Astronomy Lecture

1. Astronomy Lecture The Lives of Stars from Birth Through Middle Age Lesson 13, Chapter 12 Lesson 13, Chapter 12

2. Cosmic Mountains of Creation A Spitzer Space Telescope image of star-forming region W5, 7000 light-years away, in the constellation Cassiopeia. Young stars can be seen developing in clouds of gas and dust. (Lori Allen [Harvard- Smithsonian CfA] et al., JPL-Caltech, NASA) Cosmic Mountains of Creation A Spitzer Space Telescope image of star-forming region W5, 7000 light-years away, in the constellation Cassiopeia. Young stars can be seen developing in clouds of gas and dust. (Lori Allen [Harvard- Smithsonian CfA] et al., JPL-Caltech, NASA)

3. Protostars and Pre-Main-Sequence Stars Interstellar Medium. Stars form from the gas and dust that is scattered throughout the galaxy. Molecular Clouds. Large clouds of gas may be more massive than 1000 suns. Regions of the coldest and densest clouds may begin to gravitationally collapse. Protostars. As the central condensing regions heat up by compression, they begin to glow, and form a photosphere. Pre-Main-Sequence Stars. When the interior of the protostars become hot enough, nuclear fusion begins in their cores, and they join the main sequence on the H-R diagram. Protstars and Pre-Main-Sequence StarsProtstars and Pre-Main-Sequence Stars

4. Composition of the Interstellar Medium Composition of the Interstellar Medium

5. Reddening of Starlight by the Interstellar Medium Dust in interstellar space scatters more short-wavelength (blue) light passing through it than longer-wavelength colors. Therefore, stars and other objects seen through interstellar clouds appear redder than they would otherwise.Reddening of Starlight by the Interstellar Medium Dust in interstellar space scatters more short-wavelength (blue) light passing through it than longer-wavelength colors. Therefore, stars and other objects seen through interstellar clouds appear redder than they would otherwise.

6. Interstellar Reddening Light from these two nebulae pass through different amounts of interstellar dust and therefore they appear to have different colors. Because NGC 3603 is farther away, it appears a ruddier shade of red than does NGC 3576. (Anglo- Australian Observatory)Interstellar Reddening Light from these two nebulae pass through different amounts of interstellar dust and therefore they appear to have different colors. Because NGC 3603 is farther away, it appears a ruddier shade of red than does NGC 3576. (Anglo- Australian Observatory)

7. Stars and the Interstellar Medium This open cluster, called the Pleiades, can easily be seen with the naked eye in the constellation Taurus (the Bull). It lies about 375 light-years (116 pc) from Earth. The stars are not shedding mass. The blue glow surrounding the stars of the Pleiades is a reflection nebula created as some of the stars' radiation scatters off preexisting dust grains in their vicinity. The cluster has a diameter of about 5 light-years, is about 100 million years old, and contains about 500 stars. (Gary Goodman, 1996)Stars and the Interstellar Medium This open cluster, called the Pleiades, can easily be seen with the naked eye in the constellation Taurus (the Bull). It lies about 375 light-years (116 pc) from Earth. The stars are not shedding mass. The blue glow surrounding the stars of the Pleiades is a reflection nebula created as some of the stars' radiation scatters off preexisting dust grains in their vicinity. The cluster has a diameter of about 5 light-years, is about 100 million years old, and contains about 500 stars. (Gary Goodman, 1996)

8. The Orion Nebula The middle "star" in Orion's sword is actually the Orion Nebula, part of a huge system of interstellar gas and dust in which new stars are now forming. The Orion Nebula is a region visible to the naked eye. It is 1600 ly (490 pc) from Earth and has a diameter of roughly 16 ly (5 pc). This nebula's mass is about 300 Msun. In the center are four massive stars, brightest members of the Trapezium star cluster, which cause the nebula to glow. (Gary Goodman, 1996)The Orion Nebula The middle "star" in Orion's sword is actually the Orion Nebula, part of a huge system of interstellar gas and dust in which new stars are now forming. The Orion Nebula is a region visible to the naked eye. It is 1600 ly (490 pc) from Earth and has a diameter of roughly 16 ly (5 pc). This nebula's mass is about 300 Msun. In the center are four massive stars, brightest members of the Trapezium star cluster, which cause the nebula to glow. (Gary Goodman, 1996)

