1. Stars Chapter 30

2. Properties of the Sun The Sun is the largest object in the solar system, in both size and mass. 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.

3. Properties of the Sun The solar interior is gaseous throughout because of its high temperature—about 1 × 10 7 K in the center. The solar interior is gaseous throughout because of its high temperature—about 1 × 10 7 K in the center. Many of the gases are in a plasma state Many of the gases are in a plasma state The outer layers of the Sun are not quite hot enough to be plasma. The outer layers of the Sun are not quite hot enough to be plasma.

4. The Sun’s Atmosphere The photosphere, approximately 400 km in thickness, is the lowest layer of 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.

5. The Sun’s Atmosphere The chromosphere is above the photosphere. The chromosphere is above the photosphere. The corona, which is the top layer of the Sun’s atmosphere, extends several million kilometers outward from the top of the chromosphere. The corona, which is the top layer of the Sun’s atmosphere, extends several million kilometers outward from the top of the chromosphere.

6. The Sun’s Atmosphere Solar Wind – Gas flows outward from the corona at high speeds and forms the solar wind which consists of charged particles, or ions, that flow outward through the entire solar system. – 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.

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

8. 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. – 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.

9. Solar Activity Other Solar Features – Solar flares are violent eruptions of particles and radiation from the surface of the Sun that are associated with sunspots. – 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.

10. The Solar Interior Fusion occurs within the core of the Sun where the pressure and temperature are extremely high. 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. In the core of the Sun, helium is a product of the process in which hydrogen nuclei fuse. 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. At the Sun’s rate of hydrogen fusing, it is about halfway through its lifetime, with about another 5 billion years left.

11. The Sun

12. 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. A spectrum is visible light arranged according to wavelengths. A spectrum is visible light arranged according to wavelengths.

13. 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. The Sun consists of hydrogen, about 73.4 percent by mass, and helium, 25 percent, as well as a small amount of other elements. The Sun’s composition represents that of the galaxy as a whole. The Sun’s composition represents that of the galaxy as a whole.

14. Groups of Stars Constellations are the 88 groups of stars named after animals, mythological characters, or everyday objects. Constellations are the 88 groups of stars named after animals, mythological characters, or everyday objects. – Circumpolar constellations can be seen all year long. – Summer, fall, winter, and spring constellations can be seen only at certain times of the year.

15. Aries, Cancer, Canis Major, Draco, Hercules, Hydra, Leo, Libra, Orion, Pegasus, Pices, Taurus Ursa Minor, Virgo

16. Groups of Stars Star Clusters – 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 an open cluster, the stars are not densely packed. In a globular cluster, stars are densely packed into a spherical shape. In a globular cluster, stars are densely packed into a spherical shape.

17. 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.

18. Constellations

19. Stellar Position and Distances Astronomers use two units of measure for long 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 × 10 12 km. – A parsec (pc) is equal to 3.26 ly, or 3.086 × 10 13 km.

20. 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. 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. 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. As Earth moves from one side of its orbit to the opposite side, a nearby star appears to be shifting back and forth.

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

22. Basic Properties of Stars The diameters of stars range from as little as 0.1 times the Sun’s diameter to hundreds of times larger. 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. The masses of stars vary from a little less than 0.01 to 20 or more times the Sun’s mass.

23. Basic Properties of Stars Apparent Magnitude – 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. – 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.

24. 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.

25. Basic Properties of Stars Specta – Luminosity is the energy output from the surface of a star per second. Stars also have dark absorption lines in their spectra and are classified according to their patterns of absorption lines. Stars also have dark absorption lines in their spectra and are classified according to their patterns of absorption lines. Luminosity

26. Spectra of Stars Classification by Spectra – 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. B5 star F5 star K5 star M5 star – The differences in the appearance of their spectra are almost entirely a result of temperature effects.

27. 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 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. If the star is moving away, the wavelengths become longer, or redshifted.

28. 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.

29. 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.

