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2. Resources Chapter Presentation Bellringers Image Bank Standards Assessment Visual Concepts Math Focus

3. Stars, Galaxies, and the Universe Chapter 15 Table of Contents Section 1 Stars Section 2 The Life Cycle of the Stars Section 3 Galaxies Section 4 Formation of the Universe

4. Section 1 Stars Chapter 15 Bellringer List ways that stars differ from one another. How is the sun like other stars? How is it different? Write your answers in your Science Journal.

5. Section 1 Stars Chapter 15 What You Will Learn • Stars differ in size, temperature, composition, brightness, and color. • Distances between stars are very large and are measured in light-years.

6. Section 1 Stars Chapter 15 Color of Stars • The color of a star indicates the star’s temperature. • Red stars are the coolest, and blue stars are the hottest. • If two stars differ in color, you can conclude that they differ in temperature too.

7. Section 1 Stars Chapter 15 Composition of Stars • Astronomers use an instrument called a spectroscope to separate a star’s light into a spectrum (plural, spectra). • A spectrum is the band of colors produced when white light passes through a prism.

8. Section 1 Stars Chapter 15 Composition of Stars, continued • A continuous spectrum shows all of the colors, while an absorption spectrum shows which wavelengths of light are absorbed. • The spectrum of a star is an absorption spectrum because the atmosphere of the star absorbs certain portions of the light produced by the star.

9. Section 1 Stars Chapter 15 Composition of Stars, continued • When a chemical element emits light, only some colors in the spectrum show up. The colors that appear are called emission lines. • Every element has a unique set of emission lines that act like a fingerprint for that element.

10. Section 1 Stars Chapter 15 Composition of Stars, continued • The pattern of lines in a star’s absorption spectrum is unique to that star. • A star’s absorption spectrum can be used to determine the elements in that star’s atmosphere and the stage the star occupies in its life cycle. • Stars are made of mostly hydrogen and helium gases.

11. Stars, Galaxies, and the Universe Chapter 15 Composition of Stars, continued

12. Section 1 Stars Chapter 15 Classifying Stars • Stars are now classified by how hot they are. • Temperature differences between stars result in color differences that can be seen. For example, class O stars are blue—the hottest stars.

13. Section 1 Stars Chapter 15 Classifying Stars, continued • Magnitude is used to compare the brightness of one object with the brightness of another object. • To express the brightness of stars, astronomers use a system of magnitudes.

14. Section 1 Stars Chapter 15 Classifying Stars, continued • Positive magnitude numbers represent dim stars. Negative magnitude numbers represent bright stars. • The brightest star in the night sky, Sirius, has a magnitude of -1.4.

15. Stars, Galaxies, and the Universe Chapter 15 Classifying Stars

16. Section 1 Stars Chapter 15 How Bright Is That Star? • The apparent magnitude is the brightness of a star as seen from Earth. The absolute magnitude is the brightness that a star would have at a distance of 32.6 light-years from Earth. • If all stars were the same distance away, their absolute magnitudes would be the same as their apparent magnitudes.

17. Stars, Galaxies, and the Universe Chapter 15 Absolute and Apparent Magnitude

18. Section 1 Stars Chapter 15 Distance to the Stars • Because stars are so far away, astronomers use a unit called a light-year to measure the distance from Earth to the stars. • A light-year is the distance that light travels in one year, about 9.46 trillion kilometers.

19. Section 1 Stars Chapter 15 Distance to the Stars, continued • Parallax is an apparent shift in the position of an object when viewed from different locations. • Astronomers use parallax and trigonometry to find the actual distance to stars that are close to Earth.

20. Section 1 Stars Chapter 15 Motions of Stars • All of the stars in the sky appear to make one complete circle around Polaris every 24 h. • This apparent motion of the stars is due to Earth’s rotation and its revolution around the sun. • Each star is actually moving in space. But because stars are so distant, their actual motion is hard to see.

21. Stars, Galaxies, and the Universe Chapter 15

22. Section 2 The Life Cycle of Stars Chapter 15 Bellringer List 3 stages in the life cycle of an insect. List 3 stages in the life cycle of a human. List any stages you may know of in the life cycle of a star. Write your answers in your Science Journal.

23. Section 2 The Life Cycle of Stars Chapter 15 What You Will Learn • During star formation, gravity pulls dust and gas into a sphere, and when the sphere gets dense enough, nuclear fusion begins. • The stages of a star’s life cycle and the star’s properties at each stage can be plotted on the H-R diagram.

24. Section 2 The Life Cycle of Stars Chapter 15 Types of Stars • Stars can be classified by mass, size, brightness, color, temperature, composition, and age. • A star is classified differently as its properties change. Its properties often relate to whether the star can generate energy by nuclear fusion.

25. Section 2 The Life Cycle of Stars Chapter 15 The Life Cycle of Sunlike Stars • New stars form from gas and dust, which are pulled into a sphere by gravity. • As the sphere becomes denser, hydrogen combines into helium and energy is released in a process called nuclear fusion.

26. Section 2 The Life Cycle of Stars Chapter 15 The Life Cycle of Sunlike Stars, continued • After a sunlike star forms, it enters the main sequence stage, the longest stage of its life cycle. The main sequence is the location on the H-R diagram where most stars lie. • During the main sequence stage, energy is generated in the core of the star from nuclear fusion.

