Life Cycle of Stars
The Birth of a Star
In space, there exists huge clouds of gas and dust. These clouds consist of hydrogen and helium, and are the birthplaces of new stars. Gravity causes these clouds to shrink and become warmer. The body starts to collapse under its own gravity, and the temperature inside rises. After the temperature reaches several thousand degrees, the hydrogen molecules are ionized (electrons are stripped from them), and they become single protons. The contraction of the gas and the rise in temperature continue until the temperature of the star reaches about 10,000,000 degrees Celsius (18,000,000 degrees Fahrenheit). At this point, nuclear fusion occurs in a process called proton-proton reaction. Briefly, proton-proton reaction is when four protons join together and two are converted into neutrons; an 4He nucleus is formed. During this process, some matter is lost and converted to energy as dictated by Einstein's equation. At this point, the star stops collapsing because the outward force of heat balances the gravity.

The Hydrogen Burning Stage
The proton-proton reaction occurs during a period called the hydrogen-burning state, and its length depends on the star's weight. In heavy stars, the great amount of weight puts a large amount of pressure on the core, raising the temperature and speeding up the fusion process. These heavy stars are very bright, but only live for a short amount of time. After the energy from this deuteron-hydrogen fusion process ends, the star begins to contract again, and the temperature and pressure subsequently increase. Nuclear fusion occurs between the hydrogen and lithium & other light metals in the star, but this process soon ends. Contraction starts again, and the extreme high temperature and pressure cause the hydrogen to transform into helium through the carbon-nitrogen-oxygen cycle. When all the hydrogen has been used up, the star is at its largest size, and it is called a red giant. Different things can happen to the star now.

Scenario 1: |

Planetary NebulasOne scenario is that the star will continue to make energy by using hydrogen and helium outside of the core; its surface will rise and fall and the star will become a variable star. After it gets out of control, the layers of gas will pull away, forming a shell of gas known as a planetary nebula. | |

Scenario 2: |

White dwarf The other scenario is that the star will continue to shine through the fusion of helium nuclei, in the triple alpha process. The star is now a white dwarf, and further contraction is prevented by the repulsion of electrons in the core. | |

Supernova Very heavy stars will continue to fuse heavy elements in order to produce more energy. However, once iron is formed, it cannot be fused to make more energy since it has such a high binding energy and is therefore very stable. The core will collapse under gravity and huge amounts of gas on the surface of the star will explode out. This star is now called a supernova. | |

Neutron Star After a supernova explosion, the iron core of the star may be extremely heavy, and the force of gravity may be extremely large. It then becomes a neutron star, where the repulsion between neutrons stops the contraction caused by gravity. Neutron stars consist of matter that is 100 million times denser than white dwarf matter. | |

Pulsars
A neutron star may spin rapidly after a supernova explosion, and it may emit two beams of radio waves, light, and X-rays. These beams radiate in a circle because the star is spinning, and it appears that the star is pulsing on and off. Thus, it is given the name Pulsar.
Black Holes
Neutron-neutron repulsion can only counteract the force of gravity if the core of the dead star weighs less than three times the weight of the sun. In an extremely heavy core, no force can stop the matter from being squeezed into a smaller and smaller space. Nothing can escape these black holes; not even light.

EXPANDING UNIVERSE QUESTIONS
Pupil Worksheet -- The Expanding Universe
READ THE FOLLOWING PASSAGE CAREFULLY
Expanding Universe
Astronomers believe that the Universe is expanding. It was found that, the more distant or fainter a galaxy, the more rapidly it was moving away from us, e.g. NGC 669 is moving away from us at 5 million metres per second and is believed to be 250 million light years away. The cluster of galaxies Abel 2065 is moving away from us at 22 million metres per second and is believed to be about 1,100 million light years away. This observation was put in a law called after Erwin Hubble in which the distance is related to the speed that the galaxy or cluster of galaxies is moving away from us
Distance = redshift x Hubble constant.
There is a degree of dispute about the value of the Hubble constant. For the purposes of this project we will use the figure of 20,000 metres per second per million light years
Evidence
The evidence that stars and galaxies are moving AWAY comes mainly from the RED SHIFT of the light spectrum received from them. Most people have heard the DOPPLER effect as the sound of, say a police siren, changes from high pitch when approaching to a low pitch when receding from the observer. The change in pitch could be used to measure the SPEED of the car (in fact, RADAR guns used in speed traps do use this method).Similarly, the change in light FREQUENCY, which of course changes the COLOUR of the light, tells astronomers the SPEED of the star either towards us or away from us.
Theory
One theory about the Universe is that it all started with the BIG BANG; before that, there was NO space and NO time. It is quite easy to calculate the age of the Universe by finding the time when all the stars and galaxies must have been in one PLACE. Current measurements give the age within the range 10 to 20 thousand million years.
ANSWER THE FOLLOWING QUESTIONS
1 - a. Calculate the time (in years) that the Abel 2065 cluster of galaxies and our Sun have been moving apart. Use the formula

