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
The young super star cluster R136 is located in the Tarantula Nebula in the nearby Large Magellanic Cloud galaxy. These stars, surrounded by glowing interstellar clouds, are only a few million years old. (NASA, ESA, and F. Paresce [INAF-IASF, Bologna, Italy], R. O’Connell [UVA, Charlottesville], and Wide Field Camera 3 Science Oversight Committee)

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 FIGURE 12-1 Everything Ages
In less than 60 years (1935 to 1994), corrosive gases in the air caused this statue of George Washington in New York City to erode even more
rapidly than normal. Even without the significant pollution that humans add to the air, everything in contact with it decays. (a: 1994 NYC Parks
Photo Archive/Fundamental Photographs; b: © 1994 Kristen Brochman/Fundamental Photographs)
Stars also have a life cycle. Since even short-lived stars live for millions of years, it’s unusual to see a noticeable change in anyone’s lifetime, though it does happen occasionally.

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 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 (a reflection nebula).
FIGURE 12-2 Stars and the Interstellar Medium
(a) This open cluster, called the Pleiades, can easily be seen with the naked eye in the constellation Taurus (the Bull). Pleiades lies about 440 ly (134 pc) from Earth. The stars are not shedding mass, unlike the stars in Figures 12-15a and 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. (Anglo-Australian Observatory)

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.
FIGURE 12-2 Stars and the Interstellar Medium
(b) 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.
(NASA/JPL-Caltech/J. Stauffer[SSC/Caltech])

7. We see an emission nebula via:
A. reflected blue light from a nearby star or stars.
B. blue light emitted by hot (excited) hydrogen atoms.
C. red light emitted by hot (excited) hydrogen atoms.
D. reflected red light from a nearby star.

8. Stars and the Interstellar Medium
H-R diagram for 500 stars in the Pleiades . 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.
FIGURE 12-2 Stars and the Interstellar Medium
(c) 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.
Some of the high-mass, hot, luminous, blue stars at the upper end plot above the main sequence. We shall see that this is because they are finishing their time on the MS and are leaving it; some are already long gone.

9. A Connection to Interstellar Space
FIGURE 12-3 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. (Dena Digilio Betz)
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.

10. Interstellar Reddening
FIGURE 12-6 Interstellar Reddening
(a) 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.
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.

11. Interstellar Reddening
FIGURE 12-6 Interstellar Reddening
(b) 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 (Anglo-Australian Observatory)
Light from these two nebulae pass through different amounts of interstellar dust and therefore they a have different colors. Because NGC 3603 is farther away, its color is completely dominated by the Hα line, while NGC 3576 has some Hβ.

12. A Dark Nebula
FIGURE 12-4 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 (Anglo-Australian Observatory)
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.

13. A Gas- and Dust-Rich Region of Orion
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.
FIGURE 12-5 A Gas- and Dust-Rich Region of Orion
(a) 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. (R. Maddalena, M. Morris, J.
Moscowitz, and P. Thaddeus)

14. 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 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 1600 ly from Earth, while the star Alnitak is only 815 ly away from us.
FIGURE 12-5 A Gas- and Dust-Rich Region of Orion
(b, c) A variety of nebulae appear in the sky around Alnitak, also called ζ (zeta) Orionis, the easternmost star in the belt of Orion. To the left of Alnitak is a bright, red emission nebula called NGC 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 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 1600 ly from Earth, while the star Alnitak is only 815 ly away from us. NGC refers to the New General Catalog of stars and IC stands for Index Catalogs, two supplements to the NGC. (b: Royal Observatory, Edinburgh; c: R. C. Mitchell, Central Washington University)

15. A Supernova Remnant
FIGURE 12-7 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. (a: Nancy Levenson/NASA; b: Jeff Hester, Arizona State University and NASA)
(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.
(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.

16. Core of the Rosette Nebula – Sweeping Dust
FIGURE 12-8 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. (Anglo-Australian Observatory)
The “young stars” which are doing this are the high-mass, extremely luminous blue stars. There will also be lower-mass young stars that aren’t luminous enough to have much effect. A “Bok globule” is a small dark nebula.