9. A Gas- and Dust-Rich Region of Orion A variety of nebulae appear in the sky around Alnitak, also called ? (zeta) Orionis, the easternmost star in the belt of Orion. To the left of Alnitak is a bright, red emission nebula, (The Flame Nebula) called NGC 2024. The glowing gases in emission nebulae are excited by ultraviolet radiation from young, massive stars. Dust grains obscure part of NGC 2024, giving the appearance of black streaks, while the distinctively shaped dust cloud, called the Horsehead Nebula, blocks the light from the background nebula IC 434. The Horsehead is part of a larger complex of dark interstellar matter, seen in the lower left of this image. Above and to the left of the Horsehead Nebula is the reflection nebula NGC 2023, whose dust grains scatter blue light from stars between us and is more effectively than any other color. All of this nebulosity lies about 1600 light-years from Earth, while the star Alnitak is only 815 light-years away from us. NGC refers to the New General Catalog of stars and IC stands for Index Catalogs, two supplements to the NGC. (Gary Goodman, 1996)A Gas- and Dust-Rich Region of Orion A variety of nebulae appear in the sky around Alnitak, also called ? (zeta) Orionis, the easternmost star in the belt of Orion. To the left of Alnitak is a bright, red emission nebula, (The Flame Nebula) called NGC 2024. The glowing gases in emission nebulae are excited by ultraviolet radiation from young, massive stars. Dust grains obscure part of NGC 2024, giving the appearance of black streaks, while the distinctively shaped dust cloud, called the Horsehead Nebula, blocks the light from the background nebula IC 434. The Horsehead is part of a larger complex of dark interstellar matter, seen in the lower left of this image. Above and to the left of the Horsehead Nebula is the reflection nebula NGC 2023, whose dust grains scatter blue light from stars between us and is more effectively than any other color. All of this nebulosity lies about 1600 light-years from Earth, while the star Alnitak is only 815 light-years away from us. NGC refers to the New General Catalog of stars and IC stands for Index Catalogs, two supplements to the NGC. (Gary Goodman, 1996)

10. A Dark Nebula The dark nebula Barnard 86 is located in Sagittarius. It is visible in this photograph simply because it blocks out light from the stars beyond it. The cluster of bluish stars to the left of the dark nebula is a member of a star cluster, called NGC 6520. (Anglo-Australian Observatory) A Dark Nebula The dark nebula Barnard 86 is located in Sagittarius. It is visible in this photograph simply because it blocks out light from the stars beyond it. The cluster of bluish stars to the left of the dark nebula is a member of a star cluster, called NGC 6520. (Anglo-Australian Observatory)

11. Toward a Theory of Star Formation Cold, Dark Molecular Clouds. Star formation begins when a cold, dark cloud starts to collapse under its own weight. Minimum Size for Star-Forming Clouds. A cloud must have a mass of about 1000 suns to have enough gravity to overcome the 100 K thermal motion. Supernovas Trigger the Birth of Stars. When a star explodes, the shell of gas, expanding at supersonic speeds, rams into a giant molecular cloud, compressing the gas and causing it to contract, thus stimulating star birth. Toward a Theory of Star Formation Cold, Dark Molecular Clouds may start to collapse under their own weight. Minimum Size for Star-Forming Clouds is about 1000 suns. Supernovas Trigger the Birth of StarsToward a Theory of Star Formation Cold, Dark Molecular Clouds may start to collapse under their own weight. Minimum Size for Star-Forming Clouds is about 1000 suns. Supernovas Trigger the Birth of Stars

12. The Cygnus Loop, a Supernova Remnant This is an image of the Cygnus Loop, the remnant of a supernova that occurred nearly 20,000 years ago. The expanding spherical shell of gas now has a diameter of about 120 light-years. The entire Cygnus Loop has an angular diameter in our sky 6 times wider than the Moon. (Gary Goodman, 1996)The Cygnus Loop, a Supernova Remnant This is an image of the Cygnus Loop, the remnant of a supernova that occurred nearly 20,000 years ago. The expanding spherical shell of gas now has a diameter of about 120 light-years. The entire Cygnus Loop has an angular diameter in our sky 6 times wider than the Moon. (Gary Goodman, 1996)

