Sun & Stars

Part 2

There are five chapters in part 2:

Chapter Five - The Formation of Stars.

This chapter discusses the earliest processes in the life cycles of stars. The starting point for star formation is the gravitational contraction of interstellar clouds, and the chapter starts by discussing the conditions under which collapse occurs. The result of gravitational collapse is a protostar. The chapter also discusses the 4 observational properties of protostars and explores the link between such objects and the process of planetary formation. A star forming region in the Eagle Nebula is shown at the top left of this page, courtesy of NASA.

Comments:

I found this chapter interesting and the only maths to rear its head was the Jeans Mass equation, which is also covered in the activities. The Hertzsprung-Russell diagram is still very much in evidence throughout this chapter. The learning outcomes of this chapter are given as:

• Give brief definitions of the glossary terms from this chapter.
• Describe the main properties of dense clouds and their role in the formation of protostars.
• Explain the roles of gravity and thermal energy (of a gas) in the formation of a protostar.
• Describe the part played by cloud fragmentation in the formation of stars.
• Understand the properties of bipolar outflow sources and T Tauri stars and recognize that such phenomena are characteristics of the early stages of stellar evolution.
• Recognize that circumstellar discs around protostars are the likely site for the formation of planetary systems.

Chapter Six - The Main Sequence Life of Stars.

This chapter concerns the phase of a star's life when hydrogen burning occurs in its core and considers how astronomers build models of stellar structure. It then looks in some detail at the nuclear processes that liberate energy in the cores of stars and why there should be a minimum and a maximum mass that a star can have. This chapter also briefly considers the role of mass loss during the main sequence stage of stellar life.

Comments:

A short chapter with the main emphasis on the nuclear reactions occurring during the main sequence lifetime of small and massive stars. The learning outcomes are given as:

• Understand the ideas of the equations of stellar structure.
• Explain the concept of hydrostatic equilibrium and the mass-luminosity relationship for main sequence stars.
• Understand why stars on the main sequence are stable and the main sequence lifetimes are shortest for the most massive stars.
• Understand some basic ideas about nuclear reactions, including an appreciation of how rest-energy per nucleon depends on mass number.
• Explain the role of the pp chain and the CNO cycle, and the importance of temperature in these reactions.
• Explain why there is an upper and lower limit to the mass of main sequence stars.

Here's an amusing link entitled: Main Sequence Blues

Chapter Seven - The Life of Stars Beyond the Main Sequence.

This chapter deals with the changes that occur to stars after core hydrogen burning has ceased. Such stars generally become giants or, in the case of the most massive stars, supergiants. This chapter makes a comparison between the evolution of low- and high-mass stars, and the nuclear processes which occur therein.

Comments:

This chapter is largely descriptive in nature and covers more of the nuclear reactions that occur in the later stages of the life of stars. Not a difficult chapter. The learning outcomes are:

• Explain the main characteristics of the evolution of stars after their main sequence lifetime.
• Describe, in general terms, the nuclear processes that take place in stars after leaving the main sequence, and where and at what stage of evolution such processes occur.
• Describe the changes in structure, temperature, size and luminosity of stars and how their positions on the Hertzsprung-Russell diagram change as they evolve.
• Describe the role of stellar winds as a mechanism for mass loss in stars that have evolved off the main sequence.

Chapter Eight - The Death of Stars.

This chapter considers the very last stages of the life of a star. A common feature to the death of all stars is that some stellar material is returned to interstellar space (and is potentially available to form new stars) whereas other material gets 'locked-up' in a dense remnant of the stellar core. The way in which stars end their lives varies between stars of low and high mass, and the chapter concentrates on these two cases. The death of low-mass stars is characterized by stellar winds and the formation of planetary nebulae. The final stage in the life of a high-mass star is a supernova.

Comments:

Again, an interesting chapter with no maths to worry you and is mainly descriptive in nature. Supernova S1987A is considered in some detail. Learning outcomes are:

• Describe the evolution of a low- or intermediate-mass star from the asymptotic giant branch, through a thermal pulsing phase to a period of extreme mass loss and the formation of a planetary nebula.
• Describe the observed characteristics of Type II supernovae.
• Give details of the observation of supernova 1987A, including the detection of emitted neutrinos.
• Explain the processes that lead to the formation of a Type II supernova by core collapse.
• Describe the effects of stellar processes on the physical state and composition of the interstellar medium.

Chapter Nine - The Remnants of Stars.

This chapter considers the dense remnants of stars that are left after a star dies. These remnants take the form of white dwarfs, neutron stars and black holes. As well as reviewing isolated stellar remnants, the chapter also considers binary systems that contain such objects. Such binaries are important for many reasons, not least of which is that they can be used to establish, to a high degree of certainty, the existence of black holes.

Comments:

A longer chapter to finish off the first part of this course. Some equations do appear here, concerning angular momentum and the Schwarzschild radius. Interesting objects such as white dwarfs, pulsars, black holes, neutron stars, X-ray binaries and supernovae are discussed. Learning outcomes are:

• Describe the nature of white dwarfs, neutron stars and black holes.
• Describe the observational characteristics of isolated white dwarfs and neutron stars.
• Explain why it is believed that pulsars are neutron stars.
• Describe the properties of interacting binaries in which mass is transferred onto a stellar remnant.
• Explain the origin of Type Ia supernovae.