1. Unit 1: Space The Study of the Universe

2.  Mass governs a star’s temperature, luminosity, and diameter.  Mass Effects:  The more massive the star, the greater the force of gravity towards its center of mass (the core).  As a result, a star needs to be hotter and denser to counteract its own gravity.  The balance between gravity squeezing inward and outward pressure is maintained by heat due to nuclear reactions and compression.

3.  Star formation  The formation of a star begins with a cloud of interstellar gas and dust called a nebula.  Provided the cloud is big enough, it will begin collapsing in on itself as a result of gravity.  As it continues to contract, its rotational forces it into a disk shape with a hot and dense center.  This is called a protostar.

4.  Fusion Begins.  When the temperature at the core of the protostar becomes hot enough, nuclear fusion reactions begin.  The first fusion reactions always begin with the conversion of hydrogen into helium.  Once this happens, the star becomes stable and it takes its place along the main sequence according to its mass.

6.  Life Cycles of medium- low mass stars.  A star like the Sun will gradually become more luminous because the core density and temperature rise slowly and increase the reaction rate.  It takes about 10 billion years for a star like the Sun to convert all of the hydrogen in its core to helium.

7.  Red Giant Phase  Once a star begins burning helium in its core, it grows to become a red giant.  Red giants are so large because hydrogen continues to react in a thin layer at the edge of the helium core. The energy produced in this layer forces the outer layers of the star to expand.

8.  Red giants are so large that their cores are a great distance from the outer layers. As a result, surface gravity is low and some of the outer layers can be released by small expansions of the star due to instability.  Meanwhile, the core of the star becomes hot enough (100 million K) for helium to react and begin forming carbon.  At this point, the star contracts again and becomes more stable.

9.  The final stages  Stars that are about the same mass as the Sun never become hot enough to fuse carbon and so energy production ends.  The outer layers expand again and are expelled by pulsations that develop in the outer layers.  This shell of gas is known as a planetary nebula.

10.  White Dwarfs  A white dwarf is made of carbon and it is stable despite its lack of nuclear reactions.  It counteracts the effects of gravity with the resistance of electrons being squeezed so closely together.  This electron pressure does not require ongoing reactions so it can last indefinitely.  Eventually, the white dwarf cools and loses its luminosity: it has become a black dwarf.

11. Not this kind of black dwarf.

12.  Life Cycles of Massive Stars  Stars that are much larger than our Sun have a different life cycle.  These star may begin in the same way but, because its initial mass is greater, it is more luminous and its main sequence lifetime is much shorter.

13.  Supergiant  Supergiants can undergo many more reaction phases and, therefore, they can produce far more elements in their interior.  These stars can become red giants several times as they expand following the end of each reaction phase.  Supernova Formation  A star that begins with a mass that is 8-20 times greater than the Sun’s mass will end up with a core that is too massive to be supported by electron pressure.  Once core reactions have produced iron, no further energy producing reactions can occur.

15.  Neutron Star ▪ The iron core collapses in on itself and protons and electrons merge to form neutrons. ▪ The neutrons resist being so near to each other and this creates a tremendous amount of pressure. ▪ The core becomes a collapsed star remnant – a neutron star. ▪ A neutron star can have a mass of 1.5 to 3 times the Sun’s mass but with a radius of only about 10km. ▪ Some neutron stars are unique in that they have a pulsating pattern of light. These pulsating stars are known as pulsars.

16.  Supernova  A neutron star forms quickly while the outer portions of the star are still falling inward.  The falling gas rebounds quickly after it strikes the hard surface of the neutron star.  The entire outer portion of the star is blown off in a massive explosion called a supernova.  This explosion is the only way that elements heavier than iron are produced in the universe.

17.  Black Holes  Some stars are too massive to form neutrons stars.  The resistance of neutrons being squeezed is not great enough to counteract the force of gravity (collapse).  Matter gets compacted into an increasingly small volume.  The small, extremely dense object is known as a black hole.  In a black hole, gravity is so immense that nothing, not even light, can escape it.

18.  Homework  Answer questions 4 and 8 on page 349  Complete the Inquiry Investigation (8-B) on page 352: ▪ Answer questions 1-5 and 7 on page 353.