13. The Core of the Rosette Nebula The large, circular Rosette Nebula (NGC 2237) is near one end of a sprawling giant molecular cloud in the constellation Monoceros (the Unicorn). Radiation from young, hot stars has blown gas away from the center of this nebula. Some of this gas has become clumped in Bok globules that appear silhouetted against the glowing background gases. New star formation is taking place within these globules. The entire Rosette Nebula has an angular diameter on the sky nearly 3 times that of the Moon, and it lies some 3000 light-years from Earth. (Anglo-Australian Observatory) The Core of the Rosette Nebula The large, circular Rosette Nebula (NGC 2237) is near one end of a sprawling giant molecular cloud in the constellation Monoceros (the Unicorn). Radiation from young, hot stars has blown gas away from the center of this nebula. Some of this gas has become clumped in Bok globules that appear silhouetted against the glowing background gases. New star formation is taking place within these globules. The entire Rosette Nebula has an angular diameter on the sky nearly 3 times that of the Moon, and it lies some 3000 light-years from Earth. (Anglo-Australian Observatory)

14. Four Stages in the Process of Star Formation in Molecular Clouds Collapse of an Interstellar Cloud. When the cloud begins to collapse, it fragments into smaller and smaller clumps, which can produce a few very large stars or a whole cluster of smaller stars. Protostar. When the dense opaque region at the center of each clump reaches about 10,000 K, it is called a protostar and eventually produces a �surface� or photosphere. It is surrounded by a circumstellar disk of gas and dust. First Fusion Reactions and Bipolar Outflow. The first fusion reactions produce deuterium. The energy released creates a stellar wind of out-flowing material, which opposes the in-falling material farther out. The stellar wind rushes through the paths of least resistance at the rotational poles, leading to bipolar jets of material that flow out along the poles. Inflow Terminates. The in-flow finally terminates revealing the newly formed pre-main-sequence star. Four Stages in the Process of Star Formation in Molecular Clouds Collapse of an Interstellar Cloud Protostar First Fusion Reactions and Bipolar Outflow Inflow TerminatesFour Stages in the Process of Star Formation in Molecular Clouds Collapse of an Interstellar Cloud Protostar First Fusion Reactions and Bipolar Outflow Inflow Terminates

15. Four Stages in the Process of Star Formation in Molecular Clouds - Diagram Collapse of an Interstellar Cloud Protostar First Fusion Reactions and Bipolar Outflow Inflow TerminatesFour Stages in the Process of Star Formation in Molecular Clouds - Diagram Collapse of an Interstellar Cloud Protostar First Fusion Reactions and Bipolar Outflow Inflow Terminates

16. Evolutionary Track from Protostar to Pre-Main-Sequence Star Contraction. The protostar is still not in hydrostatic equilibrium. As heat from its inner core diffuses out from the center and radiates into space, contractions slow, but do not stop. Pre-main-sequence Star. When a protostar ceases to accumulate mass, it becomes a pre-main-sequence star. Protons begin fusing into deuterium in the core. These young stars often exhibit violent surface activity during their evolution, resulting in extremely strong stellar winds. These gas ejecting stars are called T Tauri stars. Contraction Slows Down. Luminosity decreases as the density increases because the star becomes smaller. As the luminosity decreases, so also does the contraction rate. A Newborn Star. Energy from nuclear fusion of hydrogen stops the contraction and a star is born. Pressure and gravity have finally balanced. The zero-age main-sequence (ZAMS) is the location on the H-R diagram where stars are born and spend most of their lives. Evolutionary Track from Protostar to Pre-Main-Sequence Star Contraction Pre-main-sequence Star Contraction Slows Down A Newborn StarEvolutionary Track from Protostar to Pre-Main-Sequence Star Contraction Pre-main-sequence Star Contraction Slows Down A Newborn Star

17. Pre-Main-Sequence Evolutionary Tracks This H-R diagram shows evolutionary tracks based on models of seven stars having different masses. The dashed lines indicate the stage reached after the indicated number of years of evolution. The birth line, shown in blue, is the location where each protostar stops accreting matter and becomes a pre-main-sequence star. Note that all tracks terminate on the main sequence at points that agree with the mass-luminosity relation. Pre-Main-Sequence Evolutionary Tracks This H-R diagram shows evolutionary tracks based on models of seven stars having different masses. The dashed lines indicate the stage reached after the indicated number of years of evolution. The birth line, shown in blue, is the location where each protostar stops accreting matter and becomes a pre-main-sequence star. Note that all tracks terminate on the main sequence at points that agree with the mass-luminosity relation.

