A Star is born when the contracting gas and dust from the nebula become so dense and hot that nuclear fusion starts. How long a star lives depends on its mass. • What is a nebula? A large cloud of dust and gas. • In the densest part of the nebula, gravity pulls gas and dust together.
Protostar: a contracting cloud of gas and dust with enough mass to form a star. • Proto-means first so a protostar is the first stage of the star’s life. • What causes the gas and dust in a nebula the contract? Gravity • What is the major difference between a nebula and a star? The amount of space they take up. In a star the gas is packed into a relative small volume; in a nebula the gas is spread out over a large volume. In a star the gas undergoes fusion which produces energy; in a nebula it does not.
Fusion: two hydrogen atoms, each of which has one proton in the nucleus, can join to form an atom with two protons in the nucleus. • What element has two protons in the nuclei of its atoms? Helium • This process is not a chemical reaction. It is a nuclear reaction that takes place under conditions of pressure and high temperature.
What change in the lifespan of a star is marked by the beginning of fusion? The change from nebula to protostar. • The mass of a star determines how long the start will live. Stars with more mass live less than stars with less mass. Small mass stars use up their fuel more slowly than large mass stars so they have much longer lives.
How long can a small star that uses its fuel slowly be expected to live? About 200 billion years. • How long can a medium mass star like our Sun be expected to live? About 10 billion years • How long can the most massive stars be expected to live? About 10 million years. • Students page 118 figure 2 ; then “assess your understanding” 1a,1b,1c.
What happens to a star when it runs out of fuel? • After a star rounds out if you will, it becomes a white dwarf, a neutron star, or a black hole. • What happens to a low mass star as it begins to run out of fuel? It’s outer layers expand, and a star becomes a red giant. • what happens to the core? it becomes a white dwarf. • How does a white dwarf compares to Earth? it is about the same size as Earth but has much more mass.
Does fusion occur in a white dwarf? No it has run out of fuel, fusion cannot occur. The star glows because of left over energy. • What does a white dwarf become when it stops glowing? A black dwarf. • What happens to a high mass star as it begins to run out of fuel? It becomes a super giant. • What happens to the supergiant? It explodes. • What is this explosion called? A supernova.
Summary of changes in stars: • How do stars vary? They vary in mass, temperature, and brightness. • What process begins when a protostar forms? Nuclear fusion. • What is the fuel for fusion? Hydrogen • What change leads to the end of a star’s life? It runs out of fuel. • What characteristic of a star determines what happens to a star that runs out of fuel ? The mass of the star.
Neutron star: the remains of a high mass star. • White dwarf: remains of a low mass star. • How does a neutron star compare to a white dwarf? a neutron star is much smaller and denser than a white dwarf. • How does a neutron star compare to our sun? it has more mass than the sun but is much smaller, so it is much denser than the sun. • What is the name for a neutron star that spins? Pulsar
Black holes :a black hole forms when the remains of the supernova collapse. What is the most significant property of the black hole? Very strong gravity What is the result of this strong gravity? Nothing escapes from the black hole, not even light. If the black hole cannot be seen, how do astronomers know they exist? Astronomers have detected x-rays coming from hot gas that swirls as it is pulled into a black hole.
Classwork; Using construction paper, draw and model a star’s life cycle-birth, life duration ,death.