Evolution of Our Star

[Originally submitted by me as part of the first year of my degree course]

A look at the birth, life and eventual death of the star our planet orbits, with comparisons to stars of different mass

Introduction

Star : "A self-luminous celestial body, such as the sun, that generates nuclear energy within its core. Stars are not distributed uniformly throughout the universe, but are collected together in galaxies. The age and lifetime of a star are related to its mass."

The predominant constituent of a star is hydrogen, with helium as the other major constituent. All stars follow the same general sequence through their lives: Protostar – pre- main sequence – main sequence – post- main sequence.

The minimum mass required for star formation is about 1/20 times the mass of the sun. With less mass than this, there is not enough gravitational energy released when the mass condenses to raise the temperature to the point at which the fusion of hydrogen can begin. At the other end of the spectrum, the most massive stars known are around 100 times the mass of the sun. The mass of a star is the prime factor in determining the evolution of the star, notably affecting the luminosity and temperature of the star during its main sequence stage.

The complete life cycle of a star is a process which can take anywhere between a few million years and billions of years to complete, this time scale also depending on the mass of the star involved: Less massive stars typically have longer life spans than those with greater masses: The lifetime of a star is inversely proportional to its mass.

Our star, the Sun, is a typical example of a main sequence dwarf star with a mass of 1.99*1030 kg and a radius of around 700,000 km. It is in the region of 4.55 billion years old and has used up approximately 50% of its core hydrogen supply.

The aims of this essay are to outline the birth, life and death of the Sun, and describe the differences observed with stars of masses lower and higher than that of the star at the centre of our solar system.

A star is born

When a nebula (an immense cloud of gas in space) interacts with a neighbouring nebula, or other phenomena (such as a nearby supernova), the cloud may start to collapse in on itself as it rotates. As it becomes smaller some of the energy stored in the cloud is lost as heat, keeping the cloud cool. The density of the cloud rises and it becomes more and more difficult for energy to escape, so the cloud’s temperature rises, particularly at the centre. As the centre of the cloud becomes denser, the gravitational attraction of the gas increases and so the rate of collapse increases. This means that the cloud collapses faster in the centre of the cloud than around the outer regions of the cloud which form a disk around the main centre. The cloud, if it is very large, may split into smaller pockets of gas which may themselves be sufficient for forming new stars by themselves. This gives rise to star clusters such as the Pleiades.

If the cloud contains enough matter, the temperature will continue to rise until it reaches 15 million Kelvin which is sufficient for the nuclear fusion of hydrogen in the cloud into helium to begin. This releases more energy, heating the ‘Protostar’ further, raising the internal pressure enough to halt the collapse of the Protostar.

This star has now evolved from a Protostar into a ‘true star’ at the beginning of its main sequence. The star is surrounded by a cloud composed of the remnants of the original nebula, most of which is blown away by radiation emitted by the new star. A dense rotating disc of gas and dust remains which may eventually dissipate, leaving behind a collection of planets orbiting the star.

The time period that the process of star formation up to the beginning of the main sequence occupies varies according to the mass of the material used in the formation:

For low mass stars, the entire process may take many billions of years, whereas for larger stars formation may take only a few hundred thousand years. For stars with less than 1/20 times the mass of the sun, a similar process takes place but the temperature never rises high enough to ignite the nuclear fusion reactions in the core, and so the ‘star’ becomes nothing more than a cool ball of matter in space, known as a ‘brown dwarf’.

In the case of our Sun the process of formation took around 50 million years, and the hydrogen fusion reactions began about 4.5 billion years ago.

The life of a star

The life of a sun- like star, with examples specific to our Sun
A star such as our Sun is about three- quarters hydrogen, the rest being mostly helium with some other heavier elements and spends about 80% of its total lifetime (around 10 billion years) transforming the hydrogen in its core to helium in its stable ‘main sequence’ phase. This hydrogen burning releases an enormous amount of energy.

As more helium is produced, the central regions of a sun- like star begin to contract and therefore the temperature of the central regions increases which causes an increase in luminosity- the star appears brighter to an observer.