30. Basic Structure of Stars The mass and the composition of a star determine nearly all its other properties. 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.

31. Basic Structure of Stars Fusion – Inside a star, the density and temperature increase toward the center, where energy is generated by nuclear fusion. – Stars on the main sequence all produce energy by fusing hydrogen into helium, as the Sun does. Stars that are not on the main sequence either fuse different elements in their cores or do not undergo fusion at all.

32. Stellar Evolution and Life Cycles Star Formation – A nebula (pl. nebulae) is a cloud of interstellar gas and dust. – Star formation begins when the nebula collapses on itself as a result of its own gravity. – A protostar is a hot condensed object that forms at the center of the disk that will become a new star.

33. Stellar Evolution and Life Cycles Fusion Begins – Eventually, the temperature inside a protostar becomes hot enough for nuclear fusion reactions to begin converting hydrogen to helium.

34. – When the hydrogen in its core is gone, a star has a helium center and outer layers made of hydrogen-dominated gas. – Some hydrogen continues to react in a thin layer at the outer edge of the helium core causing the outer layers to expand forming a red giant. The Sun’s Life Cycle What happens during a star’s life cycle depends on its mass. What happens during a star’s life cycle depends on its mass.

35. The Sun’s Life Cycle – While the star is a red giant, it loses gas from its outer layers while its core becomes hot enough for helium to react and form carbon. – When the helium in the core is all used up, the star is left with a core made of carbon. – The outer layers expand once again and are driven off entirely by pulsations that develop, becoming a shell of gas called a planetary nebula. – In the center of a planetary nebula, the core of the star remains as a white dwarf made of carbon.

36. The Sun’s Life Cycle A Nebula Once Again

37. The Sun’s Life Cycle Pressure in White Dwarfs – A white dwarf is stable because it is supported by the resistance of electrons being squeezed close together and does not require a source of heat to be maintained. – A star that has less mass than that of the Sun has a similar life cycle, except that helium may never form carbon in the core, and the star ends as a white dwarf made of helium.

38. A massive star undergoes many reaction phases and produces many elements in its interior. A massive star undergoes many reaction phases and produces many elements in its interior. Life Cycles of Massive Stars A massive star begins its life high on the main sequence with hydrogen being converted to helium. A massive star begins its life high on the main sequence with hydrogen being converted to helium. The star becomes a red giant several times as it expands following the end of each reaction stage. The star becomes a red giant several times as it expands following the end of each reaction stage.

39. A massive star loses much of its mass during its lifetime. A massive star loses much of its mass during its lifetime. White dwarf composition is determined by how many reaction phases the star went through before reactions stopped. White dwarf composition is determined by how many reaction phases the star went through before reactions stopped. Life Cycles of Massive Stars As more shells are formed by the fusion of different elements, the star expands to a larger size and becomes a supergiant. As more shells are formed by the fusion of different elements, the star expands to a larger size and becomes a supergiant.

40. Life Cycles of Massive Stars Neutron stars and Pulsars – A star that begins with a mass between about 8 and 20 times the Sun’s mass will end up with a core that is too massive to be supported by electron pressure. – Once no further energy-producing reactions can occur, the core of the star violently collapses in on itself and protons and electrons in the core merge to form a neutron star.

41. Life Cycles of Massive Stars Supernovae – A neutron star has a mass of 1.5 to 3 times the Sun’s mass but a radius of only about 10 km. – Infalling gas rebounds when it strikes the hard surface of the neutron star and explodes outward. – A supernova is a massive explosion in which the entire outer portion of the star is blown off and elements that are heavier than iron are created.

42. Life Cycles of Massive Stars Black Holes – A star that begins with more than about 20 times the Sun’s mass will not be able to form a neutron star. – The resistance of neutrons to being squeezed is not great enough to stop the collapse, so the core of the star simply continues to collapse forever, compacting matter into a smaller and smaller volume. – A black hole is a small, extremely dense remnant of a star whose gravity is so immense that not even light can escape its gravity field.