17. Protostar in a Bok Globule
FIGURE 12-9 Protostar in a Bok Globule
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. (a: Deep Sky Survey; b: NASA/JPL-Caltech/N. Evans, University of Texas at Austin)
This demonstrates the value of infrared astronomy in peering through dust.
(a) This visible-light image shows a small dark nebula (equivalently, Bok globule) . (b) When viewed in the infrared, a protostar is visible within the nebula.

18. Protostars are not seen in visible light telescopes because:
A. they don’t emit any radiation
B. they are surrounded by clouds of gas and dust
C. they only emit infrared radiation
D. they are all moving away from Earth so fast that their visible light is Doppler shifted into the infrared

19. A Cluster of Protostars – Same Technique
FIGURE 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 RCWv49 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. (NASA/JPL-Caltech/E. Churchwell, University of Wisconsin)

20. 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.
FIGURE 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.
(Atlas Image courtesy of 2MASS/UMASS/IPAC-Caltech/NASA/NSF)
A pre-main-sequence star has started fusion of hydrogen to helium in its core, but it is still settling.

21. Summary so far Nebulae containing gas and dust are plentiful
Protostars and stars are seen in such nebulae
Star formation is ongoing
While a protostar is collapsing, it is heating up from the gravitational potential energy. Some of the heat is radiated away, and some of it goes to heat up the core. Once the core temperature reaches about 10 million K, hydrogen fusion in the core starts, and it has become a pre-main-sequence star. The star continues to settle. Core temperature rises, thermonuclear fusion increases, and the star reaches a steady state, in which core fusion produces energy as fast as it is radiated away and the star’s size stabilizes. It is now a main sequence star. As it settles, its radiation blows away much of the gas and dust around it

22. A Brown Dwarf - a “Failed Star”
FIGURE 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. (S. Kularni, California Institute of Technology; D. Golimowski, Johns Hopkins University; NASA)
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 its companion Gliese 229A 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 is not real – it stems from an electronics overload .

23. A brown dwarf is best described as: A
A brown dwarf is best described as: A. a low mass object that doesn’t fuse in its core B. a low mass main sequence star C. a high mass main sequence star D. an object of dust too small to classify as a planet

24. Pre–Main-Sequence Evolutionary Tracks
FIGURE 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 (see Figure 11-14a).
Low mass stars (say the 0.5 Msun star) will take 30 million years to reach the main sequence, whereas the 15 Msun star gets there in a jiffy: only 100,000 years.
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..

25. 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. (Infrared Space Observatory, NASA)
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.

26. 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.
FIGURE Mass Loss from a Supermassive Star
The Quintuplet Cluster is 25,000 ly from Earth. Inset: Within the 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 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. (D. Filger, NASA)

27. Mass Loss from a Supermassive Star
FIGURE Mass Loss from a Supermassive Star
(b) The largest, most massive known star, LBV , 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. (Image by Dr. Stephen Eikenberry, Meghan
Kennedy/University of Florida)
Artist’s conception, we can’t see detail on ANY star … yet. “LBV” is Luminous Blue Variable.
The largest, most massive known star, LBV , 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.

28. An H II Region
FIGURE 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 ly 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)
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.

29. 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.
FIGURE 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 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 that infrared radiation penetrates interstellar dust that absorbs visible photons. Numerous infrared objects, many of which are stars in the early stages of formation, can be seen, 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.
(ESO—European Southern Observatory; left inset: C. R. O’Dell,
S. K. Wong, and NASA; right inset: R. Thompson, M. Rieke, G. Schneider,
S. Stolovy, E. Erickson, D. Axon, and NASA)
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.

30. The Evolution of an OB Association
FIGURE 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 (770 pc) 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. (Adapted from C. Lada, L. Blitz, and B. Elmegreen; inset:
R. Thompson, M. Rieke, G. Schneider, and NASA)
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.