18. Mass Limits, Large and Small Minimum Size. Cloud fragments that are too small will never become a real star. These fragments are called brown dwarfs. A body must have at least 0.08iM? (about 80 times the mass of Jupiter) for it to become a real star. Maximum Size. The upper limit to stellar mass appears to be about 150 M?. This limit is the result of mass loss by extremely massive stars in the early stages of their development. Mass Limits, Large and Small Minimum ~ 0.08 M? Maximum ~ 150 M?Mass Limits, Large and Small Minimum ~ 0.08 M? Maximum ~ 150 M?

19. An H II Region This emission nebula, M16, called the Eagle Nebula because of its shape, surrounds a star cluster. It is so named because it was the sixteenth object in the Messier Catalogue of astronomical objects. Star formation is presently occurring in M16, which is located 7000 lightyears from Earth in the constellation of Serpens Cauda (the Serpent's Tail). Several bright, hot O and B stars are responsible for the ionizing radiation that causes the gases to glow. (Inset) Star formation is occurring inside these dark pillars of gas and dust. Intense ultraviolet radiation from existing massive stars off to the right of this image is evaporating the dense cores in the pillars, thereby prematurely terminating star formation there. Newly revealed stars are visible at the tips of the columns. (Anglo-Australian Observatory; J. Hester and P. Scowen, Arizona State University; NASA) An H II Region This emission nebula, M16, called the Eagle Nebula because of its shape, surrounds a star cluster. It is so named because it was the sixteenth object in the Messier Catalogue of astronomical objects. Star formation is presently occurring in M16, which is located 7000 lightyears from Earth in the constellation of Serpens Cauda (the Serpent's Tail). Several bright, hot O and B stars are responsible for the ionizing radiation that causes the gases to glow. (Inset) Star formation is occurring inside these dark pillars of gas and dust. Intense ultraviolet radiation from existing massive stars off to the right of this image is evaporating the dense cores in the pillars, thereby prematurely terminating star formation there. Newly revealed stars are visible at the tips of the columns. (Anglo-Australian Observatory; J. Hester and P. Scowen, Arizona State University; NASA)

20. The Main-Sequence Stage Stars reside on the main sequence stage as long as they are converting hydrogen into helium in their cores. Stars stay at the same spot on the H-R diagram for about 90% of their lives. They do not travel up or down the sequence. When the hydrogen in their cores is depleted, stars begin to move off the main sequence. The Main-Sequence StageThe Main-Sequence Stage

21. Leaving the Main-Sequence � Low-Mass Stars Red dwarf stars, < 0.4 M?, have convective cores. The material in the star is continually mixed. Thus a helium core never develops. The lifespan of these stars is many times the age of the universe. They will eventually convert their entire mass into helium, and then they will begin to cool and move down and to the right from the main-sequence. Leaving the Main-Sequence � Low-Mass StarsLeaving the Main-Sequence � Low-Mass Stars

22. Low-Mass Fully Convective Star This drawing shows how the helium created in the cores of red dwarfs rises into the outer layers of the star by convection, while the hydrogen from the outer layers descend into the core. This process continues until the entire star is helium. Low-Mass Fully Convective Star This drawing shows how the helium created in the cores of red dwarfs rises into the outer layers of the star by convection, while the hydrogen from the outer layers descend into the core. This process continues until the entire star is helium.

23. Leaving the Main-Sequence � Higher-Mass Stars Stars with masses greater than 0.4 M? have radiative cores. Material in the core does not mix with that in the upper layers. When hydrogen is substantially depleted in the center, the burning moves to higher and higher layers in the core. The inner core becomes pure helium. During this stage the star begins to become larger and more luminous, although cooler at the surface. Leaving the Main-Sequence � Higher-Mass StarsLeaving the Main-Sequence � Higher-Mass Stars

24. Structure of a Star in the Hydrogen Shell Burning Stage

25. Red Giants � part 1 Contraction of the Helium Core. Helium cannot burn at the core temperature of 107 K. Without the energy of nuclear burning to maintain the pressure, the core begins to contract. Core Heats Up. The compression of the core heats it to higher temperature causing the hydrogen in the surrounding shell to burn faster than it did before. Thus the star grows brighter as the fuel is used up. Red Giants � part 1 Contraction of the Helium Core Core Heats UpRed Giants � part 1 Contraction of the Helium Core Core Heats Up

