The life cycle of a star

In this physics coursework, I have been asked to carry out research of my selection and to develop it. I have selected to research the life cycle of a star, and I would conduct this by gathering the necessary information in a form of a report which explains this in detail. I have chosen to explore this particular topic firstly because I am extremely fascinated in space and the universe and secondly because I do not know much about the life cycle of a star and I deem this will help extend my knowledge.

Firstly when carrying out this research before describing the life cycle of a star I need to be familiar of what a star is, and how it is formed

What is a star, and how does it form?

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Stars are basically huge balls of hydrogen gas. Hydrogen is by far the most common element in the Universe, and stars form in clusters when large clouds of hydrogen, which naturally forms a hydrogen 'molecule' (H+H=H2) with another atom, collapse.

The hydrogen clouds collapses very slowly, although they can be speeded up by the effects of a passing star, or the shockwave from a distant supernova explosion. As the cloud collapses, it speeds up its rotation, and pulls more material into the centre, where a denser ball of gas, the 'proto-star' forms. The proto-star collapses under its own weight, and the collisions between hydrogen molecules inside it generate heat. Eventually the star becomes hot enough for the hydrogen molecules to split apart, and form atoms of hydrogen.

The star keeps on collapsing under its own weight, and getting even hotter in the core, until finally it is hot enough there (roughly 10 million degrees) for it to start generating energy, by nuclear fusion - combining hydrogen atoms to form a heavier element, helium. Energy is released from the core, and pushes its way out through the rest of the star, creating an outward pressure which stops the star's collapse. When the energy emerges from the star, it is in the form of light, and the star has begun to shine.

A Star is formed from a cloud of gas, mostly hydrogen, and the dust that is initially spread over a huge volume, but which is pulled together by its own collective gravity. This gravitational collapse of the cloud creates a body of large density, and the loss of gravitational potential energy in the process is very large indeed. The result is that the original particles acquire high kinetic energy, so that the collisions between them are very violent. Atoms lose their electrons. Not only has that, collisions taken place in which electrical repulsion of nuclei is no longer strong enough to keep them apart. They can become close enough together for the strong nuclear force to take effect, so that they merge. Fusion takes place, with hydrogen as the principal key material. This begins the process of conversion of mass to energy, and much of the released energy takes the form of photons which begins to stream from the new star.

Every star then exists in a state of slowly evolving stability. On the one hand there is the trend for the material to continue to collapse under gravity. On the other hand there is a tendency for the violent thermal activity and the emission of radiation resulting from fusion to blow the material apart. The more bigger star in general, the greater is the gravitational pressure and so the higher rate of energy is released by fusion, therefore bigger stars use up their supply of fusing nuclei more quickly than do smaller stars, such that bigger stars have shorter lives.

The enormous luminous energy of the stars comes from nuclear fusion processes in their centres. Depending upon the age and mass of a star, the energy may come from proton fusion, helium fusion, or the carbon cycle. For brief periods near the end of the luminous lifetime of stars, heavier elements up to iron may fuse, but since iron is at the peak of the binding energy curve, the fusion of elements more massive than iron would soak up energy rather than deliver it. This links to the below graph:

Fusion in stars makes energy available to create radiation, consuming mass at an amazing rate. The sun, for example loses a mass of 4.5 million tonnes every second. Also, heavier nuclei are formed from smaller ones, so that the compression of a star changes. Concluding this, as the star dies the material dependant on its size is scattered in space.

The Hertzsprung - Russell Diagram

This simple graph shows ways in which to classify stars. Temperature is plotted on the x-axis. This is related to the colour as cooler stars are redder, hotter stars are bluer. Relative luminosity is plotted on the y-axis. Because of the very wide range of temperatures and stellar luminosities, logarithmic scales are used. The location of an individual star on such a graph lets us establish a loose system of classification. This graph aids us to find out what star has what temperature so we can easily classify it using the relative luminosity and temperature. Here is a diagram of the graph which shows the stars in their classified points showing their rough temperature and luminosity.

So how do the changes in the stars take place?

Very massive stars experience several stages in their cores.

o First hydrogen fuses into helium then helium to carbon creating larger nuclei. Such large stars in later life can have shells or layers with heavier nuclei towards their centres. It is not only the life expectancy of a star that depends on its mass, but also the way which it dies.

o Older stars have outer layers in which hydrogen is the fuel for fusion, while the inner layers helium is the fuel, and for massive stars there may be further layers beneath. Most stars, including the sun become red giants after the end of their equilibrium phase.

o This process is started by cooling in the inner core, resulting in reduced thermal pressure and radiation pressure and so causing gravitational collapse of the hydrogen shell. But the gravitational collapse provides energy for heating the shell, and so the rate of fusion in the shell increases. This makes the shell expand enormously.

o The outermost surface of the star becomes cooler, and its light becomes redder, but the larger surface area means that the stars luminosity increases.

o Meanwhile the gravitational collapse affects the core as well, and ultimately the process of fusion of helium in the core cause the outer shell to expand further and thin leaving the hot extremely dense core as a white dwarf.

