Discovering the Universe Ninth Edition

1. Neil F. Comins • William J. Kaufmann III Discovering the Universe Ninth Edition CHAPTER 12 The Lives of the Stars from Birth Through Middle Age

2. WHAT DO YOU THINK? • How do stars form? • Are stars still forming today? If so, where? • Do more massive stars shine longer than less massive ones? What is your reasoning? • When stars like the Sun stop fusing hydrogen and helium in their cores, do the stars get smaller or larger?

3. In this chapter you will discover… • how stars form • what a stellar “nursery” looks like • how astronomers use the physical properties of stars to learn about stellar evolution • the remarkable transformations of older stars into giants • how the Hertzsprung-Russell (H-R) diagram is your guide to the stellar life cycle • how pairs of orbiting stars change each other

4. Everything Ages (a) 1935 (b) 1994

5. 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). The Pleiades lies about 440 ly (134 pc) from Earth. 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.

6. Stars and the Interstellar Medium The same region of the sky in a false-color infrared image taken by the Spitzer Space Telescope. Gases are seen here to exist in more areas than can be detected in visible light.

7. Stars and the Interstellar Medium Each dot plotted on this H-R diagram represents a star in the Pleiades whose luminosity and surface temperature have been determined. Note that most of the cool, low-mass stars have arrived at the main sequence, indicating that hydrogen fusion has begun in their core. The cluster has a diameter of about 5 ly, is about 100 million years old, and contains about 500 stars.

8. A Connection to Interstellar Space The charred layer created by overcooking this beef contains compounds of carbon and hydrogen, called polycyclic aromatic hydrocarbons. These molecules are also found in interstellar clouds.

9. 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 bluish stars to the left of the dark nebula are members of a star cluster called NGC 6520.

10. A Gas- and Dust-Rich Region of Orion This color-coded radio map of a large section of the sky shows the extent of giant molecular clouds in Orion and Monoceros as seen in the radio part of the spectrum. The intensity of carbon monoxide (CO) emission is displayed by colors in the order of the rainbow, from violet for the weakest to red for the strongest. Black indicates no detectable emission. The locations of four prominent star-forming nebulae are indicated on the star chart overlay. Note that the Orion and Horsehead nebulae are sites of intense CO emission, indicating that stars are forming in these regions.

11. A Gas- and Dust-Rich Region of Orion A variety of nebulae appear in the sky around Alnitak, the easternmost star in the belt of Orion. To the left of Alnitak is a bright, red emission nebula called NGC 2024. The glowing gases in emission nebulae are excited by UV 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 Nebula 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 it more effectively than any other color. All of this nebulosity lies about 1600ly from Earth, while the star Alnitak is only 815 ly away from us.

12. Interstellar Reddening 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.

13. 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.

14. A Supernova Remnant (a) X-ray 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 ly. The entire Cygnus Loop has an angular diameter in our sky 6 times wider than the Moon. (b) This visible-light Hubble Space Telescope image of part of the Cygnus Loop shows emission from different atoms false-color-coded with blue from oxygen, red from sulfur, and green from hydrogen.

15. 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 ly from Earth.

16. Protostar in a Bok Globule (a) This visible-light image shows a small dark nebula (equivalently, Bok globule) called L1014 located in the constellation Cygnus. (b)When viewed in the infrared, a protostar is visible within the nebula.

17. A Cluster of Protostars Over 300 protostars (yellow circles) were observed in the infrared by the Spitzer Space Telescope. This cluster of newly forming stars is 13,700 ly away in the constellation Centaurus. The nebula, some of whose gas is being converted into stars, is called RCW 49 and contains more than 2200 stars and protostars. Most of the interior of this nebula is hidden from our eyes by the dust it contains.

18. Pre–Main-Sequence Stars Seen in infrared, the two large bright objects in the center of this image are pre–main-sequence stars. They have recently shed their cocoons of gas and dust but still have strong stellar winds that create their irregular shapes. The two stars are an optical double; that is, they are not orbiting each other.

19. A Brown Dwarf Located 18 ly (6 pc) from Earth in the constellation Lepus (the Hare), Gliese 229B was the first confirmed brown dwarf ever observed. With a surface temperature of about 1000 K, its spectrum is similar to that of Jupiter. Gliese 229B is in orbit around a star. The overexposed image of part of its companion, Gliese 229A, appears on the left. The two bodies are separated by about 43 AU. Gliese 229B has from 20 to 50 times the mass of Jupiter, but the brown dwarf is compressed to the same size as Jupiter. The spike of light was produced when Gliese 229A overloaded part of the Hubble Space Telescope’s electronics.

20. 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.

21. A Stellar Nursery Full of Brown Dwarfs Besides containing more than 100 young stars, the rho Ophiuchi cloud, located 540 ly away in the constellation Ophiuchus, contains at least 30 brown dwarfs. By studying these objects, astronomers expect to learn more about early stellar evolution. This infrared image is color coded, with red indicating 7.7-µm radiation and blue indicating 14.5-µm radiation.

22. Mass Loss from a Supermassive Star Within the Quintuplet Cluster is one of the brightest known stars, called the Pistol. Astronomers calculate that the Pistol formed nearly 3 million years ago and originally had 100–200 solar masses. The structure of the gas cloud suggests the star ejected the gas we see in two episodes, 6000 and 4000 years ago. The gas from any previous ejections is so thinly spread now that we cannot see it. The nebula shown in the inset is more than 4 ly (1.25 pc) across—it would stretch from the Sun nearly to the closest star, Proxima Centauri. The image of the Quintuplet Cluster was taken in the infrared. The name Pistol was given to the star based on early, low-resolution radio images of its gas, which initially looked like an old-fashioned pistol aimed to the left near the top of the inset.

