UNIT NINE: Matter and Motion in the Universe  Chapter 26 The Solar System  Chapter 27 Stars  Chapter 28 Exploring the Universe

Chapter Twenty-Seven: Stars  27.1 The Sun  27.2 Stars  27.3 The Life Cycle of Stars

Chapter 27.1 Learning Goals  Explore the properties of the Sun.  Identify unique zones of the Sun and describe what happens in each area.  Discuss applications of solar energy.

27.1 The Sun  The Sun is is a giant, hot ball of gas held together by gravity.  The Sun is medium- sized compared with other stars in the universe. Approximately 1 million planet Earths could fit inside the Sun!

27.1 The Sun  Gravity squeezes the density of a star so tightly in the core that the electrons are stripped away and the bare nuclei of atoms almost touch each other.  Nuclear fusion occurs.

27.1 The Sun  In the process, huge amounts of energy are given off.  Because of its mass, the Sun’s gravitational force is strong enough to hold the entire solar system in orbit.

27.1 Anatomy of the sun  The apparent surface of the Sun that we can see from a distance is called the photosphere, which means “sphere of light.”  Just above it is the chromosphere.  This is a very hot layer of plasma, a high- energy state of matter.

27.1 Anatomy of the sun  The corona is the outermost layer of the sun’s atmosphere, extending millions of kilometers beyond the sun.  Sunspots are areas of gas that are cooler than the gases around them.

27.1 Features of the sun  Occasionally, large “loops” of gas called prominences can be seen jumping up from groups of sunspots.

27.1 Features of the sun  Solar wind is an electrically charged mixture of protons and electrons that cause magnetic storms.  Auroras, called the northern lights, occur when layers of our atmosphere are energized by solar winds.

27.1 Solar energy  Solar energy is a term that refers to radiant energy from the Sun.  The radiant energy of the Sun reaches Earth in the form of electromagnetic waves.  We can use solar energy to heat buildings and generate electricity.

27.1 Solar energy  Buildings that use passive solar heating are designed to trap sunlight.  Glass traps warm air, causing a “greenhouse effect.”

27.1 Solar energy  Photovoltaic (or PV) cells are devices that convert sunlight directly into electricity.  Solar cells are found on calculators, watches, or certain outdoor light fixtures.

27.1 More about the Sun’s energy  In 1905, Albert Einstein proposed that matter can be converted into energy.  His famous equation shows how huge amounts of energy can be created from a smaller mass.

27.1 More about the Sun’s energy  The amount of this energy from the Sun that reaches the outer edge of Earth’s atmosphere is known as the solar constant.  The accepted value is 1,386 watts per square meter (W/m 2 ), or about thirteen 100-watt light bulbs per square meter of surface.

Optional* Investigation 27C  Key Question: How can we use energy from the sun to generate electricity? Efficiency of a Solar Cell