26. Red Giants � part 2 Star Expands. Gas pressure from the enhanced hydrogen burning forces the star�s non-burning outer layers to expand and cool. The star is on its way to becoming a red giant. Subgiant Branch. Initially, as the surface cools, the star�s size increases about 3 times, while its luminosity increases just a little. It moves horizontally to the right on the H-R diagram, which is called the subgiant branch. Red Giants � part 2 Contraction of the Helium Core Core Heats Up Star Expands Subgiant Branch Red Giants � part 2 Contraction of the Helium Core Core Heats Up Star Expands Subgiant Branch

27. Red Giants � part 3 Red Giant Branch. The surface temperature and that deep within has fallen so low that the material is opaque to radiation. The heat is carried out by convection. The surface temperature remains constant even as the energy output increases. The star expands to a red giant. Physical Changes. The surface temperature cools to an M-type star. The size increases about 100 times. The luminosity increases 1000 times. The change from main sequence to red giant takes about 100 million years. Red Giants � part 3 Contraction of the Helium Core Core Heats Up Star Expands Subgiant Branch Red Giant Branch Physical Changes Red Giants � part 3 Contraction of the Helium Core Core Heats Up Star Expands Subgiant Branch Red Giant Branch Physical Changes

28. The Sun Today and as a Giant In about 5 billion years, when the Sun expands to become a giant, its diameter will increase a hundredfold from what it is now, while its core becomes more compact. Today, the Sun's energy is produced in a hydrogenfusing core whose diameter is about 200,000 km. When the Sun becomes a giant, it will draw its energy from a hydrogen-fusing shell that surrounds a compact helium-rich core. The helium core will have a diameter of only 30,000 km. The Sun's diameter will be about 100 times larger, and it will be about 2000 times more luminous as a giant than it is today.The Sun Today and as a Giant In about 5 billion years, when the Sun expands to become a giant, its diameter will increase a hundredfold from what it is now, while its core becomes more compact. Today, the Sun's energy is produced in a hydrogenfusing core whose diameter is about 200,000 km. When the Sun becomes a giant, it will draw its energy from a hydrogen-fusing shell that surrounds a compact helium-rich core. The helium core will have a diameter of only 30,000 km. The Sun's diameter will be about 100 times larger, and it will be about 2000 times more luminous as a giant than it is today.

29. In about 5 billion years, our Sun will swell into a giant with a radius of about � AU, swallowing Mercury. The Earth will be scorched to a cinder. Picture of Sun from Earth during the �last days�.Picture of Sun from Earth during the �last days�.

30. Red Giants � part 4 Electron Degeneracy. For stars less than 2 M?, the density of the core is very high. Nucleii and electrons are squeezed very close together. The laws of quantum mechanics prohibit electrons from getting even closer unless they have higher velocity. This makes the core virtually incompressible, much like a liquid. The pressure is governed by the density of the tightly packed electrons (electron degeneracy pressure) not by the temperature. In this state, the pressure does not depend on the temperature. Red Giants � part 4 Contraction of the Helium Core Core Heats Up Star Expands Subgiant Branch Red Giant Branch Physical Changes Electron DegeneracyRed Giants � part 4 Contraction of the Helium Core Core Heats Up Star Expands Subgiant Branch Red Giant Branch Physical Changes Electron Degeneracy

31. Red Giants � part 5 Helium Fusion. A few hundred million years after leaving the main-sequence, the core temperature reaches 108 K, high enough for helium to fuse into carbon through the triple-alpha process. The central fires reignite. For stars with mass < 2 M?, the pressure in the core is governed by electron degeneracy. The temperature increases, but the pressure does not, so the core does not expand and cool. This results in a runaway explosion. The helium burns rapidly for a few hours � generating enough energy to where thermal pressure again dominates. Red Giants � part 5 Contraction of the Helium Core Core Heats Up Star Expands Subgiant Branch Red Giant Branch Physical Changes Electron Degeneracy Helium FusionRed Giants � part 5 Contraction of the Helium Core Core Heats Up Star Expands Subgiant Branch Red Giant Branch Physical Changes Electron Degeneracy Helium Fusion

