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Stars – from Birth to Death

Born in the cosmic clouds (which can be up to 300 light-years across) known as nebula (meaning cloud) or “stellar nurseries”, stars form almost by chance.

The Birth of Stars

Plasma

Nuclear Fusion in Stars

The Life of Stars

Hydrogen Fuelled Stars (Stage I)

Helium Fuelled Stars (Stage II)

Carbon Fuelled Stars (Stage III)

White Dwarf Stars

Supernova

Neutron Stars

Pulsars

Black Holes

 

 

 

The Birth of Stars

 

Born in the cosmic clouds (which can be up to 300 light-years across) known as nebula (meaning cloud) or “stellar nurseries”, stars form almost by chance. In these incredibly vast accumulations of gas, often the result of a supernova (the explosive death of extremely large stars), is the potential for life. A slight movement of the matter that forms these clouds can cause a small clump to form. This clump will have a slight increase in mass and therefore, in gravity drawing more matter slowly towards its centre. This seemingly insignificant event is the first step towards creating a star and even a solar system like ours.

 

Public Domain NASA The Eagle Nebula Med Zoom.PNG

Image Credit NASA

 

As the clump draws in more matter it grows larger. Other clumps may have formed which too would slowly attract each other through their gravitational pull. Either way the mass will increase over millions of years and due to the conservation of angular momentum causes the clump to spin (similar to what causes water to spin as it falls down the plug hole).

 

 

Plasma

 

The sphere that has formed at the centre of the disc grows dramatically. As its mass and density increase they cause its temperature to rise (temperature is a measurement of how fast particles are moving) and when the temperature reaches approximately 10,000 degrees centigrade, the matter stops being a gas and becomes plasma.

 

In this state the particles, which are mainly hydrogen gas, are full of energy from the immense heat. When hydrogen atoms are under enough pressure and subjected to enough heat their bonds are weak and the electrons break free of the protons.  When this happens to many atoms on a large scale they are said to be in a state of plasma (commonly referred to as a soup of particles). The mass begins to collapse in on itself due to its own gravity causing the pressure to increase very quickly. The collapsing process would continue but when the temperature reaches some 10 million degrease centigrade the process of nuclear fusion begins and the energy created from this reaction acts against the forces of gravity, postponing the gravitational collapse.

 

 

Nuclear Fusion in Stars

 

(Around 10 million degrease centigrade)

Deuterium.gifIn an environment such as this electrons and protons move at phenomenal speed colliding with one another. Normally the electromagnetic force causes protons to repel each other due to their positive charge, much like the same poles on a magnet. Electromagnetic force which causes the repulsive action quadruples as the distance between the two protons halves. This means that it takes a lot of energy to get these two protons together. If they are able to get close enough another fundamental force, The Strong Force, comes into play and this force is much stronger than the electromagnetic force. It is the force that holds all nucleons together but they must be within one femtometres (the nucleus of an average atom is 4 femtometres in diameter). However with enough force, they will fuse together creating Deutronium (a stable isotope of hydrogen). This reaction also produces energy which is explained in the higher box A

 

Higher A.PNG

 

When one of these deuterium atoms collides with a normal hydrogen ion they also fuse together to form the helium isotope He3. As helium-3 the atom has two protons and one neutron whilst also releasing another photon of energy.

 

When two of these He3 atoms collide, which they inevitably will, they fuse together to create a normal helium atom He4 and releasing two protons which rejoin the system at an earlier stage.  

 

Higher B.PNG

 

These particles fuse together to make deuterium and helium. Two hydrogen atoms will split apart to form a hydrogen atom. As the initial four hydrogen ions had greater mass then the newly formed helium-4 atom, then mass has been lost during the reaction would have been converted into energy. As Einstein stated with E=mc2 the lost mass is converted into … energy. They energy produced from these reactions is much higher than the energy required to start the process (this is the reason behind projects such as ‘K Star’ and the ‘Jet Project’ which are attempts to create a small star like reaction on earth for almost limitless power).

 

Now a self-sustaining nuclear fusion reaction has begun, turning matter into energy which is released as light and heat. A star is born.

 

Birth of Star credit NASA.PNG
Image Credit NASA

 

 

 

The Life of Stars

 

Stars live for many billions of years but just how long is dependent mainly on their size. From the moment of their creation, when the pressure of their own gravity has begun the process of nuclear fusion, an battle begins between gravity trying to collapse it and the outward force from the creation of energy.

 

First nuclear fusion begins combining hydrogen as with our own sun but eventually the hydrogen is depleted and stage two begins with helium as the fuel. This continues with carbon and when all fuel is depleted the star will take a new form depending on its size. 

 

 

Hydrogen Fuelled Stars (Stage I)

 

Basically put once enough mass, such as is found in stars, is congregated together a process is started whereby gravity forces the body to collapse in on itself creating a singularity. However this process causes other events which stop or postpone this collapse. The first event happens when pressure on the hydrogen gas causes it to reach around 10 million degrees centigrade and the process of nuclear fusion begins as explained above. This nuclear reaction, and the energy release because of it, prevents the sun from continuing to collapse, but this process doesn’t continue forever. Eventually the hydrogen gas which is used as fuel for the reaction is depleted.

 

 

Helium Fuelled Stars (Stage II)

 

After some 5 billion years when the hydrogen is all but gone the nuclear fusion reaction stops and with no outward force the sun once more continues to fall in on itself. As it collapses once more the gravity causes the atoms to move much faster and the pressure raises the temperature to around 100 million degrease centigrade allowing the helium, much of which was produced in the initial fusion process, to start its own nuclear fusion reaction. This time two helium atoms fuse to form an isotope of beryllium (beryllium-8) which has four protons and four neutrons. Beryllium-8 however doesn’t last for long and quickly decays back into two helium atoms once more and this entire reaction can take place in a minute fraction of a second. But once in a while the beryllium atom is fused with a third helium atom to produce carbon. This reaction again produces energy which counteracts the gravitational force which caused the collapse.

