Objectives

1. The Sun Objectives • Explore the structure of the Sun. • Describe the solar activity cycle and how the Sun affects Earth. • Compare the different types of spectra. Vocabulary • photosphere • chromosphere • corona • solar wind • sunspot • solar flare • prominence • fusion • fission • spectrum

2. The Sun The Sun • Through observations and probes, such as the Solar Heliospheric Observatory (SOHO) and the Ulysses mission, astronomers have begun to unravel the mysteries of the Sun. • Astronomers still rely on computer models for an explanation of the interior of the Sun because the interior cannot be directly observed.

3. The Sun Properties of the Sun • The Sun is the largest object in the solar system, in both size and mass. • The Sun contains more than 99 percent of all the mass in the solar system, which allows it to control the motions of the planets and other objects. • Models show that the density in the center of the Sun is about 1.50 × 105 kg/m3.

4. The Sun Properties of the Sun • The solar interior is gaseous throughout because of its high temperature—about 1 × 107 K in the center. • Many of the gases are in a plasma state, meaning that they are completely ionized and composed only of atomic nuclei and electrons. • The outer layers of the Sun are not quite hot enough to be plasma.

5. The Sun The Sun’s Atmosphere • The photosphere, approximately 400 km in thickness, is the lowest layer of the Sun’s atmosphere. • This is the visible surface of the Sun because most of the light emitted by the Sun comes from this layer. • The average temperature of the photosphere is about 5800 K.

6. The Sun The Sun’s Atmosphere • The chromosphere, which is above the photosphere and approximately 2500 km in thickness, has a temperature of nearly 30 000 K at the top. • The corona, which is the top layer of the Sun’s atmosphere, extends several million kilometers southward from the top of the chromosphere and has a temperature range of 1 million to 2 million degrees K.

7. The Sun The Sun’s Atmosphere Solar Wind • Gas flows outward from the corona at high speeds and forms the solar wind. • Solar wind consists of charged particles, or ions, that flow outward through the entire solar system, bathing each planet in a flood of particles. • The charged particles are trapped in two huge rings in Earth’s magnetic field, called the Van Allen belts, where they collide with gases in Earth’s atmosphere, causing an aurora.

8. The Sun Solar Activity • The Sun’s magnetic field disturbs the solar atmosphere periodically and causes new features to appear in a process called solar activity. • Sunspots are cooler areas that form on the surface of the photosphere due to magnetic disturbances, which appear as dark spots.

9. The Sun Solar Activity Solar Activity Cycle • The number of sunspots changes regularly, and on average reaches a maximum number every 11.2 years. • The length of the solar activity cycle is 22.4 years. • The solar activity cycle starts with minimum spots and progresses to maximum spots. • The Sun’s magnetic field then reverses in polarity, and the spots start at a minimum number and progress to a maximum number again. • The magnetic field then switches back to the original polarity and completes the solar activity cycle.

10. The Sun Solar Activity Other Solar Features • Coronal holes, often located over sunspot groups, are areas of low density in the gas of the corona. • Solar flares are violent eruptions of particles and radiation from the surface of the Sun that are associated with sunspots. • When these particles reach Earth, they can interfere with communications and damage satellites. • A prominence, sometimes associated with flares, is an arc of gas that is ejected from the chromosphere, or gas that condenses in the inner corona and rains back to the surface.

11. The Sun Solar Activity Impact on Earth • Some scientists have found evidence of subtle climate variations within 11-year periods. • There were severe weather changes on Earth during the latter half of the 1600s when the solar activity cycle stopped and there were no sunspots for nearly 60 years. • Those 60 years were known as the “Little Ice Age” because the weather was very cold in Europe and North America during those years.

12. The Sun The Solar Interior • Fusion occurs within the core of the Sun where the pressure and temperature are extremely high. • Fusion is the combining of lightweight nuclei, such as hydrogen, into heavier nuclei. • Fission, the opposite of fusion, is the splitting of heavy atomic nuclei into smaller, lighter atomic nuclei. • In the core of the Sun, helium is a product of the process in which hydrogen nuclei fuse. • At the Sun’s rate of hydrogen fusing, it is about halfway through its lifetime, with about another 5 billion years left.

13. The Sun The Solar Interior Energy from the Sun • The quantity of energy that arrives on Earth every day from the Sun is enormous. • Above Earth’s atmosphere, 1354 J of energy is received in 1 m2 per second (1354 W/m2). • Not all of this energy reaches the ground because some is absorbed and scattered by the atmosphere.

14. The Sun The Solar Interior Solar Zones • Energy produced in the core of the Sun gets to the surface through two zones in the solar interior. • In the radiative zone, which is above the core, energy is transferred from particle to particle by radiation, as atoms continually absorb energy and then re-emit it. • Above the radiative zone, in the convective zone, moving volumes of gas carry the energy the rest of the way to the Sun’s surface through convection.

