1. 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!

2. 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.

4. 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.

5. 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.

6. 27.1 Features of the sun  Occasionally, large “loops” of gas called prominences can be seen jumping up from groups of sunspots.

7. 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.

8. 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.

9. 27.1 Solar energy  Buildings that use passive solar heating are designed to trap sunlight.  Glass traps warm air, causing a “greenhouse effect.”

10. 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.

11. 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.

12. 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.

13. 27.2 The size of stars  Stars come in a range of sizes and masses.  Our Sun is a medium- sized star.  The largest stars, giant stars have a mass of about 60 times the mass of the Sun.

14. 27.2 The size of stars  There are two types of giant stars.  Red giants are cooler than white stars.  Blue giant stars are hot and much more massive than our sun.

15. 27.2 The size of stars  Stars that are smaller than the sun come in two main categories, dwarfs and neutron stars.  Sirius, the Dog Star, is the largest known white dwarf.

16. 27.2 Temperature and color  If you look closely at the stars on a clear night, you might see a slight reddish or bluish tint to some stars.  This is because their surface temperatures are different.

17. 27.2 Temperature and color  The color of light is related to its energy.  White light is a mixture of all colors at equal brightness.

18. 27.2 Brightness and luminosity  Brightness, also called intensity, describes the amount of light energy per second falling on a surface.

19. 27.2 Brightness and luminosity  Luminosity is the total amount of light given off by a star in all directions.  Luminosity is a fundamental property of a star whereas brightness depends on both luminosity and distance.

20. 27.2 Temperature and luminosity  In the early 1900s, Danish astronomer Ejnar Hertzsprung and American astronomer Henry Russell developed an important tool for studying stars.  Their graph showed luminosity on the y axis…  …and surface temperature on the x axis

21. 27.2 Temperature and luminosity  H-R diagrams are useful because they help astronomers categorize stars into groups:  Main sequence stars, like the Sun, are in a very stable part of their life cycle.  White dwarfs are hot and dim and cannot be seen without a telescope.  Red giants are cool and bright and some can be seen without a telescope. Can you locate blue giants on the H-R diagram?

22. 27.3 The life cycle of stars  A star, regardless of its size, begins its life inside a huge cloud of gas (mostly hydrogen) and dust called a nebula.  The Eagle Nebula is the birthplace of many stars.

23. 27.3 The life cycle of stars  A protostar is the earliest stage in the life cycle of a star.  The Orion Nebula was the birthplace of these protostars.

24. 27.3 The life cycle of stars  A star is born when temperature and pressure at its center become great enough to start nuclear fusion.  Once nuclear fusion begins, a star is in the main sequence stage of its life cycle.

25. 27.3 The life cycle of stars  The time a star stays on the main sequence depends on the star’s mass.  High-mass stars burn brighter, and hotter, using up their hydrogen faster than low-mass stars.

26. 27.2 The old age of Sun-like stars  With no more energy flowing outward, nothing prevents gravity from crushing the matter in the core together.  When hydrogen fusion stops, the core glows brightly and is called a white dwarf.

27. 27.2 The old age of Sun-like stars  A planetary nebula forms when a star blows off its outer layers leaving its bare core exposed as white dwarf.  Planetary nebulae are one of nature’s ways of recycling the matter in old stars and distributing new elements.

28. 27.3 Supernovae and synthesis of the elements  Scientists believe the early universe was mostly hydrogen, helium and a trace of lithium.  Heavier elements are created by nuclear fusion inside the cores of stars.  Nuclear fusion reactions are exothermic, releasing energy only up to iron.  When the core of the star contains mostly iron, nuclear fusion stops.

29. 27.3 Supernovae and synthesis of the elements  If a star’s iron core reaches 1.4 times the mass of the Sun, gravity becomes strong enough to combine electrons and protons into neutrons.  During this brief period, heavier elements such as gold and uranium are created, as atomic nuclei are smashed together.  The core of the star collapses and the result is a spectacular explosion called a supernova.

30. 27.3 Supernovae and synthesis of the elements  The Crab nebula is the remains of a supernova.  Chinese astronomers recorded it’s demise 1054 AD.

31. 27.3 Examining light from stars  Spectroscopy is a tool of astronomy in which the light produced by a star or other object (called its spectrum) is analyzed.

32. 27.3 Analyzing light from stars  A spectrometer splits light into a spectrum of colors and displays lines of different colors along a scale.  Each element has its own unique pattern of spectral lines.

33. 27.2 Analyzing light from stars  In 1861, Sir William Huggins used spectroscopy to determine that the Sun and the stars are made mostly of hydrogen.

34. 27.2 Analyzing light from stars  A few years later, Sir Joseph Norman Lockyer observed a line at the exact wavelength of 587.6 nm.  He concluded that this must be an undiscovered element and named it helium, after the Greek name for the Sun, Helios.