o Slowly this cools and becomes a black dwarf.

o For the stars that are several times bigger then the sun, death may be even more dramatic. A core of carbon is created by fusion of helium, and once this core is sufficiently compressed then fusion of the carbon itself takes place. The rapid release of energy makes the star briefly as bright as a galaxy, as bright as 10 billion stars.

o The star explodes into a supernova and its material spreads back into the space around. In even larger stars, fusion of carbon can continue more steadily, producing still larger nuclides and ultimately creating iron nuclei. The iron nuclei also experience fusion, but these are different as they are energy consuming meaning they keep it in. The central core of the star collapses under gravity. This increases temperature but cannot now greatly increase the rate of fusion, so collapse continues. Outer layers also collapse around the core, compressing it further. It becomes denser then an atomic nucleus, protons and electrons join together to create neutrons.

o Meanwhile, the collapse of the outer layers heats these, increasing the rate of fusion so that suddenly the star explodes as a supernova. This spreads the material of these layers into space, leaving a small hot body behind a neutron star.

o Furthermore if this supernova is big enough, its gravity continues to pull the matter towards a single point with a huge gravitational field where not even light can escape from is known as the black hole.

Star pictures obtained from Internet http://www.enchantedlearning.com/subjects/astronomy

Here is an illustration of a star life cycle followed by the theory

How long a star lives for and how it dies...

How long a star lives and how it dies, depends entirely on how massive it is when it begins. A small star can sustain basic nuclear fusion for billions of years. Our sun, for example, probably can sustain reactions for some 10 billion years. Really big stars have to conduct nuclear fusion at an enormous rate to keep in hydrostatic equilibrium and quickly falter, sometimes as fast as 40,000 years.

If the star is about the same mass as the Sun, it will turn into a white dwarf star. If it is somewhat more massive, it may undergo a supernova explosion and leave behind a neutron star. But if the collapsing core of the star is very great at least three times the mass of the Sun nothing can stop the collapse. The star implodes to form an infinite gravitational warp in space, a hole. This is exemplified in a very simple diagram highlighting the consequence of each mass of the stars and what they will revolve into.

Normal stars such as the Sun are hot balls of gas millions of kilometres in diameter. The visible surfaces of stars are called the photospheres, and have temperatures ranging from a few thousand to a few tens of thousand degrees Celsius. The outermost layer of a star's atmosphere is called the "corona", which means "crown". The gas in the coronas of stars has been heated to temperatures of millions of degrees Celsius.

Most radiation emitted by stellar coronas is in X-rays because of its high temperature. Studies of X-ray emission from the Sun and other stars are therefore primarily studies of the coronas of these stars. Although the X-radiation from the coronas accounts for only a fraction of a percent of the total energy radiated by the stars, stellar coronas provide us with a cosmic laboratory for finding out how hot gases are produced in nature and how magnetic fields interact with hot gases to produce flares, spectacular explosions that release as much energy as a million hydrogen bombs

The Orion Trapezium as observed. The colours represent energy; where blue and white indicate very high energies and therefore extreme temperatures. The size of the X-ray source in the image also reflects its brightness, i.e. more bright sources appear larger in size.

The Life Cycle of a star:

In Large Stars

In hot massive stars, the energy flowing out from the centre of the star is so intense that the outer layers are literally being blown away. Unlike a nova, these stars do not shed their outer layers explosively, but in a strong, steady stellar wind. Shock waves in this wind produce X-rays; from the intensity and distribution with energy of these X-rays, astronomers can estimate the temperature, velocity and density of this wind.

Medium sized Stars

In medium-sized stars, such as the Sun, the outer layers consist of a rolling, boiling disorder called convection. A familiar example of convection is a sea-breeze. The Sun warms the land more quickly than the water and the warm air rises and cools as it expands. It then sinks and pushes the cool air off the ocean inland to replace the air that has risen, producing a sea-breeze. In the same way, hot gas rises from the central regions of the Sun, cools at the surface and descends again.

From Red Giant To supernova

Once stars that are 5 times or more massive than our Sun reach the red giant phase, their core temperature increases as carbon atoms are formed from the fusion of helium atoms. Gravity continues to pull carbon atoms together as the temperature increases and additional fusion processes proceed, forming oxygen, nitrogen, and eventually iron.

As the shock encounters material in the star's outer layers, the material is heated, fusing to form new elements and radioactive isotopes. While many of the more common elements are made through nuclear fusion in the cores of stars, it takes the unstable conditions of the supernova explosion to form many of the heavier elements. The shock wave propels this material out into space. The material that is exploded away from the star is now known as a supernova remnant.

The White Dwarf

A star experiences an energy crisis and its core collapses when the star's basic, non-renewable energy source, hydrogen which is used up. A shell of hydrogen on the edge of the collapsed core will be compressed and heated. The nuclear fusion of the hydrogen in the shell will produce a new surge of power that will cause the outer layers of the star to expand until it has a diameter a hundred times its present value. This is called the 'red giant' phase of a star's existence.

