Stars are created out of nebulae - vast clouds of dust and gas in interstellar space. The dust and gas (mainly hydrogen, but some helium) may continue to exist in a cloud for millions of years, but if it is disturbed (by colliding with other clouds, or from the blast from a nearby supernova explosion for example) the rotating cloud may begin to collapse in on itself. As the centre of the cloud becomes more dense, the collapse accelerates due to the increasing gravitational attraction. The central region collapses faster than the outer areas, and so the outlying gas is left rotating about the centre, obscuring our view of what happens there. If the cloud is very large, it may fragment into smaller pockets, each pocket also collapsing. The collapsing clouds mark the beginning of the formation of a star. As the collapse continues, the gas in each cloud begins to warm up until the centre begins to glow. This is known as a ‘protostar.’
1H + 1H → 2H + e+ + νe
Eventually the central temperature is so high that nuclear reactions begin to occur, and the object evolves from a protostar into a true star. The star is still orbited by residual gas and dust from the original interstellar cloud. This material can then begin to form planets; but some is driven away from the region by a strong "wind" of particles and radiation from the star. We call such stars ‘T Tauri’ stars, named after the first star of this type to be observed by astronomers. By blowing away the surplus gas and dust, the young star is no longer shrouded from our view, and can be studied directly.
When stars form out of very large interstellar clouds, these may fragment into many smaller clouds. This means that we often see stars forming in groups called ‘clusters.’ One such cluster is the Pleiades.
The timescale for this formation process depends on the mass of the newly formed star. Low mass stars may take billions of years to form while high mass stars may take only a few hundred thousand years.
The nuclear reaction that begins in the star’s core is called the proton-proton
(p-p) chain. This converts hydrogen into helium in the following way:
2H + 1H → 3He
3He + 3He → 4He +21H
The energy released in this reaction is given by E=mc2, where m is the difference in mass between the helium atom produced and the two hydrogen atoms which fused to produce it. c is the speed of light.
High mass stars may also convert hydrogen to helium via the CNO cycle. This is only possible in more massive stars as a higher temperature is required for the reactions to begin.
12C + 1H → 13N + γ
13N → 13C + e+ + νe
13C + 1H → 14N + γ
14N + 1H → 15O + γ
15O → 15N + e+ + νe
15N + 1H → 12C + 4He
During this stage of their evolution, stars lie on the Main Sequence of the Hertzsprung-Russell diagram. This is where they spend the majority of their lifetime. During the Main Sequence stage, the star will remain approximately constant in size and luminosity due to a balance in gravity pulling the star inwards and thermal pressure within the gas pushing it out. Our Sun is currently half way through its Main Sequence lifetime and will remain in the same phase for approximately another 5 billion years.
The time spent on the Main Sequence depends on a star’s mass. More massive stars are hotter and will therefore have higher fusion reaction rates, so will burn their fuel more quickly than smaller, cooler stars.
Once the hydrogen in the star’s core has been depleted, the nuclear reaction will stop. This causes the core of the star to contract and the outer layers to expand and cool, forming a Red Giant. When this happens to the Sun it will expand out to a radius of approximately 1 AU. This is seen as an evolution up and slightly to the right on the H-R diagram, up the Red Giant Branch (RGB).
The contraction causes the core to heat up. If it reaches a critical temperature, helium burning will begin. In a solar type star this will happen very suddenly, in the ‘Helium Flash’. The star will then settle down to a second stable burning phase, converting helium to carbon. Hydrogen burning also still continues in a shell around the helium core. In a more massive star the Helium Flash will not occur as the burning will start more slowly.
From this point onwards the evolution of the star depends entirely on its mass.
Once the helium in the core is depleted, it will enter a second Red Giant phase. The core will contract and heat again, but this time will not reach the temperature required to start carbon burning. This causes the core to contract further, and the outer layers of the star to be thrown off in a shell known as a ‘planetary nebula’. This can be clearly seen as it glows due to heating from the stellar core.
The small, dense core of the star is left behind, and is called a white dwarf. The density is such that a sugar-cube sized piece of the white dwarf would weigh as much as two large polar bears. The evolution through planetary nebula stage to white dwarf is shown on the H-R diagram below.
The white dwarf will slowly cool until is can no longer be seen. It is then called a ‘black dwarf’ and is at the end of its lifetime.
An animation showing the evolution of a solar mass star from a gas cloud to a white dwarf can be seen by clicking here
Once the helium has been depleted, the core will contract again, leading to a second Red Giant phase. As the core contracts, it heats up, and reaches the temperature required for carbon burning. This process continues through oxygen and silicon, possibly burning all the way up to iron if the stellar mass is great enough for each new critical temperature to be reached. By this time the structure of the star will be layered like an onion. Once iron is made in the core, no further fusion reactions can take place as it is no longer energetically viable: iron is at the top of the binding energy curve, so any further fusion requires more energy to be put in than is released in the reaction.
At this point the core collapses very quickly to a critical size, where it stops suddenly, sending out a blast wave which causes the outer layers to explode in a supernova.
The very small dense core is left behind as a ‘Neutron Star’. This has a radius of approximately 10km, but a mass of up to three times the mass of the Sun. It is so dense that one cubic centimetre would weigh as much as the entire population of Earth – that’s 6 billion people packed into a volume the size of a sugar cube.
Very massive stars
If a star is more than 25 times the mass of the Sun, it will burn up to iron in the same way as a few solar-mass star. However, when the core collapses, the gravity is so great that there is no critical core radius – it just keeps collapsing to form a black hole.
Evolution on the Hertzsprung-Russell Diagram
The Hertzsprung-Russell Diagram is a plot of luminosity against temperature. As a star evolves, its luminosity and temperature change, so it moves around on the H-R diagram. The top left area represents hot, bright stars, top right is cool bright stars, bottom left is hot, dim stars, and bottom right is cool, dim stars. There are four main areas that can be identified on the diagram. The first is the Main Sequence, which lies in a band diagonally from top left to bottom right. This is where stars spend most of their life while they are burning hydrogen into helium. The Sun is currently on the Main Sequence, about half way through its MS lifetime. It is a yellow star, at a temperature of about 6000 K so can be found about half way along the MS. Hot, blue stars such as Sirius can be found near the top left, while cooler redder stars are near to the bottom right. The second area is the giant branch, where the Sun will evolve as it swells to become a red giant. Above this are the supergiants. Finally, there is the white dwarf area to the bottom left of the diagram. This is where the Sun will end up at the end of its life.
The first part of a star's life is the evolution from a gas cloud onto the Main Sequence. This is shown as the solid line on the diagram below. The luminosity of the protostar drops as it shrinks from a large cloud to a spinning ball of gas, but it heats up slightly as it does so.
Next the star will begin to burn hydrogen in its core and will settle down onto the Main Sequence for the majority of its lifetime. The star will slightly increase in temperature and luminosity while it is on the MS. The Sun is currently at this point in its lifetime.
As the hydrogen in the star's core begins to run out, the star will leave the MS and swell to become a red giant. The temperature drops, but because the star is so much bigger, the luminosity increases.
As the fuel in the star's core becomes totally depleted, the star begins to swell and shrink, throwing off its outer layers. This gas is puffed out into space and surrounds the star in a shell, forming a planetary nebula. The luminosity stays about the same during this process, but as more and more layers are shed, the very hot core becomes more visible, so the temperature of the star increases dramatically.
Finally, the core of the star is left. There is no more burning in the core and the star, now known as a white dwarf, simply cools until it can no longer be seen. Both the temperature and luminosity drop.
Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001