 

 

Carbon Fuelled Stars (Stage III)

 

When the Helium reserve run out then the nuclear reaction stops leaving no resistance to gravity. Once again the star collapses on itself and the pressure causes the temperature to increase once more to around 500 million degrease. At this temperature the carbon atoms begin a process of nuclear fusion and all the elements from carbon to iron are produced.

 

Why only up to iron? Well iron is a very special atom. When all the elements lighter than iron are produce through nuclear fusion they are lighter then the some of their parts and therefore energy is released to account for the loss of mass (energy is released when atoms are fussed together)

 

However all the elements heavier than iron weigh more than the sum of there parts and so when these heavier elements are split through nuclear fission energy is released to account for the loss of mass (energy is released when the atoms are split apart).

 

Iron on the other hand is perfectly stable so when the star has predominantly iron in its core then no fuel is available to continue nuclear fusion.

 

 

White Dwarf Stars

 

With no more fuel to counter act the gravity the sun’s collapse continues and the atoms become pact incredibly tightly. At this point a singularity seems inevitable but there is another law of physics that comes into play. Electrons that are packed tightly together produce an outward force and as the atoms are squashed close and closer the force from the electrons grows. At some point this force equalises with that of the gravity and a perfect equilibrium is reached. In this state the sun may remain for all eternity and is the likely fate of our own sun and any star that is less than 1.4 solar masses (our sun is 1 solar mass).

 

 Dwarf Stars Higher C.PNG

 

 

Supernova

 

Suoernova of a Star 
On January 21, 2014, astronomers witnessed a supernova soon after it exploded in the Messier 82, or M82, galaxy

 

If a star is larger than 1.4 solar masses then there exist two possible fates for the star based on its size. First a huge explosion known as a supernova occurs, where the outer layers of the star are blown away leaving just the core of the star. This core may become a neutron star or a black hole. It is believed that a supernova explosion occurred and our solar system was born from the expelled matter. As previously stated a sun will only produce elements as heavy as iron yet on earth we have many elements that are much heavier so where did they appear from. Well when a giant star dies and a supernova explosion occurs the energy from the explosion if sufficient enough to produce all the elements that are heavier than iron

 

(A star won’t produce them because it requires energy and if the star doesn’t produce more energy than it uses then there is nothing to counteract its collapse. But the energy from the supernova could be considered ‘lost’ energy and is therefore available to produce heavier elements).

 

 

Neutron Stars

 

Smaller stars but those that are still at least four times that of our own will form a neutron star. The core left over after a supernova continues its collapse the electrons and protons reach phenomenal speeds (close to the speed of light which is 299,792,458 m/s) they have enough energy to create their own nuclear fusion. At this sort of speed protons and electrons can fuse together and create neutrons and this process continues until neutrons are all that remain.

 

In an atom nearly all the mass is made up by the nucleus but most of its size is empty space. In a neutron star the electrons and protons no longer exist, only neutrons and with no electrons there is no empty space. This means that the neutrons are packed together in one giant nucleus.

 

Neutron stars are very massive but very small stars packed so tightly together a star that was once more massive than or own would take up the space of a large city. Many are no more than 20 kilometres in diameter.

 

Neutron Star Credit NASA.jpg
Image Credit NASA/Dana Berry (a neutron star forming in the centre of a supernova of a very large star)

 

 

Pulsars

 

Pulsar Credit NASA.PNG
Image Credit NASA

 

The most common type of neutron star is called a pulsar. they were first detected in 1967 by a team of physicist led by Jocelyn Bell at Cambridge University. Using a radio telescope they detected a pulse at a precise rate of 30.2 times a second. It later became apparent that what they detected was a type of neutron star. Neutron stars produce a very powerful electric fields similar to the magnetosphere of earth but vastly more intense. This magnetic field spins by the force of the spinning star and occasionally the spin of the magnetic field has the same spinning axis as that of the Star. In this case neutron stars are very difficult to detect due to their minute size in space. But more often than not the spin axis of the magnetic fields is tilted compared to the spin axis of the star and this creates funnels at either end of high energy radiation. These high energy beams are shot into space and can be detected with radio telescopes as pulses produced by the spin. This is how they gained the name pulsars.

 

 

Black Holes

 

Neutron stars, on their own, will not collapse in on themselves completely and will remain as a neutron star spinning in space for all time. However if a neutron star is part of a binary system (a solar system that has two stars in its centre that orbit each other) then some of the matter from the other star begins to be attracted to the gravity of the neutron star. This additional gravity can push the star over the edge and cause the star to collapse into itself forming a black hole.

 

Alternatively when supermassive stars that are around 3 solar masses go supernova and die they will become black holes directly and not become a neutron star. After the supernova explosion the supermassive core is so great that nothing will prevent its entire collapse and in this event they become black holes. A small point in space that is so dense and produces so much gravity that not even light can escape. In fact the only way that we know they exist is from the bending of light caused by their immense gravitational pull or from observing stars that obit them.  Many such black holes exist in the centre of galaxies including our own Milky Way.

 

Black Hole Credit NASA.png
Image Credit NASA/Dana Berry

 

 

Sources:

Why does E=mc2? (Professors Brian Cox and Jeff Forshaw – Da Capo Press – UK ISBN: 978-0-306-81911-7)

http://www.nasa.gov/mission_pages/GLAST/science/neutron_stars.html