There are other possible conditions that allow astronomers to observe X-rays from a white dwarf. These opportunities occur when a white dwarf is capturing matter from a nearby companion star. As captured matter falls onto the surface of the white dwarf, it accelerates and gains energy. This energy goes into heating gas on or just above the surface of the white dwarf to temperatures of several million degrees. The hot gas glows brightly in X-rays. A careful analysis of this process can reveal the mass of the white dwarf, its rate of rotation and the rate at which matter is falling onto it. In some cases, the matter that gathers on the surface can become so hot and dense that nuclear reactions occur. When that happens, the white dwarf suddenly becomes 10,000 times brighter as the explosive outer layers are blown away in what is called a nova outburst. After a month or so, the excitement is over and the cycle begins anew.

The Supernova

Every 50 years or so, a massive star in our galaxy blows itself apart in a supernova explosion. Supernovas are one of the most violent events in the universe, and the force of the explosion generates a blinding flash of radiation, as well as shock waves analogous to sonic booms.

There are two types of supernovas:

o Type II, where a massive star explodes

o Type I, where a white dwarf collapses because it has pulled too much material from a nearby companion star onto itself.

The general picture for a Type II supernova is when the nuclear power source at the centre or core of a star is exhausted, the core collapses. In less than a second, a neutron star (or black hole, if the star is extremely massive) is formed. When matter crashes down on the neutron star, temperatures rise to billions of degrees Celsius. Within hours, a disastrous explosion occurs, and all but the central neutron star is blown away at speeds in excess of 50 million kilometres per hour.

A thermonuclear shock wave races through the now expanding stellar debris, fusing lighter elements into heavier ones and producing a brilliant visual outburst that can be as intense as the light of ten billion Suns. The matter thrown off by the explosion flows through the surrounding gas producing shock waves that create a shell of multimillion degrees gas and high energy particles called a supernova remnant. The supernova remnant will produce intense radio and X-radiation for thousands of years.

In several young supernova remnants the rapidly rotating neutron star at the centre of the explosion gives off pulsed radiation at X-ray and other wavelengths, and creates a magnetized bubble of high-energy particles whose radiation can dominate the appearance of the remnant for a thousand years or more.

Eventually, after rumbling across several thousand light years, the supernova remnant will disperse.

The Neutron Stars

The nucleus contains more than 99.9 percent of the mass of an atom, yet it has a diameter of only 1/100,000 that of the electron cloud. The electrons themselves take up little space, but the pattern of their orbit defines the size of the atom, which is therefore 99.9% open space. What we perceive as solid when we bump against a rock is really a disorder of electrons moving through empty space so fast that we can't see or feel the emptiness. Such extreme forces occur in nature when the central part of a massive star collapses to form a neutron star. The atoms are crushed completely, and the electrons are jammed inside the protons to form a star composed almost entirely of neutrons.

The result is a tiny star that is like a gigantic nucleus and has no empty space. Neutron stars are strange and fascinating objects. They represent an extreme state of matter that physicists are eager to know more about. The intense gravitational field would pull your spacecraft to pieces before it reached the surface. The magnetic fields around neutron stars are also extremely strong. Magnetic forces squeeze the atoms into the shape of cigars. Even if a spacecraft carefully stayed a few thousand miles above the surface neutron star so as to avoid the problems of intense gravitational and magnetic fields, you would still face another potentially fatal hazard. If the neutron star is rotating rapidly, as most young neutron stars are, the strong magnetic fields combined with rapid rotation create an amazing generator that can produce electric potential differences of trillions of volts.

Such voltages, which are 30 million times greater than those of lightning bolts, create deadly blizzards of high-energy particles. If a neutron star is in a close orbit around a normal companion star, it can capture matter flowing away from that star. This captured matter will form a disk around the neutron star from which it will spiral down and fall, or accrete, onto the neutron star. The in falling matter will gain an enormous amount of energy as it accelerates. Much of this energy will be radiated away at X-ray energies. The magnetic field of the neutron star can funnel the matter toward the magnetic poles, so that the energy release is concentrated in a column, or spot of hot matter. As the neutron star rotates, the hot region moves into and out of view and produces X-ray pulses.

Black Holes

When a star runs out of nuclear fuel, it will collapse. If the core, or central region, of the star has a mass that is greater than three Suns, no known nuclear forces can prevent the core from forming a deep gravitational damage in space called a black hole. A black hole does not have a surface in the usual sense of the word. There is simply a region, or boundary, in space around a black hole beyond which we cannot see.

This boundary is called the event horizon. Anything that passes beyond the event horizon is doomed to be crushed as it descends ever deeper into the gravitational well of the black hole. No visible light, nor X-rays, nor any other form of electromagnetic radiation, or any particle, no matter how energetic, can escape. The radius of the event horizon (proportional to the mass) is very small, only 30 kilometres for a non-spinning black hole with the mass of 10 Suns.

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