1 billion years from now, an observer on Earth will see the sun as roughly 10% brighter than it is today, and in 3.5 billion years the sun will appear 40% brighter than it is today, causing conditions on the Earth to be similar to those on Venus at the present time.

Eventually the core is made entirely of helium, and the fusion process which converts the hydrogen into helium occurs in a shell which surrounds it. As more helium is produced, the central core is made larger, pushing the reaction shell closer to the surface of the star.

The energy given out by this shell pushes the outermost regions away from the star. These regions cool, and the star expands. At this stage the star is known as a Red Giant and may be hundreds of times larger than when it first appeared on the main sequence.

After another 1.9 billion years the sun will use up the last of the hydrogen in its core and as a result it will expand to over 1.5 times its present size, and will be more than twice as bright. Over the next 700 million years its brightness will stay at a similar level, but the radius will increase from 1.6 to 2.3 times the current radius of the sun, and it will cool from 5517 to 4902Kelvin.

When the sun is about 12 billion years old the radius will increase to over 165 times its present radius, destroying Mercury. The sun will become more than 2 thousand times brighter than it is today and the temperature will drop again to 3107Kelvin.

As the helium core becomes denser, the temperature increases. Eventually the temperature reaches about 100 million Kelvin which is sufficient for the star to begin new reactions, converting the helium core into carbon.

The shell of hydrogen around the helium core will become weaker causing the sun to shrink slightly, ceasing to be a Red Giant. This will continue over a period of 1 million years and then settle into a period of stability as a helium burning star with a radius of about 9.5 times that of its current measurement. Its temperature will also increase to around 4724 Kelvin and will become fainter at around 41 times the suns present luminosity. The helium- burning stage will continue for a time period in the region of 110 million years, and the helium core will be transformed into a carbon core surrounded by a shell of helium. Again, the core will increase in size, and the helium shell will push the outer layers of the sun away, causing them to expand and cool once more making the star a Red Giant.

The sun will expand once more to 18 times its current radius, becoming 110 times brighter and cooling to 4450Kelvin.
As the star grows, the outer layers of the sun evaporate off into space and consequently the mass of the star is reduced during this phase.
The sun will experience a reduction in mass so that it is now around 46% of its original mass.

Around 1 million years later the nuclear reactions in the core cease as the temperature never becomes great enough to begin a new phase of reactions, so only the shells of helium and hydrogen burning close to the surface provide energy for the star.

After this point, any observer from our solar system is truly hypothetical, as all of the inner planets have been consumed by the incomprehensibly hot atmosphere of the sun in its red giant phases.
The helium shell becomes unstable, and once every few hundred thousand years, the star pulses violently, each pulse throwing off more mass than the previous. It is predicted that there are four pulses in total.
Every 100 thousand years the sun will pulse up to almost the radius of the Earth’s orbit, and become many times brighter.

The life of stars with less mass than that of our sun
A less massive star burns its supply of hydrogen more slowly than larger stars, lasting longer but being smaller and less bright throughout its lifetime- a star with 1/10 times the mass of the sun will be approximately 1/10,000 times as bright and will last 1,000,000,000,000 years- this time period is longer than the current age of the universe.

The life of stars with more mass than that of our sun
The more massive a star is, the faster it uses its supply of hydrogen so its life on the main sequence is shorter but it is bigger, brighter and hotter than a star of similar mass to the sun- a star 10 times as massive as the sun will burn 10 thousand times more brightly, but its life will only last around 100 million years.

Another difference is as follows: if the star is massive enough, where a sun- like star would stop with a hydrogen core and helium and hydrogen shells burning close to the surface, the increased mass allows a new set of reactions to commence and carbon burning commences in the core. This will be discussed further in the star death section of this essay.

Star death

Death of the sun and sun- like stars (up to 1.4 times the mass of the sun)

Over a period of time encompassing thousands of years, the Red Giant’s inner regions shrink, heating up. This throws off the outer layers of the star. What is observed is called a Planetary Nebula- a small, dense core surrounded by a cloud of gas which appears to be expanding. Planetary nebula can take many forms: they can be ring shaped, circular, dumbbell- like or irregular.