23. Mass Loss from a Supermassive Star The largest, most massive known star, LBV 1806-20, is 5 million times brighter and apparently some 150 times more massive than the Sun. This drawing shows the star’s color and its size compared to the Sun.

24. An H II Region The Eagle Nebula, M16, surrounds a star cluster. Star formation is presently occurring in M16. 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.

25. 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. This nebula’s mass is about 300 solar masses. Left inset: This view at visible wavelengths shows the inner regions of the Orion Nebula. At the lower left are four massive stars, the brightest members of the Trapezium star cluster, which cause the nebula to glow. Right inset: This view shows numerous infrared objects—many of which are stars in the early stages of formation—along with shock waves caused by matter flowing out of protostars faster than the speed of sound waves in the nebula. Shock waves from the Trapezium stars may have helped trigger the formation of the protostars in this view.

26. The Evolution of an OB Association High-speed particles and ultraviolet radiation from young O and B stars produce a shock wave that compresses gas farther into the molecular cloud, stimulating new star formation deeper in the cloud. Meanwhile, older stars are left behind. Inset: Stars forming around a massive star 2500 ly away in the constellation Monoceros’s Cone Nebula. The stars (small dots on the right side of the inset) arrayed around the bright, massive central star are believed to have formed as a result of the central star compressing surrounding gas with high-speed particles and radiation. The younger stars are just 0.04–0.08 ly from the central star.

27. Plotting the Ages of Stars This photograph shows a region of ionized hydrogen and the young star cluster NGC2264 in the constellation Monoceros. The red nebulosity is located about 2600 ly from Earth and contains numerous stars that are about to begin hydrogen fusion in their cores.

28. Plotting the Ages of Stars Each dot plotted on this H-R diagram represents a star in NGC 2264 whose luminosity and surface temperature have been measured. Note that most of the cool, low-mass stars have not yet arrived at the main sequence. Calculations of stellar evolution indicate that this star cluster started forming about 2 million years ago.

29. A Summary of the Star Formation Process

30. 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 descends into the core. This process continues until the entire star is helium.

31. Evolution of Stars Off the Main Sequence (a) Hydrogen fusion occurs in the core of main-sequence stars. (b) 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. (c) Eventually, the core reaches 108 K, whereupon core helium fusion begins. This activity causes the core to expand, slowing the hydrogen shell fusion and thereby forcing the outer layers of the star to contract.

32. A Mass-Loss Star A red giant star is shedding its outer layers, thereby creating this reflection nebula, labeled IC 2220 and called Toby Jug, located in the constellation Carina. The star is embedded inside the nebula and is not visible in this image.

33. 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 hydrogen-fusing 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.

34. The Sun Today and as a Giant This composite of visible and infrared images shows red giant stars in the open cluster M50 in the constellation of Monoceros (the Unicorn).

35. 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.

36. 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.

37. 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.

38. Analogy for Cepheid Variability (a) As pressure builds up in this pot, the force on the lid (analogous to a Cepheid’s outer layers) increases. (b) When the pressure inside the pot is sufficient, it lifts the lid off (expands the star’s outer layers) and thereby allows some of the energy inside to escape. This process cycles (two cycles are shown here), as do the luminosity and temperature of Cepheid stars.

39. 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 (δ Cephei stars) are brighter, more massive, and more metal-rich stars than Type II Cepheids. The greater brightness of the Type I Cepheids is a result of their higher mass.

40. A Globular Cluster This cluster, M10, is about 85 ly across and is located in the constellation Ophiuchus (the Serpent Holder), roughly 16,000 ly 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.

41. 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.

42. Structure of the H-R Diagram (a) Data taken by the Hipparcos satellite placed 41,453 stars more precisely on the H-R diagram than any previous observations. This figure shows the overall structure of the H-R diagram. The thickness of the main sequence is due in large part to stars of different ages turning off the main sequence at different places, as shown in (b). (b) 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.

43. The Evolution of a Theoretical Cluster of 100 Stars

44. 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 Hγ (410 nm) and Hδ (434 nm).

45. Detached, Semidetached, Contact, and Over-Contact Binaries (a) In a detached binary, neither star fills its Roche lobe. (b) If one star fills its Roche lobe, the binary is semidetached. Mass transfer is often observed in semidetached binaries. (c) In a contact binary, both stars fill their Roche lobes. (d) The two stars in an over-contact binary both overfill their Roche lobes. The two stars actually share the same outer atmosphere.

46. Three Close 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 β 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. (b) β 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. (c) W Ursae Majoris is an over-contact 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.

47. 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.

48. Summary of Key Ideas

49. Protostars and Pre–Main-Sequence Stars • Enormous, cold clouds of gas and dust, called giant molecular clouds, are scattered about the disk of the Galaxy. • Star formation begins when gravitational attraction causes clumps of gas and dust, called protostars, to coalesce in Bok globules within a giant molecular cloud. As a protostar contracts, its matter begins to heat and glow. When the contraction slows down, the protostar becomes a pre–main-sequence star. When the pre–main-sequence star’s core temperature becomes high enough to begin hydrogen fusion and stop contracting, it becomes a main-sequence star.

50. Protostars and Pre–Main-Sequence Stars • The most massive pre–main-sequence stars take the shortest time to become main-sequence stars (O and B stars). • In the final stages of pre–main-sequence contraction, when hydrogen fusion is about to begin in the core, the pre–main-sequence star may undergo vigorous chromospheric activity that ejects large amounts of matter into space. G, K, and M stars at this stage are called T Tauri stars. • A collection of a few hundred or a few thousand newborn stars formed in the plane of the Galaxy is called an open cluster. Stars escape from open clusters, most of which eventually dissipate.