The Star Garden
Stars
Approximately three generations of stars have formed since the big bang and the Sun is an example of the latest, having formed about four and a half billion years ago.
Stars form when clouds of hydrogen are disturbed. Turbulence pushes matter together and this forms rotating, dense regions which eventually collapse under their own gravitation, becoming protostars. Magnetic fields prevent clouds from collapsing in their entirety and reduce the angular momentum of new stars, which stops them flying apart. As the clouds collapse, gravitational potential energy is converted to kinetic and then thermal energy, causing the centre to heat up. As it becomes more massive, it collapses even further and this triggers nuclear fusion at the centre. This energy prevents the new star from collapsing any further and makes it shine.
Stars that are at least eight times as massive as the Sun are usually known as high mass stars. It is not currently known how high mass stars form as they begin emitting nuclear energy before they have enough mass to remain intact and so they should fly apart. It is important that we come to understand how high mass stars form as they are responsible for producing most of the heavy elements in the universe and vastly effect the structure and evolution of galaxies.
When protostars become hot enough to fuse hydrogen into helium at their core, they are known as 'main sequence' stars. The term main sequence refers to a stars position on the H-R diagram, a method of representation first devised by Danish astronomer Ejnar Hertzsprung in 1911 and independently discovered by American astronomer Henry Norris Russell in 1913. The H-R diagram is a scatter graph which plots the luminosity of stars, which is related to their mass, against their temperature, which is related to their colour. Massive main sequence stars are brighter and bluer than less massive ones, they also have shorter lifetimes.
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The H-R Diagram Image credit: Atlas of the Universe
The Sun is a relatively low mass main sequence star. In five billion years time, when it runs out of hydrogen to fuse in its core, it will no longer be producing enough nuclear energy to counterbalance the force of gravity and so it will contract slightly. This allows hydrogen fusion to begin in the shell around the core. The outer atmosphere of the Sun will become larger, redder and more luminous, extending to the Earth. At this point the Sun is no longer referred to as a main sequence star, stars like this are known as red giants.
The helium core will continue to collapse until it becomes dense enough for fusion to begin again, creating carbon. After one hundred million years or so, the core will be entirely made of carbon. When the fusion stops, the core will contract again and the outer atmosphere will expand, extending to Jupiter. Stars like this are referred to as red supergiants. In the next few tens of thousands of years, the Sun's outer atmosphere will be driven away, leaving a planetary nebula, ionised by the hot carbon core. As the core cools, it will become a white dwarf about the size of the Earth. When a white dwarf stops producing light it is known as a black dwarf. White dwarfs are expected to burn for well over fourteen billion years and so the universe is not old enough for any to exist yet.
Planetary Nebula; NGC 6751, Butterfly Nebula, Stingray Nebula, Cat's Eye Nebula, Eskimo Nebula, Ring Nebula
A high mass star will continue to fuse carbon into oxygen, and go on to create a core of neon, oxygen, silicon and eventually iron. Iron is so massive that fusion would consume rather than produce energy, the core rapidly collapses. It becomes so dense that electrons and protons combine to form neutrons and is then known as a neutron star. Neutron stars are typically about twenty five kilometres in diameter and can rotate over six hundred times a second. If a neutron star is rotating at the right angle, then a strong magnetic force can be detected at its poles. These stars are known as 'pulsars'.
When it can become no denser, in-falling matter bounces off in a supernova explosion, producing most of the natural elements heavier than iron. These will contribute to the planets, moons, asteroids and comets which will orbit the next generation of stars. For about a month, the star will be brighter than the whole galaxy.
An even more massive star will collapse even further, becoming a black hole. Anything which travels past a point known as the 'event horizon' will no longer be able to escape. A non-rotating black hole about ten times the mass of the Sun will have an event horizon of about sixty kilometres in diameter, this diameter will increase by about six kilometres for every Solar mass that falls into it.
References
See NASA's profile of stars.