The core of the planetary nebula is the core of the original star. This is still very hot, with a temperature of about 100 thousand Kelvin, but gradually cools and contracts to form a white dwarf . Over billions of years the core cools further to eventually form a non- luminous dead star called a black dwarf, which is not much larger than the planet Earth.

The hydrogen in the outer shell of the star continues to burn, but is eventually used up completely. This star cools over a period of billions of years to become a brown dwarf.

Death of stars of greater mass than the sun

Stars with a mass up to 1.4 times that of the sun end their lives in the same way as sun- mass stars- they end as white dwarfs, and cool to black dwarfs.

Stars which are slightly more massive end their lives in much the same way, except that the final white dwarf is made up of a different mixture of elements, including some of the heavier elements due to the carbon burning stage introduced by the greater gravitational energy provided by a greater mass.

Stars more than five times more massive than the sun end their lives in a much more spectacular fashion- as already stated in the previous section, stars many times more massive than the sun develop carbon burning in their core. During this stage, temperatures can reach 100 million Km and many new chemical elements are manufactured. Once the core of the star consists mainly of iron, no more energy can be generated- manufacture of the heavy elements up to iron creates energy, adding to the balance between gravity and radiation. After iron, the manufacture of elements requires more energy, so there is no more energy available to act against the pull of gravity. The stars core then collapses in less than one second. Protons and electrons in the atoms in the core are crushed together to form neutrons, releasing a torrent of neutrinos which carry away most of the energy from the explosion. This collapse releases enough energy to blow away all of the outer layers of the star and all elements heavier than iron in a violent and colourful explosion which is brighter than the collective light of all the other stars in the galaxy. This is a supernova.

What is left over is a neutron star- the central core of the star composed entirely of neutrons, and a rapidly dispersing cloud of gas, called a supernova remnant. The neutron star has a mass close to the sun’s, but a radius of only about 5 kilometres. A pulsar is a rapidly rotating neutron star, with a beam of radio emission out of each pole. The pulses are extremely regular, and all pulsars slow down as they lose rotational energy.

Overview

To simplify, our sun will go through the following seven phases during its lifetime:

1. Hydrogen burning: The temperature of the collapsing cloud of gas increases sufficiently to ignite the hydrogen fusion reactions, converting hydrogen from the core into helium. This phase will last about 10.6 billion years.
2. First red giant: The last of the hydrogen is used up in the core, and the sun swells to 1.5 times its original size, and more than twice as bright. Over the following 700 million years the star swells gradually. This phase will last about 1.3 billion years.
3. Helium burning: The temperature of the core becomes sufficient to ignite the fusion reactions converting helium in the core into carbon. This phase will last about 100 million years.
4. Second red giant: The sun expands, becomes many times brighter and cools. This phase will last about 20 million years.
5. Unstable pulsation: The sun pulses violently between 100 thousand year intervals, throwing off more matter each time. This phase lasts about 400 thousand years.
6. Planetary nebula: The outer layers of the sun are thrown off, exposing the inner core. This is surrounded by an expanding cloud of gas which eventually dissipates. This phase lasts for about 10 thousand years.
7. White dwarf/ black dwarf: The sun contracts and cools, turning into a white dwarf, and eventually a brown dwarf. This phase is the last, and lasts indefinitely.

Bibliography

• http://www.star.le.ac.uk/edu/stars/evolution.html ("Stella Evolution")
• http://www.rog.nmm.ac.uk/leaflets/star/star.html ("What Is A Star?")
• http://www-astronomy.mps.ohio-state.edu/~pogge/Lectures/vistas97.html ("The Once And Future Sun")
• http://www.seds.org/messier/m/m001.html ("Supernovae")
• Introductory Astronomy and Astrophysics (Fourth Edition), Zeilik and Gregory, Saunders College Publishing, 1998, Pgs 318-326
• Oxford Reference Shelf, Oxford University Press, 1994
• Dictionary of Astronomy (Third Edition), Jacqueline Mitton, Penguin, 1998
• Lecture notes