1. Birth and Life of a Star What is a star? A star is a really hot ball of gas, with hydrogen fusing into helium at its core. Stars spend the majority of their lives fusing hydrogen, and when the hydrogen fuel is gone, stars fuse helium into carbon. The more massive stars can fuse carbon into even heavier elements, which is where most of the heavy elements in the universe are made. Throughout this whole process is that battle between gravity and gas pressure, known as equilibrium. Birth- the birth of a star begins in the Stellar Nursery (nebula) where gas and dust begin to combine to form the star. A nebula is a cloud of dust and gas, composed primarily of hydrogen (97%) and helium (3%). Within a nebula, there are varying regions when gravity causes this dust and gas gather more atoms (mass), their gravitational attraction to other atoms increases, pulling more atoms into the “clump” (star). A Protostar is the beginning stages of star formation when gravity pulls in the gas and dust from the star nursery. When the gas and dust are combined, the protostar reaches a temperature of 1,800,000 degrees F! Main Sequence Phase starts when nuclear fusion starts. Nuclear Fusion is the joining of the nuclei of 4 Hydrogen atoms into a Helium atom. A small amount of mass (0.7%) is lost at this point and this mass is converted into energy; light and heat. (Grade A) A nebula is usually made up of hydrogen gas. It is the first stage of a star's cycle A star spends a brief childhood as a protostar, a star powered purely by its own gravitational contraction. Energy is generated within a protostar and so it has some luminosity. The radiant forces generated by nuclear fusion and the forces of gravity from its mass are equal during the main sequence phase. This phase can last billions of years, brighter starts use Hydrogen more quickly and so die younger. (Grade B) Nebula Protostar Nebula Main sequence star

2. Death of a Medium/Small star At the onset of nuclear fusion the star is born and begins the process of fusing Hydrogen into Helium and then Helium into Lithium. Our sun has insufficient mass to fuse heavier elements than Carbon. Heavier elements like gold, lead and iron are created in much larger stars. The Expansion Phase: The Hydrogen in the core of the star has all been used and the star starts to ‘burn’ Helium this has greater radiant forces and so the outer layers expand to form a red giant. One and a half billion years later, the surface of the star is 3.3 times the size it is now, and its temperature about 4300 degrees. As seen from earth, our Sun will look like a big orange disk. The problem however is that the temperature on earth has increased by about 100 degrees because of this. So all the seas will have evaporated by that time. Many white dwarfs are about the same size as the Earth, and about 100 times smaller than the Sun. They may weigh the same as the sun, which would make them very dense. The heavier the white dwarf is, then the smaller its size will be. A star like our Sun will become a white dwarf when it has run out of fuel. Near the end of its life, it will go through a red giant stage, and then lose most of its gas, until what is left settles down and becomes a young white dwarf. White dwarf stars are extremely hot; so they emit bright white light. This heat is what is left of the heat made when the star collapsed. Because white dwarfs are extremely small, it takes them a long time to cool down. Eventually, all white dwarfs will cool down into what is called a black dwarf. These are what is left of the star after all of its heat and light has gone away. A white dwarf is what stars like the Sun become after they have exhausted their nuclear fuel. Near the end of its nuclear burning stage, this type of star expels most of its outer material. Only the hot core of the star remains. This core becomes a very hot white dwarf, with a temperature exceeding 100,000 Kelvin, the white dwarf cools down over the next billion years or so. Red Giant White dwarf Black dwarf

3. Death of a Large Star Super Red Giant Red supergiants (RSGs) They are the largest stars in the universe in terms of volume, although they are not the most massive. after burning their hydrogen large stars become red supergiants during their helium-burning phase. A supernova is when a very big star explodes. This happens when a star totally runs out of energy to make heat and light. They are very big and because of gravity they press on their centers very hard and use up their energy very quickly, so they usually only live for a few million years. Then they become a black hole or a neutron star. Small stars do not explode. They cool and shrink down into a white dwarf star. Supernovas are very big explosions. Some of the dust and gas from the supernova can become a new nebula (Solar nursery). Supernovas and life Without supernovas there would be no life on Earth. This is because many of the chemical elements were made in supernova explosions. These are called "heavy elements". Heavy elements are needed to make living things. The supernova is the only way heavy elements can be made. Other elements were made by fusion in stars. Heavy elements need very high temperature and pressure to form. In a supernova explosion the temperature and pressure are so high that heavy elements can be made. Supernova New nebula Neuton Star Black hole A neutron star is a very small and dense star made almost completely of neutrons. They are what is left from the core of a massive star after it has exploded as a supernova, Neutron stars have a radius of about 10 kilometres (6.2 mi) and a mass from about 1.4 to 5 times the mass of our Sun. A black hole is a region of space from which nothing, including light, can escape. It is has such an enormous mass that its own gravity has caused it to collapse in on itself. Around a black hole there is an undetectable surface which marks the point of no return, called an event horizon. It is called "black" because it absorbs all the light that hits it