27. Section 2 The Life Cycle of Stars Chapter 15 The Life Cycle of Sunlike Stars, continued • When a main-sequence star uses all of the hydrogen in its core, the atmosphere of the star grows very large and cools. The star may become a red giant or red supergiant. • In the final stage of its life cycle, a sunlike star becomes a white dwarf. A white dwarf no longer generates energy by nuclear fusion.

28. Section 2 The Life Cycle of Stars Chapter 15 A Tool for Studying Stars • The H-R diagram is a tool for studying the life cycles of stars. The H-R diagram is a graph that shows the relationship between the star’s surface temperature and absolute magnitude. • The stages of a star’s life cycle and its properties at each stage can be plotted on an H-R diagram.

29. Stars, Galaxies, and the Universe Chapter 15

30. Stars, Galaxies, and the Universe Chapter 15

31. Stars, Galaxies, and the Universe Chapter 15 H-R Diagram

32. Section 2 The Life Cycle of Stars Chapter 15 The Aging of Massive Stars • Massive stars use their hydrogen very fast and tend to have shorter lives than other stars do. • A supernova is a gigantic explosion in which a massive star collapses and throws its outer layers into space.

33. Section 2 The Life Cycle of Stars Chapter 15 The Aging of Massive Stars, continued • Following a supernova, the center of the collapsed star may contract into a very small dense ball of neutrons called a neutron star. • If a neutron star is spinning, it is called a pulsar. Pulsars send out beams of radiation that sweep across space.

34. Section 2 The Life Cycle of Stars Chapter 15 The Aging of Massive Stars, continued • If the center of a collapsed massive star has a mass that is several times the mass of the sun, the star may contract further and leave a black hole. • A black hole is an object that is so dense and massive that light cannot escape its gravity.

35. Section 3 Galaxies Chapter 15 Bellringer What do you think galaxies are made of? What is the name of the galaxy in which we live? Draw a sketch of what you think a spiral galaxy looks like when viewed from outside that galaxy. Record your answers in your Science Journal.

36. Section 3 Galaxies Chapter 15 What You Will Learn • The three types of galaxies are spiral galaxies, elliptical galaxies, and irregular galaxies. • Galaxies are composed of stars, planetary systems, nebulas, and star clusters. • Looking at distant galaxies reveals what young galaxies looked like.

37. Section 3 Galaxies Chapter 15 Types of Galaxies • A galaxy is a collection of stars, dust, and gas bound together by gravity. • Astronomers classify a galaxy as spiral, elliptical, or irregular according to its shape and rate of star formation.

38. Section 3 Galaxies Chapter 15 Types of Galaxies, continued • Spiral galaxies have a bulge at the center and spiral arms. • The spiral arms are made of gas, dust, and new stars that have formed. • The galaxy in which we live is a spiral galaxy called the Milky Way.

39. Section 3 Galaxies Chapter 15 Types of Galaxies, continued • Most elliptical galaxies are round or oval and seem to have stopped making new stars. • Elliptical galaxies are among the largest galaxies in the universe. • Irregular galaxies have no definite shape and form new stars slowly.

40. Section 3 Galaxies Chapter 15 Contents of Galaxies • Some galaxies contain nebulas and star clusters. • A nebula is a large cloud of gas and dust in interstellar space. • Nebulas are a region in space where stars are born.

41. Section 3 Galaxies Chapter 15 Contents of Galaxies, continued • A globular cluster is a highly concentrated group of up to 1 million stars that looks like a ball. • Open clusters are groups of 100 to 1,000 stars that are close together relative to other stars.

42. Stars, Galaxies, and the Universe Chapter 15 Contents of Galaxies

43. Section 3 Galaxies Chapter 15 Quasars • Quasars are among the most distant objects in the universe and are located in the centers of galaxies. • Quasars generate energy at a high rate and are among the most powerful energy sources in the universe.

44. Section 3 Galaxies Chapter 15 Origin of Galaxies • Because light takes time to travel through space, looking through a telescope is like looking back in time. • Looking at distant galaxies reveals what early galaxies looked like.

45. Section 4 Formation of the Universe Chapter 15 Bellringer Write a paragraph that describes how you think the universe formed. Write your answers in your Science Journal.

46. Section 4 Formation of the Universe Chapter 15 What You Will Learn • The big bang theory is the standard theory to explain the formation of the universe. • Gravity controls the size and shape of the universe.

47. Section 4 Formation of the Universe Chapter 15 The Big Bang Theory • Cosmology is the study of the origin, structure, processes, and evolution of the universe. • Careful measurements have shown that most galaxies are moving away from each other and that the universe is expanding. With these findings, scientists have worked backward in time to figure out how the universe formed.

48. Section 4 Formation of the Universe Chapter 15 The Big Bang Theory, continued • The big bang theory is the theory that all matter and energy in the universe was compressed into an extremely small volume that about 14 billion years ago exploded and began expanding in all directions. • Scientists believe that radio “noise” coming from all directions in space is cosmic background radiation left over from the big bang.

49. Stars, Galaxies, and the Universe Chapter 15

50. Section 4 Formation of the Universe Chapter 15 Gravity and the Universe • After the big bang, gravitational attraction caused the matter distributed throughout the universe to form galaxies. • Because gravity acts over such great distances, gravity controls the size and shape of the universe. • Every object in the universe is part of a larger system and gravity acts within each system.