“Teaching about Astronomy–The Sun and Stars”

LDEO NGSS Summer Institutes: “Teaching about Astronomy” (GED 7214)

Lesson 7:  The Sun and Stars—Energy and Stellar Evolution

Expect Time Required: 3 – 4 hr

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Selected NGSS Connections:

PS3.D:            Energy in Chemical Processes and Everyday Life
Nuclear Fusion processes in the center of the sun release the energy that ultimately reaches Earth as radiation. 

Selected PS/ES Core Curriculum Concepts:

1.2b Stars form when gravity causes clouds of molecules to contract until nuclear fusion of light elements into heavier ones occurs. Fusion releases great amounts of energy over millions of years.
• The stars differ from each other in size, temperature, and age.
• Our Sun is a medium-sized star within a spiral galaxy of stars known as the Milky Way. Our galaxy contains billions of stars, and the universe contains billions of such galaxies.

2.1a Earth systems have internal and external sources of energy, both of which create heat.

2.2a Insolation (solar radiation) heats Earth’s surface and atmosphere unequally due to variations in:
• the intensity caused by differences in atmospheric transparency and angle of incidence which vary with time of day, latitude, and season
• characteristics of the materials absorbing the energy such as color, texture, transparency, state of matter, and specific heat
• duration, which varies with seasons and latitude

Key Ideas

Stars evolve and change over time. The temperature and luminosity of a star vary throughout its lifetime.
Depending on its temperature and its mass, a star will follow a particular path of development, as depicted on the “Temperature and Luminosity” chart (ESRT p. 15)
The Sun is a Main Sequence star. It formed out of a gaseous nebula about 4.6 billion years ago, when temperatures became hot enough so that hydrogen was transformed in helium through nuclear fusion. The Sun is thought to be about halfway through its life cycle.
Stars seem to go from the Main Sequence into the Red Giant stage. During this stage, heavier elements form, including carbon, nitrogen, oxygen, silicon, and iron. Eventually, they explode in a nova or supernova. During this explosion, atoms of the heaviest elements, up to Uranium, are created and sprayed into space. These atoms may be gathered in new nebula gas and dust clouds, and begin the process again.

Introduction

The Sun is the largest object in the Solar System and the source of almost all energy powering Earth. Yet the Sun is also just an “average” star, one of billions in the Milky Way Galaxy. In this Lesson, you will examine some basic concepts astronomers have discovered about the Sun and other stars. These include the characteristics of the Sun, how the Sun and Stars create their energy, and a star’s life cycle.

Characteristics of the Sun
Begin by viewing the slideshow “The Sun.”

Then learn more through the SOHO (SOlar and Heliospheric Observatory) website (https://www.nasa.gov/mission_pages/soho/index.html)

Responses:
1. Make a simple chart explaining what happens in the Sun’s photosphere, chromosphere, core, and corona.

2. By what process does solar energy reach Earth?
3. What are coronal mass ejections? How can they affect Earth?
4. What are two other important concepts about the Sun do you think students should know?

How the Sun Creates Its Energy
Until recently, many people thought that the Sun created energy by “burning” some kind of fuel, just as a fire here on Earth burns. Now we know that solar energy results from “nuclear fusion,” a process in which energy is created by converting the mass of atoms into pure energy.

 View the slideshow “What You Should Know about Stars.”

Focusing for now on “Nuclear Fusion,” view “Solar Energy – Nuclear Fusion in the Sun – Simplified Version”   (https://www.youtube.com/watch?v=pusKlK1L5To)

Responses:
5. Briefly explain the difference between “burning” and “nuclear fusion.”

6. Briefly explain the difference between “fusion” and “fission.” 
7. Briefly explain what happens during nuclear fusion in the Sun to create energy. 
8. What kinds of comparisons can be made about the amount of energy created in the Sun and terrestrial processes?

The Sun Compared with Other Stars
One of the most interesting stories about how “Science” makes progress involves our understanding of the Life Cycle of Stars. A little more than a century ago, two scientists independently decided to plot what was known about the “luminosity” (how much energy it’s emitted) versus star color, an indication of temperature. Hertzspring worked in Denmark and Russell in the US. They published their similar results within a few months of each other, so today we connect their achievements as the H-R diagram.

http://www.atnf.csiro.au/outreach/education/senior/cosmicengine/stars_hrdiagram.html

You should become familiar with the version provided on p. 15 of the Earth Science reference Tables.  One example of lessons based on this is provided by HMXEarthScience. There are many others available ranging in complexity.

  One reason the H-R diagram is important is that it was the first time that people plotted two sets of data against each other without knowing what they would find and discover a significant pattern which led to an explanation of how the Universe works. In this case, they found that plotting luminosity against temperature revealed that stars fall into three “groups.”

Most stars, including the Sun, form a zone that runs from the lower right (cool, low luminosity) to the upper left (hot, high luminosity). The Sun falls in the middle because it is an “average” star compared with the others. A few fall into the upper right—large, cool, but giving off a lot of energy—and are called “Red Giants.”   Others fall in the lower left—small, hot, and emitting relatively little energy—the “White Dwarfs.” 

So what did this pattern mean? Astronomers realized it was a clue to stellar evolution. They gradually developed a story about how stars form, spend most of their existence, and end.

Review the slideshow “What You Should Know about Stars.” Pay particular attention to understanding the stages in a star’s life cycle.

Responses:

9A. Complete the table on the next page to demonstrate your understanding of the events of a star’s life story. Give brief explanations of what happens in each stage. Space is also provided if you want to make any comments about how you would teach this.

9B. What happens after that depends on the size of the star. The fate of the Sun and other “average stars” differs from the fate of a much more massive star. Complete the tables on the following page to demonstrate your understanding of these sequences.

{Note: The following tables did not copy correctly, so use the Word of pdf versions when submitting assignments.}

9A. Stages in the Life Story of All Stars

Comments:

9B-1. Stages in the Life Story of the Sun and other “Average” Stars

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9B-2. Stages in the Life Story of a Massive Star

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10. Elements are created within stars through the processes called “nucleosynthesis. The elements formed depend greatly on the mass of the star and the stage in its Life Story. Complete the table below to summarize in which stage each of these groups of elements are created.

Comments:

Examine questions in recent Regents Earth Science exams.
Select 5 questions pertaining to Minerals and explain what students need to know and do to answer these correctly.