15. The Sun The Solar Interior Solar Zones

16. The Sun Spectra • A spectrum is visible light arranged according to wavelengths. • There are three types of spectra: • A continuous spectrum is a spectrum that has no breaks in it that can be produced by a glowing solid or liquid, or by a highly compressed, glowing gas. • An emission spectrum has bright lines in it called emission lines that depend on the element being observed. • An absorption spectrum has dark lines called absorption lines which are caused by different chemical elements that absorb light at specific wavelengths.

17. The Sun Spectra • Absorption is caused by a cooler gas in front of a source that emits a continuous spectrum. • By comparing laboratory spectra of different gases with the dark lines in the solar spectrum, it is possible to identify the elements that make up the Sun’s outer layers.

18. The Sun Spectra A continuous spectrum is produced by a hot solid, liquid, or dense gas. When a cloud of gas is in front of this hot source, an absorption spectrum is produced. A cloud of gas without a hot source behind it will produce an emission spectrum.

19. The Sun Solar Composition • The Sun consists of hydrogen, about 73.4 percent by mass, and helium, 25 percent, as well as a small amount of other elements. • This composition is very similar to that of the gas giant planets. • The Sun’s composition represents that of the galaxy as a whole.

20. The Sun Section Assessment 1. Match the following terms with their definitions. ___ photosphere ___ corona ___ chromosphere ___ sunspot D B A C A. the middle layer of the Sun’s atmosphere B.the outermost layer of the Sun’s atmosphere C.cooler region on the Sun’s surface that forms due to magnetic irregularities D. the lowest layer of the Sun’s atmosphere

21. The Sun Section Assessment 2. How can we determine what gases are in the outer layers of the Sun’s atmosphere? Dark bands in the solar spectrum represent light that has been absorbed by the gases of its atmosphere. By comparing laboratory spectra of different gases with the dark lines in the solar spectrum, it is possible to identify the elements that make up the Sun’s outer layers.

22. The Sun Section Assessment 3. Identify whether the following statements are true or false. ______ The Sun contains more than 99 percent of all mass in the solar system. ______ Most visible light from the sun originates in the chromosphere. ______ The energy released by the Sun originates through nuclear fission. ______ Mass can be converted into energy. true false false true

23. End of Section 1

24. Measuring the Stars Objectives • Describe star distribution and distance. • Classify the types of stars. • Summarize the interrelated properties of stars. Vocabulary • constellation • binary star • parallax • apparent magnitude • absolute magnitude • luminosity • Hertzsprung-Russell diagram • main sequence

25. Measuring the Stars Groups of Stars • Constellations are the 88 groups of stars named after animals, mythological characters, or everyday objects. • Circumpolar constellations can be seen all year long as they appear to move around the north or south pole. • Summer, fall, winter, and spring constellations can be seen only at certain times of the year because of Earth’s changing position in its orbit around the Sun.

26. Measuring the Stars Groups of Stars Star Clusters • Although stars may appear to be close to each other, very few are gravitationally bound to one other. • By measuring distances to stars and observing how they interact with each other, scientists can determine which stars are gravitationally bound to each other. • A group of stars that are gravitationally bound to each other is called a cluster. • In an open cluster, the stars are not densely packed. • In a globular cluster, stars are densely packed into a spherical shape.

27. Measuring the Stars Groups of Stars Binaries • A binary star is two stars that are gravitationally bound together and that orbit a common center of mass. • More than half of the stars in the sky are either binary stars or members of multiple-star systems. • Astronomers are able to identify binary stars through several methods. • Accurate measurements can show that its position shifts back and forth as it orbits the center of mass. • In an eclipsing binary, the orbital plane of a binary system can sometimes be seen edge-on from Earth.

28. Measuring the Stars Stellar Position and Distances • Astronomers use two units of measure for long distances. • A light-year (ly) is the distance that light travels in one year, equal to 9.461 × 1012 km. • A parsec (pc) is equal to 3.26 ly, or 3.086 × 1013 km.

29. Measuring the Stars Stellar Position and Distances • To estimate the distance of stars from Earth, astronomers make use of the fact that nearby stars shift in position as observed from Earth. • Parallax is the apparent shift in position of an object caused by the motion of the observer. • As Earth moves from one side of its orbit to the opposite side, a nearby star appears to be shifting back and forth.

30. Measuring the Stars Stellar Position and Distances • The distance to a star, up to 500 pc using the latest technology, can be estimated from its parallax shift.

31. Measuring the Stars Basic Properties of Stars • The basic properties of stars include diameter, mass, brightness, energy output (power), surface temperature, and composition. • The diameters of stars range from as little as 0.1 times the Sun’s diameter to hundreds of times larger. • The masses of stars vary from a little less than 0.01 to 20 or more times the Sun’s mass.

32. Measuring the Stars Basic Properties of Stars Magnitude • One of the most basic observable properties of a star is how bright it appears. • The ancient Greeks established a classification system based on the brightnesses of stars. • The brightest stars were given a ranking of +1, the next brightest +2, and so on.

