Birth of a Star
The ages of stars vary from young to old but most of the stars that you see in the sky are actually not there anymore as they are really far away. Our closest star the sun was formed about 4.5 billion years ago.
Stars are created as the nebula slowly contracts under its own gravity caused by gravitational attraction and a clump of matter forms inside the cloud. Gravity continues pulling in more matter and the clump collapses inward. The clump becomes denser and begins to heat up. Eventually the core of the collapsing clump of matter becomes so compressed and so hot that nuclear reactions start. Hydrogen fuses to form helium which releases energy and causes the star to shine.
STAR
A star is a luminous ball of gas held together by gravity. It produces its own heat and light by nuclear fusion. They are born from nebulae and contain mostly of hydrogen and helium gas. Surface temperatures of stars range from 2000oC to above 30,000oC. The brightest stars have masses 100 times that of the Sun and produce as much light as millions of Suns. They live for less than a million years before exploding as supernovae. The dimmest stars are the red dwarfs, less than one-thousandth of the brightness of the Sun.
The smallest mass possible for a star is about 8% that of the Sun (80 times the mass of the planet Jupiter), or else nuclear reactions don’t take place. Objects with mass less than 8% shine faintly and are dubbed brown dwarfs or a large planet. Towards the end of its life, a star like the Sun swells up into a red giant until it finally shrinks, becoming a white dwarf.
RED GIANT
This is a large bright star with a cool surface. It is formed during the later stages of the evolution of a star, as it runs out of hydrogen fuel at its centre. Red giants have diameters between 10 and 100 times that of the Sun. They are very bright because they are so large, although their surface temperature is lower than that of the Sun, about 2000-3000oC.
Very large stars (red giants) are often called Super Giants. These stars have diameters up to 1000 times that of the Sun.
RED DWARF
These are very cool, dim and small stars, roughly one tenth the mass and diameter of the Sun. They burn very slowly and have estimated lifetimes of 100 billion years. Barnard's Star is a red dwarf.
WHITE DWARF
This is very small, hot star, the last stage in the life cycle of a star like the Sun. White dwarfs have a mass alike to the Sun’s, but only 1% of the Sun's diameter; totalling to a white dwarf roughly the diameter of the Earth’s. The surface temperature of a white dwarf is 8000oC or more.
White dwarfs are the shrunken leftovers of normal stars, whose nuclear energy have been used up. White dwarfs consist of matter with a very high density due to gravitational effects, example: one spoonful of its matter has a mass of several tonnes. White dwarfs cool and fade over several billion years.
SUPERNOVA This is the scorching, explosive death of a star, and often results in the star gaining the brightness of 100 million suns for a short time. There are two general types of Supernova:
1) These occur in dual star systems in which gas from one star falls on to a white dwarf, causing it to explode.
2) These occur in stars ten times or more as massive as the Sun, which suffer internal nuclear fusions at the ends of the stars lives, leading to an explosion. They leave behind neutron stars and black holes.
NEUTRON STARS
These stars are formed primarily of neutrons and are produced when a supernova explodes, making the protons and electrons to combine to produce a neutron star. Neutron stars are very dense. Usual stars have a mass of three times the Sun but a diameter of only 20 km. If its mass is any greater, its gravity will be so strong that it will shrink further to become a black hole.
BLACK HOLES
Black holes are believed to form from massive stars at the end of their lifetimes. The gravitational attraction in a black hole is so huge that nothing can escape from it, not even light. The density of matter in a black hole cannot be measured. Black holes alter the space around them, and can often suck neighbouring matter (planets, asteroids, rock) into them including stars.
Death of a Star
When the hydrogen is used up the helium core collapses and the outer layer expands and cools. The star shines less brightly and is now called a red giant. The death of a star after the red giant stage depends on the size of the star.
Small Star: After a small e.g. our Sun has become a red giant its outer layer drifts away into space. The hot dense core that remains is called a white dwarf. The white dwarf cools and eventually stops shining.
Massive Star: After a massive star has become a red giant, nuclear reactions in the helium core continue and form other elements around an iron core. The collapse of this core causes an explosion called a supernova. The remains of the core form a tiny, very dense neutron star.
Really Massive Star: These stars are similar to massive stars. From the red giant stage nuclear reactions create an iron core which explodes as a supernova and forms a neutron star. The difference for really massive stars is that the core of the neutron star collapses further and this results in a black hole.