32. Evolution of Stars Off the Main Sequence Hydrogen fusion occurs in the cores of main-sequence stars. When the core is converted into helium, fusion there ceases and then begins in a shell that surrounds the core. The star expands into the giant phase. This newly formed helium sinks into the core, which heats up. Eventually, the core reaches 108 K, whereupon core helium fusion begins. This causes the core to expand, slowing the hydrogen shell fusion and thereby forcing the outer layers of the star to contract. Evolution of Stars Off the Main Sequence Hydrogen fusion occurs in the cores of main-sequence stars. When the core is converted into helium, fusion there ceases and then begins in a shell that surrounds the core. The star expands into the giant phase. This newly formed helium sinks into the core, which heats up. Eventually, the core reaches 108 K, whereupon core helium fusion begins. This causes the core to expand, slowing the hydrogen shell fusion and thereby forcing the outer layers of the star to contract.

33. Post-Main-Sequence Evolution The luminosity of the Sun changes as our star evolves. It began as a protostar with decreasing luminosity. On the main sequence today, it gradually brightens. Giant phase evolution occurs more rapidly, with faster and larger changes of luminosity. Note the change in scale of the horizontal axis scale at 12 billion years.Post-Main-Sequence Evolution The luminosity of the Sun changes as our star evolves. It began as a protostar with decreasing luminosity. On the main sequence today, it gradually brightens. Giant phase evolution occurs more rapidly, with faster and larger changes of luminosity. Note the change in scale of the horizontal axis scale at 12 billion years.

34. Post-Main-Sequence Evolution Model-based evolutionary tracks of five stars are shown on this H-R diagram. In the high-mass stars, core helium fusion ignites smoothly where the evolutionary tracks make a sharp turn upward into the giant region of the diagram. Post-Main-Sequence Evolution Model-based evolutionary tracks of five stars are shown on this H-R diagram. In the high-mass stars, core helium fusion ignites smoothly where the evolutionary tracks make a sharp turn upward into the giant region of the diagram.

35. Giant Stars Horizontal Branch. While a star is burning helium in its core, the star again becomes stable at a higher surface temperaure and slightly lower luminosity. In the H-R diagram, it resides in a region called the horizontal branch. Variable Stars. Stars on the horizontal branch undergo a period of instability in their atmospheres, which rhythmically expand and contract, causing them to vary in brightness. These are pulsating variable stars. Giant StarsGiant Stars

36. The Instability Strip The instability strip occupies a region between the main sequence and the giant branch on the H-R diagram. A star passing through this region along its evolutionary track becomes unstable and pulsates.The Instability Strip The instability strip occupies a region between the main sequence and the giant branch on the H-R diagram. A star passing through this region along its evolutionary track becomes unstable and pulsates.

37. The Rate of Stellar Evolution Stars must evolve because they are gradually using up their fuel supply. The mass of a star is a measure of how much fuel it starts out with. The luminosity of a star is a measure of how fast it is burning its fuel. The lifetime of a star is proportional to its mass divided by its luminosity. The Rate of Stellar EvolutionThe Rate of Stellar Evolution

38. Main-Sequence Lifetimes We know that the main-sequence lifetime of the Sun is about 9 billion years. Using the mass-luminosity relationship (from the previous chapter), with mass and luminosity relative to the Sun,  Main-Sequence LifetimesMain-Sequence Lifetimes

39. Main-Sequence Lifetimes - TableMain-Sequence Lifetimes - Table

40. Formation of Star Clusters Massive hot stars form first. These are of spectral types O and B, so they are called OB Associations. They produce lots of UV radiation, which ionizes the surrounding gas. Stellar winds and ionizing radiation from the O and B stars carve out a cavity in the surrounding nebula. Where the out-flow is supersonic, it creates a shock wave, which forms along the outer edge of the cavity, compressing the gas as it passes, and therefore stimulating a new round of star birth. This results in a group of stars, all formed about the same time, lying in the same region of space. This is called a star cluster. Formation of Star ClustersFormation of Star Clusters

41. Types of Star Clusters Open Clusters. These are also called galactic clusters because they all lie in the galactic plane. Most open clusters contain a few tens to about a 1000 stars. They are relatively young compared to the age of the galaxy. Globular Clusters. These are giant clusters of hundreds of thousands of tightly knit stars. They are roughly spherical and are typically about 50 pc in diameter. They lie in a spherical region surrounding our galaxy. These are very old � as old as the galaxy itself. Types of Star ClustersTypes of Star Clusters