31. Plotting the Ages of Stars
This photograph shows a region of ionized hydrogen and the young star cluster NGC 2264 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.
FIGURE Plotting the Ages of Stars
(a) This photograph shows a region of ionized hydrogen and the
young star cluster NGC 2264 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. (David Malin/Anglo-Australian Observatory)

32. Plotting the Ages of Stars-Cluster Only 2 Million yr
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.
FIGURE Plotting the Ages of Stars
(b) Each dot plotted on this H-R diagram represents a star in this cluster 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.

33. Why are A-type main sequence stars hotter than G-type main sequence stars?
A. A-type stars have cores of metal, whereas G-type stars do not
B. A-type stars have more fusion on their surface than G-type stars
C. A-type stars have more fusion in their cores than G-type stars
D. A-type stars fuse in their cores and near their surfaces, while G-type stars only fuse in their cores.

34. A Summary of the Star Formation Process
FIGURE A Summary of the Star Formation Process
This set of drawings takes you through the star formation process for the Sun and other stars with less than about 1.5 solar masses.

35. Some Differences From Sun: Fully Convective Star
FIGURE 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.
A red dwarf is a low-mass main-sequence star (0.4 Msun and less).
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.

36. The star is on the main sequence
It fuses hydrogen to helium, just as our Sun does. (Some stars have a variation.)
It spends 80-90% of its lifetime on main sequence.
It very slowly brightens.
… then life gets exciting

37. Evolution of Stars Off the Main Sequence
FIGURE 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, and 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.
Helium (atomic number = 2, mass = 4) fuses to carbon (atomic number = 6, mass = 12). Three helium nuclei at a time have to fuse, because two helium nuclei cannot be fused to anything which liberates energy.
(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.

38. What is Helium Fusion
4He + 4He + 4He  12C Three Heliums fuse to Carbon
4He + 12C  16O Some of the C picks up one more He
It takes 3 He’s: two of them won’t create any energy, no matter how you fuse them
It takes a temperature of 100 million K
It begins with a spike for low-mass stars
The spike is the “helium flash,” for stars up to 3 Msun or so. The turn-on is gradual for higher-mass stars. See textbook about “electron degeneracy pressure;” the cause of the helium flash. Electron degeneracy pressure will come back next chapter with a bang.

39. A Mass-Loss Star
FIGURE 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. (Anglo-Australian Observatory)
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.

40. Red giants burn helium via nuclear fusion in their core
Red giants burn helium via nuclear fusion in their core. The ash (end product) of this nuclear fusion is: A. iron. B. hydrogen. C. lithium and carbon. D. carbon and oxygen.

41. The Sun Today and as a Giant
FIGURE The Sun Today and as a Giant
(a) 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.
Good news: 1 AU diameter means 0.5 AU radius so it doesn’t engulf Earth (yet). Bad news: 2000 times increase in luminosity incinerates Earth. Worse news: final exam in course comes sooner, so end of the world won’t save you.
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.

42. Red Giant Stars
FIGURE The Sun Today and as a Giant
(b) This composite of visible and infrared images shows red giant stars in the open cluster M50 in the constellation of Monoceros (the Unicorn). (T. Credner and S. Kohle, Astronomical Institutes of the University of Bonn)
This composite of visible and infrared images shows red giant stars in the open cluster M50 in the constellation of Monoceros (the Unicorn).

43. Post–Main-Sequence Evolution
FIGURE 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.
The Sun is a red giant from the point marked “sun is a red giant” until the point marked “Helium flash.” It just gets larger and cooler, in such a way that luminosity increases
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.

44. 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.
FIGURE Post–Main-Sequence Evolution
(b) Model-based evolutionary tracks of five stars are shown onthis 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.
See the textbook about “electron degeneracy pressure” and the helium flash. The helium flash is of moderate interest for Astro 101, but understanding electron degeneracy pressure will be essential to understand supernovae.