VELOCITY = DISTANCE � TIME

where 1 light year = (3 x 108 ) x (3.7 x 107 )metres velocity of x seconds in light a year

b. What assumptions have been made about the distances and the velocity?
2 - The diagrams show the sound waves produced by a siren on a car

a. State why the two people hear the same note when the car is stationary.
b. Now the car moves from A to B. Copy the diagram and complete the sound waves.

c. Use the formula WAVE VELOCITY = FREQUENCY X WAVELENGTH to explain why A hears a low pitched sound and B hears a high pitched sound.
3 - Explain why a light spectrum shifted to the RED end of the spectrum tells us that the light source is moving away from us.
THE LIFE CYCLE OF A STAR
Outlined below are the many steps involved in a stars evolution, from its formation in a nebula, to its death as a white dwarf or neutron star.
NEBULA

A nebula is a cloud of gas (hydrogen) and dust in space. Nebulae are the birthplaces of stars. There are different types of nebula. An Emission Nebula e.g. such as Orion nebula, glows brightly because the gas in it is energised by the stars that have already formed within it. In a Reflection Nebula, starlight reflects on the grains of dust in a nebula. The nebula surrounding the Pleiades Cluster is typical of a reflection nebula. Dark Nebula also exist. These are dense clouds of molecular hydrogen which partially or completely absorb the light from stars behind them e.g. the Horsehead Nebula in Orion.
Planetary Nebula are the outer layers of a star that are lost when the star changes from a red giant to a white dwarf.
STAR
A star is a luminous globe of gas producing its own heat and light by nuclear reactions (nuclear fusion). They are born from nebulae and consist mostly of hydrogen and helium gas. Surface temperatures range from 2000�C to above 30,000�C, and the corresponding colours from red to blue-white. The brightest stars have masses 100 times that of the Sun and emit as much light as millions of Suns. They live for less than a million years before exploding as supernovae. The faintest stars are the red dwarfs, less than one-thousandth 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), otherwise nuclear reactions do not take place. Objects with less than critical mass shine only dimly and are termed brown dwarfs or a large planet. Towards the end of its life, a star like the Sun swells up into a red giant, before losing its outer layers as a Planetary Nebula and finally shrinking to become 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 like the Sun, as it runs out of hydrogen fuel at its centre. Red giants have diameter's 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 2000-3000�C.
Very large stars (red giants) are often called Super Giants. These stars have diameters up to 1000 times that of the Sun and have luminosities often 1,000,000 times greater than the Sun.
RED DWARF
These are very cool, faint and small stars, approximately one tenth the mass and diameter of the Sun. They burn very slowly and have estimated lifetimes of 100 billion years. Proxima Centauri and Barnard's Star are red dwarfs.
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 similar to that of the Sun, but only 1% of the Sun's diameter; approximately the diameter of the Earth. The surface temperature of a white dwarf is 8000�C or more, but being smaller than the Sun their overall luminosity's are 1% of the Sun or less.
White dwarfs are the shrunken remains of normal stars, whose nuclear energy supplies have been used up. White dwarf consist of degenerate matter with a very high density due to gravitational effects, i.e. one spoonful has a mass of several tonnes. White dwarfs cool and fade over several billion years.
SUPERNOVA
This is the explosive death of a star, and often results in the star obtaining the brightness of 100 million suns for a short time. There are two general types of Supernova:-
Type I These occur in binary star systems in which gas from one star falls on to a white dwarf, causing it to explode.
Type II These occur in stars ten times or more as massive as the Sun, which suffer runaway internal nuclear reactions at the ends of their lives, leading to an explosion. They leave behind neutron stars and black holes. Supernovae are thought to be main source of elements heavier than hydrogen and helium.
NEUTRON STARS
These stars are composed mainly of neutrons and are produced when a supernova explodes, forcing the protons and electrons to combine to produce a neutron star. Neutron stars are very dense. Typical stars having 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. Pulsars are believed to be neutron stars that are spinning very rapidly.
BLACK HOLES

Black holes are believed to form from massive stars at the end of their life times. The gravitational pull in a black hole is so great that nothing can escape from it, not even light. The density of matter in a black hole cannot be measured. Black holes distort the space around them, and can often suck neighbouring matter into them including stars.
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