a life cycle; it is initially formed and eventually dies. During the course of its life, a star manufactures progressively heavier chemical elements because of the nuclear reactions taking place in its core.

The eventual fate of a star is virtually sealed by the mass it contains when it is formed. A star which contains less than five times the mass of the Sun will fuse hydrogen into helium for many millions - in some cases billions – of years. When its supply of hydrogen runs out, internal conditions will change and the star will become a red giant star, fusing helium into carbon.

As the helium runs out, so the star will die. It will return its outer layers of hydrogen and helium into space, causing something which is deceptively called a planetary nebula. What is lei) is a small, compact object known as a white dwarf. Stars of mass greater than five times that of the Sun are individually more significant on the cosmological scale. They are present in the Universe in far fewer numbers, and have much shorter lives than their less massive cousins.

Whereas smaller stars cannot fuse the carbon produced by helium fusion, these larger mass stars can. In fact, they can fuse a great many chemical elements through a complex pattern of interactions which are made possible by the extreme conditions in their interiors. Iron, however, is a chemical element which marks a watershed in nature. All elements lighter than iron can be fused in order to give out energy, but from iron upwards, energy needs to be supplied in order for the elements to be fused.

In the hearts of stars, iron builds up like ash in a fire. This is because nowhere near enough of the amount of energy necessary to fuse iron is available. When the amount of iron reaches about one-and-a-half times the mass of the Sun, its atomic structure breaks down and causes the star to collapse. Material from the star’s outer layers falls downwards towards the shrunken core and impacts its surface.

A huge explosion, known as a supernova, takes place which blows the star to pieces and supplies all the excess energy required to fuse all the elements heavier than iron. If anything remains of the original star, if may be a fragment of its collapsed core known as a neutron star. Alternatively, the collapsed core may have been so large that it becomes a black hole.

When they explode, these massive stars seed space with heavier elements. In short, they change the composition of the Universe. We know from measurement that the composition of the Universe today is 75 per cent hydrogen, 23 per cent helium and 2 per cent (at most) everything else. This 2 per cent consists of all the atoms apart from the hydrogen and helium, which make us, the Earth and the Solar System.

So the composition of the Universe has changed with time due to the action of stars. An immediate consequence of this is that stars in the early Universe could not possibly have had planets such as the Earth in orbit around them because the Universe did not contain enough heavier elements from which to make rocky worlds. Planets, when they did form, would have been gaseous bodies, similar to Jupiter.

The distribution of stars within the Universe was of major concern to cosmologists of the 1700s and 1800s. The issue was of such great contention that it was only finally decided in the second decade of this century, thanks to the work of Harlow Shapley.