33. Measuring the Stars Basic Properties of Stars Apparent Magnitude • Apparent magnitude is based on the ancient Greek system of classification which rates how bright a star appears to be. • In this system, a difference of 5 magnitudes corresponds to a factor of 100 in brightness. • Negative numbers are assigned for objects brighter than magnitude +1.

34. Measuring the Stars Basic Properties of Stars Absolute Magnitude • Apparent magnitude does not actually indicate how bright a star is, because it does not take distance into account. • Absolute magnitude is the brightness an object would have if it were placed at a distance of 10 pc.

35. Measuring the Stars Basic Properties of Stars Luminosity • Luminosity is the energy output from the surface of a star per second. • The brightness we observe for a star depends on both its luminosity and its distance. • Luminosity is measured in units of energy emitted per second, or watts. • The Sun’s luminosity is about 3.85 × 1026 W.

36. Measuring the Stars Spectra of Stars • Stars also have dark absorption lines in their spectra and are classified according to their patterns of absorption lines.

37. Measuring the Stars Spectra of Stars Classification • Stars are assigned spectral types in the following order: O, B, A, F, G, K, and M. • Each class is subdivided into more specific divisions with numbers from 0 to 9. • The classes correspond to stellar temperatures, with the O stars being the hottest and the M stars being the coolest. • The Sun is a type G2 star, which corresponds to a surface temperature of about 5800 K.

38. B5 star K5 star F5 star M5 star Measuring the Stars Spectra of Stars Classification • All stars, including the Sun, have nearly identical compositions—about 73 percent of a star’s mass is hydrogen, about 25 percent is helium, and the remaining 2 percent is composed of all the other elements. • The differences in the appearance of their spectra are almost entirely a result of temperature effects.

39. Measuring the Stars Spectra of Stars Wavelength Shift • Spectral lines are shifted in wavelength by motion between the source of light and the observer due to the Doppler effect. • If a star is moving toward the observer, the spectral lines are shifted toward shorter wavelengths, or blueshifted. • If the star is moving away, the wavelengths become longer, or redshifted.

40. Measuring the Stars Spectra of Stars Wavelength Shift

41. Measuring the Stars Spectra of Stars Wavelength Shift • The higher the speed, the larger the shift, and thus spectral line wavelengths can be used to determine the speed of a star’s motion. • Astronomers can learn only about the portion of a star’s motion that is directed toward or away from Earth.

42. Measuring the Stars Spectra of Stars H-R Diagrams • A Hertzsprung-Russell diagram, or H-R diagram, demonstrates the relationship between mass, luminosity, temperature, and the diameter of stars. • An H-R diagram plots the absolute magnitude on the vertical axis and temperature or spectral type on the horizontal axis.

43. Measuring the Stars Spectra of Stars H-R Diagrams • The main sequence, which runs diagonally from the upper-left corner to the lower-right corner of an H-R diagram, represents about 90 percent of stars. • Red giants are large, cool, luminous stars plotted at the upper-right corner. • White dwarfs are small, dim, hot stars plotted in the lower-left corner.

44. Measuring the Stars Spectra of Stars H-R Diagrams

45. Measuring the Stars Section Assessment 1. Match the following terms with their definitions. ___ binary star ___ absolute magnitude ___ luminosity ___ parallax B C A D A. the energy output from the surface of a star per second B.when two stars are gravitationally bound and orbit a common center of mass C.the brightness an object would have if placed at a set distance D. an apparent shift in the position of an object caused by the motion of the observer

46. Measuring the Stars Section Assessment 2. How can astronomers measure the speed at which a star is moving? Spectral lines are shifted in wavelength by motion between the source of light and the observer. If a star is moving toward the observer, spectral lines are blueshifted. If a star is moving away, spectral lines are redshifted. The higher the speed, the larger the shift.

47. Measuring the Stars Section Assessment 3. Identify whether the following statements are true or false. ______ The full Moon has less brightness than Venus on the absolute magnitude scale. ______ Luminosity of stars is a relatively consistent stellar property. ______ Around two-thirds of the stars in the sky are either binary stars or members of multi-star systems. ______ The Sun is part of the main sequence. true false false true

48. End of Section 2

49. Stellar Evolution Objectives • Explain how astronomers learn about the internal structure of stars. • Describe how the Sun will change during its lifetime and how it will end up. • Compare the evolutions of stars of different masses. Vocabulary • nebula • protostar • neutron star • supernova • black hole

50. Stellar Evolution Basic Structure of Stars • The mass and the composition of a star determine nearly all its other properties. • Hydrostatic equilibrium is the balance between gravity squeezing inward and pressure from nuclear fusion and radiation pushing outward. • This balance, which is governed by the mass of a star, must hold for any stable star; otherwise, the star would expand or contract.