42. A Globular Cluster This cluster, M10, is about 85 light-years across and is located in the constellation Ophiuchus (the Serpent Holder), roughly 16,000 lightyears from Earth. Most of the stars here are either red giants or blue, horizontal-branch stars with both core helium fusion and hydrogen shell fusion. (T. Credner and S. Kohle, Astronomical Institutes of the University of Bonn)A Globular Cluster This cluster, M10, is about 85 light-years across and is located in the constellation Ophiuchus (the Serpent Holder), roughly 16,000 lightyears from Earth. Most of the stars here are either red giants or blue, horizontal-branch stars with both core helium fusion and hydrogen shell fusion. (T. Credner and S. Kohle, Astronomical Institutes of the University of Bonn)

43. H-R Diagram of a Star Cluster Over Time Structure of the H-R Diagram These charts summarize the evolution of a theoretical cluster, as shown by their locations on the H-R diagram. (In principle, each star's evolution could be followed separately.)Structure of the H-R Diagram These charts summarize the evolution of a theoretical cluster, as shown by their locations on the H-R diagram. (In principle, each star's evolution could be followed separately.)

44. An H-R Diagram of Open Clusters The black bands indicate where data from various star clusters fall on the H-R diagram. The ages of turnoff points (in years) are listed in red alongside the main sequence. The age of a cluster can be estimated from the location of the turnoff point, where the cluster's most massive stars are just now leaving the main sequence.An H-R Diagram of Open Clusters The black bands indicate where data from various star clusters fall on the H-R diagram. The ages of turnoff points (in years) are listed in red alongside the main sequence. The age of a cluster can be estimated from the location of the turnoff point, where the cluster's most massive stars are just now leaving the main sequence.

45. An H-R Diagram of a Globular Cluster Each dot on this graph represents the absolute magnitude and surface temperature of a star in the globular cluster M55. Note that the upper half of the main sequence is missing. The horizontal branch stars are stars that recently experienced the helium flash in their cores and now exhibit core helium fusion and hydrogen shell fusion.An H-R Diagram of a Globular Cluster Each dot on this graph represents the absolute magnitude and surface temperature of a star in the globular cluster M55. Note that the upper half of the main sequence is missing. The horizontal branch stars are stars that recently experienced the helium flash in their cores and now exhibit core helium fusion and hydrogen shell fusion.

46. Population I and Population II Stars Open clusters consist of young stars formed from material that originally came from stars that exploded long ago, enriching the interstellar gases with the heavy elements formed in their cores. Metal-rich stars are called Population I stars. Globular clusters contain the oldest stars in the galaxy. Their spectra show only weak lines of heavy elements, indicating they are metal-poor. Metal-poor stars are called Population II stars. Population I and Population II StarsPopulation I and Population II Stars

47. Spectra of a Metal-Poor and a Metal-Rich Star These spectra compare (a) a metal-poor (Population II) and (b) a metal-rich (Population I) star (the Sun) of the same surface temperature. Numerous spectral lines prominent in the solar spectrum are caused by elements heavier than hydrogen and helium. Note that corresponding lines in the metal-poor star's spectrum are weak or absent. Both spectra cover a wavelength range that includes two strong hydrogen absorption lines, labeled H? (410 nm) and Hd (434 nm). (Lick Observatory) Spectra of a Metal-Poor and a Metal-Rich Star These spectra compare (a) a metal-poor (Population II) and (b) a metal-rich (Population I) star (the Sun) of the same surface temperature. Numerous spectral lines prominent in the solar spectrum are caused by elements heavier than hydrogen and helium. Note that corresponding lines in the metal-poor star's spectrum are weak or absent. Both spectra cover a wavelength range that includes two strong hydrogen absorption lines, labeled H? (410 nm) and Hd (434 nm). (Lick Observatory)

48. Pulsating Variable Stars There are two classes of pulsating variable stars, RR Lyrae and Cepheid variables. RR Lyrae variables pulsate in 0.5 to 1 day. Cepheid variable stars have periods ranging from about 1 to 100 days. Cepheid variable stars have an important property that astronomers can use to find the distance to other galaxies. Their luminosity is related to their period of pulsation in what is called the Period-Luminosity Relationship. Pulsating Variable StarsPulsating Variable Stars