45. 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.
FIGURE 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.
In 1912, Henrietta Leavitt showed that Cepheid Variables have a simple relationship between their luminosity and pulsation period.

46. 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.
FIGURE 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. (a–d: Janet Horton)

47. The Period-Luminosity Relation for Cepheids
FIGURE 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. (Adapted from H. C. Arp)
So why is this so important? Measuring distances! Remember, parallax, the only direct way, is only good to a several hundred LY. If a star can be identified as a Cepheid, its period is straightforward: just plot its light curve for a few weeks or months (virtually nothing in astronomy is really easy) and measure the period. Then read the graph above to get its luminosity. Now, its apparent brightness (“easily” measured) is proportional to luminosity divided by the square of the distance. So, solve for distance. We’ll see how Edwin Hubble used Cepheids about 1924 to revolutionize our concept of the universe.
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.

48. A Globular Cluster
FIGURE 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. (T. Credner and S. Kohle,
Astronomical Institutes of the University of Bonn)
Back to the theme of the star’s life cycle. “Horizontal branch” means stars after they start core He fusion. They shrink and heat up in such a way as to keep their luminosity approximately constant. Hence their position on the H-R diagram is approximately horizontal from right to left, as shown in fig
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.

49. An H-R Diagram of a Globular Cluster
FIGURE 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.
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.

50. Mass, Temperature, Luminosity, and Lifetime
That 25 Msun star has 25 times as much “gas in the tank” to start life, compared to the Sun. But … it burns it 80,000 times faster. That means its lifetime should be 25/80,000 that of the Sun. 25/80000=1/3200. The Sun’s lifetime is 10,000 million hears, so 10,000 x 1/3200 = million years. The table says 3 million years, clearly close enough. The universe is 14 billion years old, so no star 0.75 Msun or less has left the main sequence yet.
First point: high-mass stars live much shorter lives. They also are much more luminous (cause of short life), larger when on main sequence (chapter 11), hotter, and hence bluer, radiating much of their energy in the UV.
Second point: if the stars in a cluster start forming about the same time (and there is plenty of reason to believe that they do), the high-mass stars will go through their entire lifetime before the lowest mass stars have finished settling onto the main sequence. This can be used to determine the age of a star cluster, as the next slide shows.
High-mass stars consume their fuel MUCH faster.
The age of a cluster can be determined by examining an H-R diagram of its stars.

51. Structure of the H-R Diagram and Main Sequence Turnoff
FIGURE Structure of the H-R Diagram
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).
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.
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.

52. The Evolution of a Theoretical Cluster of 100 Stars
FIGURE 12-31
(c–j) Summary of the evolution of a theoretical cluster of 100 stars, as shown by their locations on the H-R diagram. (In principle, each star’s evolution could be followed separately.) After a star passes through the red giant phase, it is deleted from the diagram.

53. 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 (434 nm) and Hδ (410 nm).
FIGURE 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). (Lick Observatory)
Metal – in astronomy, any element with atomic number = 3 or higher is called a “metal”. Terrible chemistry. We are stuck with the usage.
“Metal-Rich” would mean maybe 1% “metals” and 99% hydrogen and helium. “Metal-poor” would be more like 0.01% “metals”.
Population I and Population II are the terms used by astronomers for “Metal-Rich” and “Metal-Poor” respectively.

54. Detached, Semidetached, Contact, and Over-Contact Binaries
FIGURE 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.
(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.

55. 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.
FIGURE 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.

56. Mass Exchange Between Close Binary Stars
FIGURE 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.
A case history. The two stars can be far enough apart when both are on the main sequence that they each run through main sequence as if they were independent. But when one turns into a red giant, they interact. They are the same distance apart, but the swelling to a red giant spans that distance.
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.

57. Summary of Key Ideas

58. 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.

59. 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.

