1. < BackNext >PreviewMain Chapter 15 Stars, Galaxies, and the Universe Preview Section 1 StarsStars Section 2 The Life Cycle of the StarsThe Life Cycle of the Stars Section 3 GalaxiesGalaxies Section 4 Formation of the UniverseFormation of the Universe Concept Map

2. < BackNext >PreviewMain 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.

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

4. < BackNext >PreviewMain 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.

5. < BackNext >PreviewMain 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.

6. < BackNext >PreviewMain 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.

7. < BackNext >PreviewMain 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.

8. < BackNext >PreviewMain 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.

9. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15 Composition of Stars, continued

10. < BackNext >PreviewMain 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.

11. < BackNext >PreviewMain 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.

12. < BackNext >PreviewMain 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.

13. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15 Classifying Stars

14. < BackNext >PreviewMain 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.

15. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15 Absolute and Apparent Magnitude

16. < BackNext >PreviewMain 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.

17. < BackNext >PreviewMain 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.

18. < BackNext >PreviewMain 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.

19. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15

20. < BackNext >PreviewMain 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.

21. < BackNext >PreviewMain 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.

22. < BackNext >PreviewMain 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.

23. < BackNext >PreviewMain 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.

24. < BackNext >PreviewMain 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.

25. < BackNext >PreviewMain 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.

26. < BackNext >PreviewMain 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.

27. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15

28. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15

29. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15 H-R Diagram

30. < BackNext >PreviewMain 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.

31. < BackNext >PreviewMain 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.

32. < BackNext >PreviewMain 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.

33. < BackNext >PreviewMain 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.

34. < BackNext >PreviewMain 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.

35. < BackNext >PreviewMain 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.

36. < BackNext >PreviewMain 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.

37. < BackNext >PreviewMain 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.

38. < BackNext >PreviewMain 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.

39. < BackNext >PreviewMain 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.

40. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15 Contents of Galaxies

41. < BackNext >PreviewMain 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.

42. < BackNext >PreviewMain 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.

43. < BackNext >PreviewMain 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.

44. < BackNext >PreviewMain 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.

45. < BackNext >PreviewMain 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.

46. < BackNext >PreviewMain 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.

47. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15

48. < BackNext >PreviewMain 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.

49. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15 Structure of the Universe

50. < BackNext >PreviewMain Section 4 Formation of the Universe Chapter 15 How Old Is the Universe? Scientists use white dwarfs to estimate the age of the universe. The oldest white dwarfs are 12 billion to 13 billion years old. It took about 1 billion years after the big bang for the first white dwarf to form. So, the universe must be about 14 billion years old.

51. < BackNext >PreviewMain Section 4 Formation of the Universe Chapter 15 A Forever-Expanding Universe? The universe is composed of matter, dark matter, and dark energy. Dark matter does not give off light but has gravity and can be detected. Dark energy seems to be accelerating the expansion of the universe. Scientists think that the universe may keep growing and expanding forever.

52. < BackNext >PreviewMain Chapter 15 Stars, Galaxies, and the Universe Use the terms below to complete the concept map on the next slide. spectra absolute magnitude brightness color temperature spectroscope stars emission lines Concept Map

53. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15 Concept Map

54. < BackNext >PreviewMain Stars, Galaxies, and the Universe Chapter 15 Concept Map