49. The Period-Luminosity Relation The period of a Cepheid variable is directly related to its average luminosity: The more luminous the Cepheid, the longer its period and the slower its pulsations. Type I Cepheids (d Cephei stars) are metal-rich stars. They are brighter than the Type II Cepheids (W Virginis stars), which are metal-poor stars. (Adapted from H. C. Arp)The Period-Luminosity Relation The period of a Cepheid variable is directly related to its average luminosity: The more luminous the Cepheid, the longer its period and the slower its pulsations. Type I Cepheids (d Cephei stars) are metal-rich stars. They are brighter than the Type II Cepheids (W Virginis stars), which are metal-poor stars. (Adapted from H. C. Arp)

50. Mass Transfer in Close Binary Systems Can Produce Unusual Double Stars More massive stars evolve more quickly than less massive stars. As they evolve off the main-sequence, they swell in size. In a binary star system, the more massive star can become so large that its atmosphere can be pulled away by a close companion star�s gravity. Mass Transfer in Close Binary Systems Can Produce Unusual Double StarsMass Transfer in Close Binary Systems Can Produce Unusual Double Stars

51. Detached, Semidetached, Contact, and Overcontact Binaries In a detached binary, neither star fills its Roche lobe. If one star fills its Roche lobe, the binary is semidetached. Mass transfer is often observed in semidetached binaries. In a contact binary, both stars fill their Roche lobes. The two stars in an overcontact binary both overfill their Roche lobes. The two stars actually share the same outer atmosphere. Detached, Semidetached, Contact, and Overcontact Binaries In a detached binary, neither star fills its Roche lobe. If one star fills its Roche lobe, the binary is semidetached. Mass transfer is often observed in semidetached binaries. In a contact binary, both stars fill their Roche lobes. The two stars in an overcontact binary both overfill their Roche lobes. The two stars actually share the same outer atmosphere.

52. Three Close Eclipsing Binaries Sketches of and light curves for three eclipsing binaries are shown. The phase denotes the fraction of the orbital period from one primary minimum to the next. (a) Algol, also known as � Persei, is a semidetached binary. The deep eclipse occurs when the giant star (right) blocks the light from the smaller, but more luminous, main-sequence star.Three Close Eclipsing Binaries Sketches of and light curves for three eclipsing binaries are shown. The phase denotes the fraction of the orbital period from one primary minimum to the next. (a) Algol, also known as � Persei, is a semidetached binary. The deep eclipse occurs when the giant star (right) blocks the light from the smaller, but more luminous, main-sequence star.

53. Three Close Eclipsing Binaries Sketches of and light curves for three eclipsing binaries are shown. The phase denotes the fraction of the orbital period from one primary minimum to the next. (b) � Lyrae is a semidetached binary in which mass transfer has produced an accretion disk that surrounds the detached star. This disk is so thick and opaque that it renders the secondary star almost invisible.Three Close Eclipsing Binaries Sketches of and light curves for three eclipsing binaries are shown. The phase denotes the fraction of the orbital period from one primary minimum to the next. (b) � Lyrae is a semidetached binary in which mass transfer has produced an accretion disk that surrounds the detached star. This disk is so thick and opaque that it renders the secondary star almost invisible.

54. Three Close Eclipsing Binaries Sketches of and light curves for three eclipsing binaries are shown. The phase denotes the fraction of the orbital period from one primary minimum to the next. (c) W Ursae Majoris is an overcontact binary. Both stars therefore share their outer atmospheres. The short, 8-h period of this binary indicates that the stars are very close to each other.Three Close Eclipsing Binaries Sketches of and light curves for three eclipsing binaries are shown. The phase denotes the fraction of the orbital period from one primary minimum to the next. (c) W Ursae Majoris is an overcontact binary. Both stars therefore share their outer atmospheres. The short, 8-h period of this binary indicates that the stars are very close to each other.

55. Mass Exchange Between Close Binary Stars This sequence of drawings shows how close binary stars can initially be isolated but, as they age, grow and exchange mass. Such mass exchange leads to different fates than if the same stars had evolved in isolation.Mass Exchange Between Close Binary Stars This sequence of drawings shows how close binary stars can initially be isolated but, as they age, grow and exchange mass. Such mass exchange leads to different fates than if the same stars had evolved in isolation.