1a.) What is the source of the heat produced by the Sun?
A sun is million times Earth's volumes and 333,000 Earth's mass. The source of heat that is produced from the Sun comes from the hydrogen and helium fusion in its core which is called thermal nuclear fusion. The Sun's thermal nuclear fusion produces about 400 trillion-trillion watts of energy that is eventually radiates through its surface into space in the form of light. (Universe and Beyond, pp 73 - 99, 2004)
1b.) How long (approximately) will the Sun maintain the current levels of heat and brightness?
The Sun will maintain the current levels of heat and brightness approximately 5 - 6 billion years. (Universe and Beyond, pp 73 - 99, 2004)
1c.) What is likely to happen afterwards?
The Sun will evolve into a red dwarf into an eventual white dwarf. This death will come gradually due to its depletion of hydrogen. The depletion of hydrogen will start at the core and work its way out word while at the same time contracting the core. This gradual gnawing away at the hydrogen will produce more energy/heat/light; whereby, the sun becomes brighter and larger in size like an overfilled balloon. It continues to get so hot that the helium around the core starts heating up and fuses with the carbon while at the same time, continues to enlarge the sun along with increasing its brightness. This process compresses the sun's mass at the same time. Eventually over 100 million years, from the beginning dying sequence to the end, the Sun will become a elderly white dwarf with its stellar wind acting as its last burp. It will maintain its original path in the galaxy except it will no longer produce heat and eventually will turn into a black dwarf, cold and lifeless. (Universe and Beyond, pp 73 - 99, 2004)
1d.) What is likely to happen to Earth as the Sun evolves?
Meanwhile, the expanding dying Sun is impacting Earth by means of a gradual climate change. With the gradual increased temperatures, it will have a disastrous effect on Earth; thereby, temperatures will eventually climb to above boiling point making oceans evaporate into the sky, polar ice caps and the atmosphere toxic volcanic activity. Eventually, Earth will become a rock of charcoal while being invaded by stellar winds produced by the dying Sun. At this point, no one really knows if Earth will exist. (Universe and Beyond, pp 73 - 99, 2004)
2.) What is the "main sequence?"
The main sequence shows stellar temperatures and luminosities and is a plotted from high temperatures/luminosity to low temperature/luminosity. The main sequence is an graphical curve that 90% of the stars in our solar system fall into. These are viable stars that are still burning hydrogen in their cores. They will maintain their present position on the main sequence band for most of their life time. Stars that deviate from the sequence are stars with changes in their core physics and life span. This usually indicates the star's hydrogen core is becoming depleted along with the change in their energy source. The star's mass determines its brightness, its life span, its temperature and its size. All stars are fixed into the sequence accordingly. Stars in the blue zone on the left side of the diagram are the hottest, while stars in the red zone on the right side are the coolest. The sun is fixed in the yellow zone of the sequence. The other regions include the brightest supergiants that are located on upper part of diagram, then the cluster of stars called giants located above the sequence and below the sequence band are the dimmest, coolest and smaller stars called the red dwarfs. (Universe and Beyond, pp 86 - 87, 2004)
3.Name the possible ways a star can evolve after the main sequence.
The star won't fluctuate from the main sequence until all its hydrogen fuel has been depleted. When this happens, it will no longer be a main sequence star and the star will migrate to the upper red zone. For example, our sun will become a red giant during its dying days that will land it between Aldabaran and Antares in the red zone. Afterwards, the Sun will migrate down to the white dwarf area and finally rest as a black dwarf. (Universe and Beyond, pp 73 - 99, 2004)
Stars with 1% luminosity of our Sun are already red in color and will last 100 billion years. Stars evolve naturally through their life span depending on their mass . The larger its mass the hotter and shorter its lifespan is in the main sequence. The cooler smaller stars maintain a longer life in the main sequence zone. The main reason for this difference is the larger, hotter star depletes their hydrogen core at a faster rate. (Universe and Beyond, pp 73 - 99, 2004)
Dying six solar mass stars usually will abrupt into a supernova. In this case, the explosion is about million times hotter than the Sun's surface. Near the star's ending stage, the continued struggle with its depletion of hydrogen, helium and finally the heavier elements such as, carbon, nitrogen, oxygen, etc; it is finally left with only iron to produce its energy. With the residual iron element becomes no fuel for the star; thereby, gravity wins the battle and produces the big bang. It usually takes the explosion to create new stars. In 1054 AD, a supernova exploded and now it is the crab nebula, home of a pulsar/neutron star. A teaspoon of neutron star would equal a 3 KM mountain. If the star has a certain type of mass, or becomes a giant red dwarf, it may become a black hole instead of white dwarf of a smaller sun. (Universe and Beyond, pp 90 - 93, 2004)
Neutron stars are created by collapsing stars supernovas and have degenerated neutrons. Neutron stars exist in main sequence forever until otherwise proven. Dark energy in space is also negatively charged and should why neutron stars never die. The star from the super nova must be less than 3 times the Sun to become a neutron star.
Black holes are created by collapsing stars from supernovas that are approximately 3 times or greater. The spaceship needs to generate speeds greater than speed of light to escape the event horizon's gravity. (Universe and Beyond, pp 73 - 99, 2004)