Earth Science, 12e

1. Earth Science, 12e Beyond Our Solar SystemChapter 22

2. After reading, studying, and discussing the chapter, students should be able to: • Discuss the principle of parallax and explain how it is used to measure the distance to a star. • List and describe the major intrinsic properties of stars. • Describe the different types of nebulae. • Describe the most plausible model for stellar evolution and list the stages in the life cycle of a star. • Describe the possible final states that a star may assume after it consumes its nuclear fuel and collapses. • List and describe the major types of galaxies. • Describe the big bang theory of the origin of the universe. Learning Objectives

3. Properties of stars • Distance • Measuring a star’s distance can be very difficult • Stellar parallax • Used for measuring distance to a star • Apparent shift in a star’s position due to the orbital motion of Earth • Measured as an angle • Near stars have the largest parallax • Largest parallax is less than one second of arc

4. Properties of stars • Distance • Distances to the stars are very large • Units of measurement • Kilometers or astronomical units are too cumbersome to use • Light-year is used most often • Distance that light travels in 1 year • One light-year is 9.5 trillion km (5.8 trillion miles) • Other methods for measuring distance are also used

5. Properties of stars • Stellar brightness • Controlled by three factors • Size • Temperature • Distance • Magnitude • Measure of a star’s brightness

6. Properties of stars • Stellar brightness • Magnitude • Two types of measurement • Apparent magnitude • Brightness when a star is viewed from Earth • Decreases with distance • Numbers are used to designate magnitudes – dim stars have large numbers and negative numbers are also used

7. Properties of stars • Stellar brightness • Magnitude • Two types of measurement • Absolute magnitude • “True” or intrinsic brightness of a star • Brightness at a standard distance of 32.6 light-years • Most stars’ absolute magnitudes are between –5 and +15

8. Properties of stars • Color and temperature • Hot star • Temperature above 30,000 K • Emits short-wavelength light • Appears blue • Cool star • Temperature less than 3,000 K • Emits longer-wavelength light • Appears red

9. Properties of stars • Color and temperature • Between 5,000 and 6,000 K • Stars appear yellow • e.g., Sun • Binary stars and stellar mass • Binary stars • Two stars orbiting one another • Stars are held together by mutual gravitation • Both orbit around a common center of mass

10. Properties of stars • Binary stars and stellar mass • Binary stars • Visual binaries are resolved telescopically • More than 50% of the stars in the universe are binary stars • Used to determine stellar mass • Stellar mass • Determined using binary stars – the center of mass is closest to the most massive star

11. Binary stars orbit each other around their common center of mass Figure 24.4

12. Properties of stars • Binary stars and stellar mass • Stellar mass • Mass of most stars is between 1/10 and 50 times the mass of the Sun

13. Hertzsprung-Russell diagram • Shows the relation between stellar • Brightness (absolute magnitude) and • Temperature • Diagram is made by plotting (graphing) each star’s • Luminosity (brightness) and • Temperature

14. Hertzsprung-Russell diagram • Parts of an H-R diagram • Main-sequence stars • 90% of all stars • Band through the center of the H-R diagram • Sun is in the main sequence • Giants (or red giants) • Very luminous • Large • Upper-right on the H-R diagram

15. Hertzsprung-Russell diagram • Parts of an H-R diagram • Giants (or red giants) • Very large giants are called supergiants • Only a few percent of all stars • White dwarfs • Fainter than main-sequence stars • Small (approximately the size of Earth) • Lower-central area on the H-R diagram • Not all are white in color • Perhaps 10% of all stars

16. Idealized Hertzsprung-Russell diagram Figure 24.7

17. Variable stars • Stars that fluctuate in brightness • Types of variable stars • Pulsating variables • Fluctuate regularly in brightness • Expand and contract in size • Eruptive variables • Explosive event • Sudden brightening • Called a nova

18. Interstellar matter • Between the stars is “the vacuum of space” • Nebula • Cloud of dust and gases • Two major types of nebulae • Bright nebula • Glows if it is close to a very hot star • Two types of bright nebulae • Emission nebula • Reflection nebula

19. A faint blue reflection nebula in the Pleiades star cluster Figure 24.9

20. Interstellar matter • Nebula • Two major types of nebulae • Dark nebula • Not close to any bright star • Appear dark • Contains the material that forms stars and planets

21. Stellar evolution • Stars exist because of gravity • Two opposing forces in a star are • Gravity – contracts • Thermal nuclear energy – expands • Stages • Birth • In dark, cool, interstellar clouds • Gravity contracts cloud and temperature rises • Radiates long-wavelength (red) light • Becomes a protostar

22. Stellar evolution • Stages • Protostar • Gravitational contraction of gaseous cloud continues • Core reaches 10 million K • Hydrogen nuclei fuse • Become helium nuclei • Process is called hydrogen burning • Energy is released • Outward pressure increases • Outward pressure balanced by gravity pulling in • Star becomes a stable main-sequence star

23. Stellar evolution • Stages • Main-sequence stage • Stars age at different rates • Massive stars use fuel faster and exist for only a few million years • Small stars use fuel slowly and exist for perhaps hundreds of billions of years • 90% of a star’s life is in the main sequence