60. Main-Sequence and Giant Stars
The Sun has been a main-sequence star for 4.6 billion years and should remain so for about another 5 billion years. Less massive stars than the Sun evolve more slowly and have longer main-sequence lifetimes. More massive stars than the Sun evolve more rapidly and have shorter main-sequence lifetimes.
Main-sequence stars with a solar mass between 0.08 and 0.4 convert all of their mass into helium and then stop fusing. Their lifetimes last hundreds of billions of years, so none of these stars has yet left the main sequence.
Core hydrogen fusion ceases when hydrogen in the core of a main-sequence star with a solar mass greater than 0.4 is gone, leaving a core of nearly pure helium surrounded by a shell where hydrogen fusion continues. Hydrogen shell fusion adds more helium to the star’s core, which contracts and becomes hotter. The outer atmosphere expands considerably, and the star becomes a giant.

61. Main-Sequence and Giant Stars
When the central temperature of a giant reaches about 100 million K, the thermonuclear process of helium fusion begins. This process converts helium to carbon, then to oxygen. In a massive giant, helium fusion begins gradually. In a less massive giant, it begins suddenly in a process called helium flash.
The age of a stellar cluster can be estimated by plotting its stars on an H-R diagram. The upper portion of the main sequence disappears first, because more massive main-sequence stars become giants before low-mass stars do.
Giants undergo extensive mass loss, sometimes producing shells of ejected material that surround the entire star.
Relatively young stars are metal rich (Population I); ancient stars are metal poor (Population II).

62. Clusters of Stars
Groups of between a few hundred and a few thousand stars, formed together from a single interstellar cloud in the disk of our Galaxy, are called open clusters.
Star in open clusters go their separate ways.
Groups of hundreds of thousands to millions of stars formed together from a common interstellar cloud are called globular clusters.
Stars in globular clusters remain bound together.

63. Variable Stars
When a star’s evolutionary track carries it through a region called the instability strip in the H-R diagram, the star becomes unstable and begins to pulsate.
RR Lyrae variables are low-mass, pulsating variables with short periods. Cepheid variables are high-mass, pulsating variables exhibiting a regular relationship between the period of pulsation and luminosity.
Mass can be transferred from one star to another in close binary systems. When this occurs, the evolution of the two stars changes.

64. Key Terms accretion disk birth line Bok globule brown dwarf
Cepheid variable
contact binary
core helium fusion
dark nebulae
dense core
detached binary
electron degeneracy pressure
emission nebula
evolutionary track
giant molecular cloud
globular cluster
H II regions
helium flash
horizontal-branch star
hydrogen shell fusion
instability strip
interstellar extinction
interstellar medium
interstellar reddening
Jeans instability
molecular cloud
nebula (plural nebulae)
OB association
open cluster
over-contact binary
Pauli exclusion principle
period-luminosity relation
Population I star
Population II star
pre–main-sequence
star
protostar
red dwarf
reflection nebula
Roche lobe
RR Lyrae variable
semidetached binary
supernova remnant
T Tauri stars
turnoff point
Type I Cepheid
Type II Cepheid
variable stars
zero-age main sequence
(ZAMS)

65. WHAT DID YOU THINK? How do stars form?
Stars form from the collective gravitational attraction of a clump of gas and dust inside a giant molecular cloud.

66. WHAT DID YOU THINK? Are stars still forming today? If so, where?
Yes. Astronomers have seen stars that have just arrived on the main sequence, as well as infrared images of gas and dust clouds in the process of forming stars. Most stars in the Milky Way form in giant molecular clouds in the disk of the Galaxy.

67. WHAT DID YOU THINK?
Do more massive stars shine longer than less massive ones? What is your reasoning?
No. Lower-mass stars last longer because the lower gravitational force inside them causes fusion to take place at slower rates compared to the fusion inside higher-mass stars. These latter stars therefore use up their fuel more rapidly than do lower mass stars.

68. WHAT DID YOU THINK?
When stars like the Sun stop fusing hydrogen and helium in their cores, do the stars get smaller or larger?
They get larger. Such stars start fusing hydrogen and helium outside their cores. This new fusion, closer to the star’s surface, is able to push the star’s outer layers out farther than they had been before.