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What is a star

The Hertzsrung - Russell Diagram
Hertzsprung-Russell diagram. A plot of luminosity against temperature, showing the location of some well known stars and the future evolutionary track of the Sun. Both scales are logarithmic.

Our Sun, like most of the stars we see in the sky, is converting hydrogen into helium. In the process it is converting 4 million metric tons of mass into energy every second. The relation between mass and energy is given by Einstein="s" famous equation E="mc2. " The nuclear fusion of hydrogen takes place at the centre of the Sun where the temperature is about 15 million degrees Kelvin and the density is 160 thousand kilograms per cubic metre. The outer layers of the Sun act as an enormous insulating blanket through which it may take a photon a million years to find its way to the surface, where the temperature is 5,800 K.

The Sun is in a state of perfect equilibrium between the force of gravity, which holds it together, and the central outward pressure resulting from the fusion of hydrogen into helium. This is a stable equilibrium. For example, if we imagine adding mass to the Sun, then the central inward pressure due to gravity would increase. This would increase the rate of nuclear fusion, which would in turn increase the central temperature and provide an increased balancing outward pressure. Similarly, reducing the mass of the Sun would reduce the central inward pressure due to gravity and thereby in turn reduce the rate of nuclear reactions, energy release and temperature.

Our Sun lies in the middle of the range of star masses and brightness. There are many stars more massive than the Sun.

In the 1920s Sir Arthur Eddington showed that there is an upper limit to the luminosity of a star of a given mass, set by the fact that at its surface the outward pressure of the radiation coming from inside the star will equal the inward pressure caused by the force of gravity. If the star is any brighter then it will blow material away from its surface and so reduce its mass and luminosity. This effectively limits the maximum mass of a star to 100 times the mass of the Sun. The luminosity of such a star is one million times that of the Sun and it will live out its entire life in a mere four million years. At the other end of the scale are the faintest brown dwarfs, barely massive enough to briefly sustain nuclear fusion. They are about 0.014 the mass of the Sun and only fifteen times the mass of Jupiter.

How long a star shines depends on its mass. Stars like the Sun can remain almost unchanged for ten thousand million years, more than long enough for life to develop on a nearby planet. Meanwhile a star like Eta Carina, 100 times the mass of the Sun may only last four million years.

In many ways stars are very simple objects; round spheres of gas in hydrostatic equilibrium, hot and dense at the centre, much cooler at the surface and as the Astronomer Royal Sir Martin Rees says, much simpler than ants! As a consequence it is possible to describe a star by just two numbers, its surface temperature and its luminosity, often expressed as a multiple or fraction of the luminosity of the Sun. When a graph of luminosity against temperature is drawn, each star can be represented by a single point. With very few exceptions, stars which lie in the same place on the graph not only have the same temperature and luminosity but also the same mass, central density and age. This graph was first plotted by H. N. Russell in 1913 and is now called an Hertzsprung-Russell or HR diagram. Stars spend most of their lives along a diagonal strip called the 'main sequence'. For stars that lie on the main sequence there is a simple relationship between their luminosity (L) and mass (M) of the form; L is proportional to Mx, where x varies between 3.5 and 2.7 as the mass increases from 1 to 25 solar masses.

As a star lives out its life or evolves, it follows a well defined track in the HR diagram. We have not been around long enough to see any significant changes in the Sun or any other star, but we can predict their future using theoretical models of stellar evolution, based on the laws of physics and a great deal of data about the properties of gases at high temperatures.

We test the truth of these stellar evolution models by comparing their predictions with observations of clusters of stars that were all born at the same time but are at different stages of their lives, because of their wide range of masses. They form a very distinct pattern in the HR diagram.

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