24. Stellar evolution • Stages • Red giant stage • Hydrogen burning migrates outward • Star’s outer envelope expands • Surface cools • Surface becomes red • Core is collapsing as helium is converted to carbon • Eventually all nuclear fuel is used • Gravity squeezes the star

25. Stellar evolution • Stages • Burnout and death • Final stage depends on mass • Possibilities • Low-mass star • 0.5 solar mass • Red giant collapses • Becomes a white dwarf

26. Stellar evolution • Stages • Burnout and death • Final stage depends on mass • Possibilities • Medium-mass star • Between 0.5 and 3 solar masses • Red giant collapses • Planetary nebula forms • Becomes a white dwarf

27. H-R diagram showing stellar evolution Figure 24.11

28. Stellar evolution • Stages • Burnout and death • Final stage depends on mass • Possibilities • Massive star • Over 3 solar masses • Short life span • Terminates in a brilliant explosion called a supernova • Interior condenses • May produce a hot, dense object that is either a neutron star or a black hole

29. Stellar remnants • White dwarf • Small (some no larger than Earth) • Dense • Can be more massive than the Sun • Spoonful weighs several tons • Atoms take up less space • Electrons displaced inward • Called degenerate matter • Hot surface • Cools to become a black dwarf

30. Stellar remnants • Neutron star • Forms from a more massive star • Star has more gravity • Squeezes itself smaller • Remnant of a supernova • Gravitational force collapses atoms • Electrons combine with protons to produce neutrons • Small size

31. Stellar remnants • Neutron star • Pea-size sample • Weighs 100 million tons • Same density as an atomic nucleus • Strong magnetic field • First one discovered in early 1970s • Pulsar (pulsating radio source) • Found in the Crab nebula (remnant of an A.D. 1054 supernova)

32. Crab Nebula in the constellation Taurus Figure 24.14

33. Stellar remnants • Black hole • More dense than a neutron star • Intense surface gravity lets no light escape • As matter is pulled into it • Becomes very hot • Emits X-rays • Likely candidate is Cygnus X-1, a strong X-ray source

34. Galaxies • Milky Way Galaxy • Structure • Determined by using radio telescopes • Large spiral galaxy • About 100,000 light-years wide • Thickness at the galactic nucleus is about 10,000 light-years • Three spiral arms of stars • Sun is 30,000 light-years from the center

35. Face-on view of the Milky Way Galaxy Figure 24.18 A

36. Edge-on view of the Milky Way Galaxy Figure 24.18 B

37. Galaxies • Milky Way Galaxy • Rotation • Around the galactic nucleus • Outermost stars move the slowest • Sun rotates around the galactic nucleus once about every 200 million years • Halo surrounds the galactic disk • Spherical • Very tenuous gas • Numerous globular clusters

38. Galaxies • Other galaxies • Existence was first proposed in mid-1700s by Immanuel Kant • Four basic types of galaxies • Spiral galaxy • Arms extending from nucleus • About 30% of all galaxies • Large diameter up to 125,000 light-years • Contains both young and old stars • e.g., Milky Way

39. The Andromeda Galaxy is an example of a large spiral galaxy Figure 24.20

40. Galaxies • Other galaxies • Four basic types of galaxies • Barred spiral galaxy • Stars arranged in the shape of a bar • Generally quite large • About 10% of all galaxies • Elliptical galaxy • Ellipsoidal shape • About 60% of all galaxies • Most are smaller than spiral galaxies; however, they are also the largest known galaxies

41. A barred spiral galaxy Figure 24.22

42. Galaxies • Other galaxies • Four basic types of galaxies • Irregular galaxy • Lacks symmetry • About 10% of all galaxies • Contains mostly young stars • e.g., Magellanic Clouds

43. Galaxies • Galactic cluster • Group of galaxies • Some contain thousands of galaxies • Local Group • Our own group of galaxies • Contains at least 28 galaxies • Supercluster • Huge swarm of galaxies • May be the largest entity in the universe

44. Red shifts • Doppler effect • Change in the wavelength of light emitted by an object due to its motion • Movement away stretches the wavelength • Longer wavelength • Light appears redder • Movement toward “squeezes” the wavelength • Shorter wavelength • Light shifted toward the blue

45. Red shifts • Doppler effect • Amount of the Doppler shift indicates the rate of movement • Large Doppler shift indicates a high velocity • Small Doppler shift indicates a lower velocity • Expanding universe • Most galaxies exhibit a red Doppler shift • Moving away

46. Raisin bread analogy of an expanding universe Figure 24.24

47. Red shifts • Expanding universe • Most galaxies exhibit a red Doppler shift • Far galaxies • Exhibit the greatest shift • Greater velocity • Discovered in 1929 by Edwin Hubble • Hubble’s Law – the recessional speed of galaxies is proportional to their distance • Accounts for red shifts

48. Big Bang theory • Accounts for galaxies moving away from us • Universe was once confined to a “ball” that was • Supermassive • Dense • Hot

49. Big Bang theory • Big Bang marks the inception of the universe • Occurred about 15 billion years ago • All matter and space was created • Matter is moving outward • Fate of the universe • Two possibilities • Universe will last forever • Outward expansion will stop and gravitational contraction will follow

50. Big Bang theory • Fate of the universe • Final fate depends on the average density of the universe • If the density is more than the critical density, then the universe would contract • Current estimates point to less than the critical density and predict an ever-expanding, or open, universe